JEE Electromagnetic Induction Questions

Magnetic flux $$\varphi$$ through a coil with $$N$$ turns and area $$A$$ in a uniform magnetic field $$\overrightarrow{B}$$ is given by the formula $$\varphi = NAB\cos\theta\quad\text{-(1)}$$ where $$\theta$$ is the angle between the normal to the coil and the magnetic field.

Since the coil rotates with angular velocity $$\omega$$ about an axis perpendicular to $$\overrightarrow{B}$$, the angle varies as $$\theta = \omega t\,.$$ Substituting into $$(1)$$ gives $$\varphi = NAB\cos(\omega t)\quad\text{-(2)}$$

The induced emf $$\varepsilon$$ is given by Faraday’s law: $$\varepsilon = -\frac{d\varphi}{dt}\,. $$ Using $$(2)$$, we find $$\varepsilon = -\frac{d}{dt}\bigl(NAB\cos(\omega t)\bigr) = NAB\,\omega\sin(\omega t)\quad\text{-(3)}$$

When $$\overrightarrow{B}$$ is parallel to the plane of the coil, its normal is perpendicular to $$\overrightarrow{B}$$ so that $$\theta = 90^\circ = \frac{\pi}{2}\,.$$ Then from $$(2)$$, $$\cos\bigl(\tfrac{\pi}{2}\bigr)=0\implies \varphi = 0\,,$$ and from $$(3)$$, $$\sin\bigl(\tfrac{\pi}{2}\bigr)=1\implies \varepsilon = NAB\,\omega\,.$$ Hence at that instant $$\varphi = 0\,,\quad \varepsilon = NAB\,\omega\,,$$ which matches Option C.

We start by noting that when a conducting disc rotates in a magnetic field perpendicular to its plane, the free electrons experience a force due to their velocity from rotation; this generates an emf between the center and the rim (Faraday disc/homopolar generator).

Next, consider a small radial element at distance $$r$$ from the center with width $$dr$$; its tangential velocity is $$v = \omega r$$, and the motional emf across this element is $$dE = Bv\,dr = B\omega r\,dr$$. To find the total emf between the center ($$r=0$$) and the rim ($$r=R$$), we integrate:

$$ E = \int_0^R B\omega r\,dr = B\omega \left[\frac{r^2}{2}\right]_0^R = \frac{1}{2}B\omega R^2 $$

Substituting the given values $$B = 0.4$$ T, $$\omega = 10\pi$$ rad/s, and $$R = 20$$ cm = 0.2 m into $$ E = \frac{1}{2}B\omega R^2 $$ yields

$$ E = \frac{1}{2} \times 0.4 \times 10\pi \times (0.2)^2 $$

$$ = \frac{1}{2} \times 0.4 \times 10\pi \times 0.04 $$

$$ = \frac{1}{2} \times 0.4 \times 0.4\pi $$

$$ = \frac{1}{2} \times 0.16\pi $$

$$ = 0.08\pi $$

$$ = 0.08 \times 3.14 = 0.2512 \text{ V} $$

Therefore, the potential difference between the axis and the rim is 0.2512 V, corresponding to Option C.

The magnetic energy stored in any region is obtained from the energy density formula for a magnetic field inside a linear medium.

Energy density in a magnetic material is given by
$$u = \frac{1}{2} B H$$

For a linear medium, $$B = \mu H$$, where $$\mu = \mu_0 \mu_r$$. Substituting $$H = \frac{B}{\mu}$$ in the energy density expression:

$$u = \frac{1}{2} B \left(\frac{B}{\mu}\right) = \frac{B^2}{2\mu}$$ $$-(1)$$

The solenoid is completely filled with a material of relative permeability $$\mu_r = 2$$, so
$$\mu = \mu_0 \mu_r = 2\mu_0$$.

Substituting this value of $$\mu$$ in equation $$(1)$$:

$$u = \frac{B^2}{2(2\mu_0)} = \frac{B^2}{4\mu_0}$$ $$-(2)$$

The total magnetic energy $$U$$ stored in the solenoid equals the energy density multiplied by the volume of the field region.
Volume of the solenoid, $$V = A l$$.

Therefore,
$$U = u \, V = \frac{B^2}{4\mu_0} \, (A l) = \frac{B^2 A l}{4\mu_0}$$.

Thus, the magnetic energy stored in the solenoid is $$\frac{B^2 A l}{4\mu_0}$$.

Correct option: Option D

Let the uniform magnetic field $$\mathbf{B}$$ be perpendicular to the plane of a rectangular loop of breadth $$b$$ (side perpendicular to the direction of motion) and length $$\ell$$ (side parallel to the direction of motion).
The loop is pulled out of the field with a constant speed $$v$$.

Magnetic flux linked with the loop is
$$\Phi = B \times \text{(area of loop that still lies inside the field)}$$

1. For $$t \lt t_1$$ : the whole loop is inside the field, so the enclosed area is constant ( $$= b\ell$$ ).
 $$\displaystyle \frac{d\Phi}{dt}=0 \;\Rightarrow\; \varepsilon = 0$$

2. From $$t_1$$ to $$t_2$$ : the front edge has crossed the field boundary, the rear edge is still inside.
 At an instant when the front edge has moved a distance $$x = v(t-t_1)$$ out of the field, the length still inside is $$\ell - x$$.
 Enclosed area $$A = b(\ell - x)$$, therefore
 $$\Phi = B b(\ell - x)$$

Using Faraday’s law $$\varepsilon = \left|\dfrac{d\Phi}{dt}\right|$$:
 $$\varepsilon = \left|B b \dfrac{d(-x)}{dt}\right| = B b v$$

• $$B$$, $$b$$ and $$v$$ are constants ⇒ $$\varepsilon$$ remains constant in this entire interval.
• The interval lasts until the rear edge reaches the boundary: $$x = \ell \Rightarrow t_2 - t_1 = \dfrac{\ell}{v}$$.

3. For $$t \gt t_2$$ : the whole loop is outside, flux is zero again, so $$\varepsilon = 0$$.

Hence the magnitude-time graph is
• zero up to $$t_1$$,
• a horizontal line of height $$B b v$$ from $$t_1$$ to $$t_2$$,
• zero after $$t_2$$.

This is a rectangular (top-hat) pulse, matching the sketch shown as Option D (image 4).
Therefore, the correct choice is Option D.

Let us analyze each statement about self-inductance:

Statement A: The self-inductance of a coil depends on its geometry (shape, size, number of turns). This is correct. For example, for a solenoid, $$L = \mu_0 n^2 A l$$.

Statement B: Self-inductance does not depend on the permeability of the medium. This is incorrect. Self-inductance is directly proportional to the permeability. For a solenoid with a core, $$L = \mu n^2 A l$$, where $$\mu = \mu_0 \mu_r$$.

Statement C: Self-induced e.m.f. opposes any change in the current in a circuit. This is correct (Lenz's law). The self-induced emf is $$\varepsilon = -L\frac{dI}{dt}$$.

Statement D: Self-inductance is the electromagnetic analogue of mass in mechanics. This is correct. In the energy analogy, $$\frac{1}{2}LI^2$$ (electromagnetic) corresponds to $$\frac{1}{2}mv^2$$ (mechanical), so $$L$$ is analogous to mass $$m$$.

Statement E: Work needs to be done against self-induced e.m.f. in establishing the current. This is correct. The energy stored in the inductor $$\frac{1}{2}LI^2$$ is the work done against the self-induced emf.

Statements A, C, D, E are correct.

The correct answer is Option 3: A, C, D, E only.

$$|\varepsilon| = L|dI/dt|$$. $$dI = 2-(-2) = 4$$ A, $$dt = 0.2$$ s.

$$0.1 = L×4/0.2 = 20L$$. $$L = 0.005$$ H = 5 mH.

The correct answer is Option (3): 5 mH.

A rectangular loop of length 2.5 m and width 2 m is placed at an angle of 60° with a magnetic field of 4 T and is removed from the field in 10 s.

The magnetic flux through the loop initially is given by $$\Phi = BA\cos\theta = 4 \times (2.5 \times 2) \times \cos 60° = 4 \times 5 \times 0.5 = 10$$ Wb.

According to Faraday’s law, the average induced emf is $$\mathcal{E} = -\frac{\Delta\Phi}{\Delta t} = -\frac{0 - 10}{10} = \frac{10}{10} = +1$$ V.

The correct answer is Option C: +1 V.

$$|emf| = N\frac{d\Phi}{dt} = N \cdot A \cdot \frac{\Delta B}{\Delta t}$$.

$$A = \pi(0.01)^2 = \pi \times 10^{-4}$$. $$\Delta B = 5000-3000 = 2000$$ T. $$\Delta t = 2$$ s.

$$22 = N \times \pi \times 10^{-4} \times 1000 = N \times 0.1\pi$$.

$$N = \frac{22}{0.1\pi} = \frac{220}{\pi} \approx 70$$.

The answer is Option (2): 70.

Find the electric flux through a surface of area 4 m$$^2$$.

$$\Phi = \vec{E} \cdot \vec{A} = \vec{E} \cdot (A\hat{n})$$

$$\vec{E} = \frac{2\hat{i} + 6\hat{j} + 8\hat{k}}{\sqrt{6}}$$, $$\hat{n} = \frac{2\hat{i} + \hat{j} + \hat{k}}{\sqrt{6}}$$, $$A = 4$$ m$$^2$$.

$$\vec{E} \cdot \hat{n} = \frac{1}{\sqrt{6}} \times \frac{1}{\sqrt{6}}(2\times2 + 6\times1 + 8\times1) = \frac{4+6+8}{6} = \frac{18}{6} = 3$$

$$\Phi = (\vec{E}\cdot\hat{n}) \times A = 3 \times 4 = 12 \text{ Vm}$$

The correct answer is 12.

Electric potential at the surface of a nucleus:

$$V = \frac{1}{4\pi\epsilon_0}\frac{Ze}{R}$$

$$Z = 50$$, $$R = 9 \times 10^{-13}$$ cm $$= 9 \times 10^{-15}$$ m.

$$V = 9 \times 10^9 \times \frac{50 \times 1.6 \times 10^{-19}}{9 \times 10^{-15}}$$

$$= 9 \times 10^9 \times \frac{80 \times 10^{-19}}{9 \times 10^{-15}}$$

$$= 10^9 \times \frac{80 \times 10^{-19}}{10^{-15}}$$

$$= 10^9 \times 80 \times 10^{-4} = 80 \times 10^5 = 8 \times 10^6$$ V

So $$\alpha = 8$$.

The answer is $$\boxed{8}$$.

The EMF induced in a conducting rod rotating about one end in a magnetic field is given by:

$$\varepsilon = \frac{1}{2}B\omega L^2$$

Here, $$B$$ is the component of the magnetic field perpendicular to the plane of rotation, $$\omega$$ is the angular velocity, and $$L$$ is the length of the rod (blade). In this problem, each blade has length $$L = 80 \text{ cm} = 0.8 \text{ m}$$, the fan rotates at $$\omega = 1200 \text{ rpm} = \frac{1200 \times 2\pi}{60} = 40\pi \text{ rad/s}$$, the magnitude of Earth's magnetic field is $$B = 0.5 \text{ G} = 0.5 \times 10^{-4} \text{ T}$$, and the angle of dip is $$\delta = 30°$$.

Since the fan rotates in the horizontal plane, the perpendicular (vertical) component of the magnetic field is

$$B_v = B \sin\delta = 0.5 \times 10^{-4} \times \sin 30° = 0.5 \times 10^{-4} \times 0.5 = 0.25 \times 10^{-4} \text{ T}$$

Substituting this into the expression for the induced EMF in one blade gives

$$\varepsilon = \frac{1}{2} \times B_v \times \omega \times L^2$$

$$\varepsilon = \frac{1}{2} \times 0.25 \times 10^{-4} \times 40\pi \times (0.8)^2$$

$$\varepsilon = \frac{1}{2} \times 0.25 \times 10^{-4} \times 40\pi \times 0.64$$

$$\varepsilon = \frac{1}{2} \times 0.25 \times 40 \times 0.64 \times \pi \times 10^{-4}$$

$$\varepsilon = \frac{1}{2} \times 6.4 \times \pi \times 10^{-4} = 3.2\pi \times 10^{-4} = 32\pi \times 10^{-5} \text{ V}$$

Comparing this result with the form $$N\pi \times 10^{-5}$$ shows that $$N = 32$$.

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We have a rectangular loop of sides 12 cm and 5 cm moving with velocity $$v = 5$$ cm/s in the positive x-direction. The magnetic field is in the positive z-direction with a spatial gradient of $$\frac{dB}{dx} = -10^{-3}$$ T/cm (along the negative x-direction) and is decreasing in time at a rate $$\frac{dB}{dt} = -10^{-3}$$ T/s. The sides of the loop are parallel to the x- and y-axes, so we define $$\ell_x = 12$$ cm and $$\ell_y = 5$$ cm.

The motional EMF due to the spatial variation of the magnetic field arises because the two sides parallel to the y-axis, each of length $$\ell_y$$, experience different values of $$B$$. This EMF is given by $$\varepsilon_{\text{motional}} = \frac{dB}{dx} \cdot \ell_x \cdot v \cdot \ell_y$$ and numerically evaluates to $$\varepsilon_{\text{motional}} = 10^{-3} \times 12 \times 5 \times 5 = 300 \times 10^{-3} \text{ T cm}^2/\text{s}$$.

To express this in standard SI units, we convert $$\frac{dB}{dx} = 10^{-3} \text{ T/cm} = 0.1 \text{ T/m},\quad \ell_x = 12 \text{ cm} = 0.12 \text{ m},\quad \ell_y = 5 \text{ cm} = 0.05 \text{ m},\quad v = 5 \text{ cm/s} = 0.05 \text{ m/s},$$ so that $$\varepsilon_{\text{motional}} = 0.1 \times 0.12 \times 0.05 \times 0.05 = 3 \times 10^{-5} \text{ V}$$.

The EMF induced by the time variation of the magnetic field is $$\varepsilon_{\text{time}} = -\frac{dB}{dt}\cdot A = 10^{-3} \times 0.12 \times 0.05 = 6 \times 10^{-6} \text{ V},$$ where the negative sign indicates that $$B$$ is decreasing in time.

Since both the motional and time-varying contributions act to oppose the decrease of magnetic flux through the loop, the total induced EMF is $$\varepsilon = \varepsilon_{\text{motional}} + \varepsilon_{\text{time}} = 3 \times 10^{-5} + 6 \times 10^{-6} = 3.6 \times 10^{-5} \text{ V}$$.

The resistance of the loop is $$R = 6 \text{ m}\Omega = 6 \times 10^{-3} \,\Omega$$, so the power dissipated as heat is $$P = \frac{\varepsilon^2}{R} = \frac{(3.6 \times 10^{-5})^2}{6 \times 10^{-3}} = \frac{12.96 \times 10^{-10}}{6 \times 10^{-3}} = 2.16 \times 10^{-7} \text{ W} = 216 \times 10^{-9} \text{ W}$$.

The answer is 216.

We need to find the potential difference between the two ends of a rod rotating about its perpendicular bisector in a magnetic field parallel to the axis of rotation.

Rod length = 60 cm = 0.6 m, angular velocity $$\omega = 20$$ rad/s, magnetic field $$B = 0.5$$ T. The rod rotates about its perpendicular bisector, and the magnetic field is parallel to the axis of rotation.

When a rod rotates about its center (perpendicular bisector), the EMF induced between the center and one end is:

$$\varepsilon = \frac{1}{2}B\omega\left(\frac{L}{2}\right)^2 = \frac{B\omega L^2}{8}$$

Both halves of the rod generate EMF from center to their respective ends. The EMF from center to each end is the same in magnitude. Since both halves sweep in opposite directions relative to the center, the induced EMF from one end to center equals that from center to the other end.

End 1 is at higher potential than center by $$\frac{B\omega(L/2)^2}{2}$$, and end 2 is also at higher potential than center by the same amount.

So the potential difference between the two ends = 0.

Alternatively: By symmetry, both ends are at the same potential (both are at the same distance from the axis). The rod is symmetric about its center, so the motional EMF from center to end 1 equals the motional EMF from center to end 2. Both ends are at the same potential.

The potential difference between the two ends is $$0$$ V.

Therefore, the answer is 0.

We are given that the current in an inductor is $$I = (3t + 8)$$ A (where $$t$$ is in seconds) and the magnitude of the induced EMF is $$12$$ mV, and we need to find the self-inductance.

Since the EMF induced in an inductor is given by $$|emf| = L \frac{dI}{dt}$$ where $$L$$ is the self-inductance and $$\frac{dI}{dt}$$ is the rate of change of current, we first compute this rate.

For $$I = 3t + 8$$, differentiating with respect to time gives $$\frac{dI}{dt} = \frac{d}{dt}(3t + 8) = 3 \text{ A/s}$$, so the rate of change of current is constant at 3 A/s.

Substituting into the EMF formula yields $$12 \times 10^{-3} = L \times 3$$, which gives $$L = \frac{12 \times 10^{-3}}{3} = 4 \times 10^{-3} \text{ H} = 4 \text{ mH}$$.

The answer is 4 mH.

$$\phi=5t^2-36t+1$$. $$emf=-d\phi/dt=-(10t-36)$$. At $$t=2$$: $$emf=-(20-36)=16$$ V.

$$I=emf/R=16/8=2$$ A.

The answer is $$\boxed{2}$$.

The mutual inductance is $$M = 0.002$$ H. The current in the first coil is $$i = i_0 \sin\omega t$$ with $$i_0 = 5$$ A and $$\omega = 50\pi$$ rad/s.

The induced EMF in the second coil:

$$\varepsilon = -M\frac{di}{dt} = -M i_0 \omega \cos\omega t$$

Maximum EMF:

$$\varepsilon_{max} = M i_0 \omega = 0.002 \times 5 \times 50\pi = 0.5\pi$$ V $$= \frac{\pi}{2}$$ V

So $$\frac{\pi}{\alpha} = \frac{\pi}{2}$$, giving $$\alpha = 2$$.

The answer is $$\boxed{2}$$.

When a bar magnet falls through a copper tube, changing magnetic flux induces eddy currents in the tube. These currents create a magnetic field opposing the magnet's motion (Lenz's law), providing a retarding force. Eventually, the retarding force balances gravity, and the magnet moves with almost constant (terminal) speed.

The correct answer is Option 3.

Solution :

Given :

Area of square loop,

$$A = 25\text{ cm}^2$$

$$= 25 \times 10^{-4}\text{ m}^2$$

Resistance of loop,

$$R = 10\ \Omega$$

Magnetic field,

$$B = 40\text{ T}$$

Time taken,

$$t = 1\text{ s}$$

Initial magnetic flux through the loop :

$$\phi_i = BA$$

Final magnetic flux after removing the loop :

$$\phi_f = 0$$

Therefore,

$$\Delta \phi = BA$$

Induced emf :

$$\mathcal{E} = \frac{\Delta \phi}{t}$$

$$= \frac{BA}{t}$$

Substituting values :

$$\mathcal{E}=\frac{40 \times 25 \times 10^{-4}}{1}$$

$$=\frac{1000}{10^4}$$

$$=0.1\text{ V}$$

Induced current :

$$I = \frac{\mathcal{E}}{R}$$

$$= \frac{0.1}{10}$$

$$= 0.01\text{ A}$$

Work done in pulling the loop slowly equals heat produced :

$$W = I^2Rt$$

$$= (0.01)^2 \times 10 \times 1$$

$$= 10^{-3}\text{ J}$$

Final Answer :

$$10^{-3}\text{ J}$$

We need to evaluate the Assertion-Reason pair about a magnet falling through a metallic pipe.

Assertion A: A bar magnet dropped through a metallic cylindrical pipe takes more time to come down compared to a non-magnetic bar with same geometry and mass.

This is TRUE. When a magnet falls through a conducting pipe, the changing magnetic flux through the pipe induces electric currents (eddy currents) in the pipe walls. By Lenz's law, these currents create a magnetic field that opposes the change in flux -- i.e., opposes the motion of the falling magnet. This creates a retarding force on the magnet, slowing it down significantly. A non-magnetic bar of the same mass and geometry does not produce changing magnetic flux, so no eddy currents are induced, and it falls freely under gravity.

Reason R: For the magnetic bar, eddy currents are produced in the metallic pipe which oppose the motion of the magnetic bar.

This is TRUE. The physical mechanism is:

1. The falling magnet creates a changing magnetic flux through any cross-section of the pipe.

2. By Faraday's law, this changing flux induces an EMF in the pipe.

3. Since the pipe is a conductor, the EMF drives circular (eddy) currents in the pipe walls.

4. By Lenz's law, these eddy currents produce a magnetic field that opposes the cause (the falling magnet), creating an upward force on the magnet.

R is the correct and direct explanation of A. The eddy currents and their opposing effect are precisely why the magnet takes more time to fall through the pipe.

The correct answer is Option 2: Both A and R are true and R is the correct explanation of A.

The induced emf is produced when there is a change in magnetic flux through the coil. By Faraday's law: $$\varepsilon = -\frac{d\Phi}{dt}$$

A. Moving the coil with uniform speed inside uniform magnetic field:

The flux through the coil remains constant (same B, same area, same orientation), so no emf is induced. ✘

B. Moving the coil with non-uniform speed inside uniform magnetic field:

Even with non-uniform speed, if the coil remains entirely within the uniform field, the flux doesn't change. No emf is induced. ✘

C. Rotating the coil inside the uniform magnetic field:

Rotation changes the angle between the area vector and B, so $$\Phi = BA\cos\theta$$ changes with time. Emf is induced. ✔

D. Changing the area of the coil inside the uniform magnetic field:

Since $$\Phi = BA$$, changing $$A$$ changes the flux. Emf is induced. ✔

The correct answer is C and D only.

The induced emf in a loop is given by Faraday’s law: $$ \varepsilon = -\frac{d\phi}{dt} $$, where $$ \phi $$ is the magnetic flux and $$ \phi = B \cdot A $$. Since the loop is circular and perpendicular to the magnetic field, $$ A = \pi r^2 $$. The magnitude is $$ |\varepsilon| = \left| \frac{d\phi}{dt} \right| $$.

Given radius $$ r = \frac{10}{\sqrt{\pi}} $$ cm, converting to meters gives $$ r = \frac{10}{\sqrt{\pi}} \times 10^{-2} = \frac{0.1}{\sqrt{\pi}} $$ m, so the area is $$ A = \pi r^2 = \pi \left( \frac{0.1}{\sqrt{\pi}} \right)^2 = \pi \times \frac{0.01}{\pi} = 0.01 $$ m².

The magnetic field decreases linearly from 0.5 T to 0 T in 0.5 s at a steady rate, so $$ \frac{dB}{dt} = \frac{\Delta B}{\Delta t} = \frac{0 - 0.5}{0.5} = -1 \, \text{T/s} $$ and thus $$ \left| \frac{dB}{dt} \right| = 1 \, \text{T/s} $$.

Since $$ \phi = B \cdot A $$ and $$ A $$ is constant, $$ \frac{d\phi}{dt} = A \frac{dB}{dt} $$. The magnitude of induced emf is $$ |\varepsilon| = A \left| \frac{dB}{dt} \right| = 0.01 \times 1 = 0.01 \, \text{V} = 10 \, \text{mV} $$.

Because the rate of change of $$ B $$ is constant, the induced emf remains constant, so at $$ t = 0.25 $$ s it is still 10 mV.

The correct option is B. emf = 10 mV.

We need to find the velocity of a metal rod that produces an EMF of $$\varepsilon = 0.08$$ V when moving perpendicular to a uniform magnetic field $$B = 0.4$$ T. The rod has length $$l = 10$$ cm $$= 0.1$$ m.

When a conducting rod moves with velocity $$v$$ perpendicular to a magnetic field, the free charges experience a Lorentz force $$F = qvB$$ that drives them to opposite ends, creating a potential difference. The motional EMF is given by:

$$\varepsilon = Blv$$

Rearranging to solve for $$v$$ and substituting:

$$v = \frac{\varepsilon}{Bl} = \frac{0.08}{0.4 \times 0.1} = \frac{0.08}{0.04} = 2 \text{ m/s}$$

Hence, the correct answer is Option 4.

Solution :

Magnetic field at the centre due to one side of the square loop is :

$$B_1 = \frac{\mu_0 I}{4\pi (L/2)}(\sin45^\circ + \sin45^\circ)$$

$$= \frac{\mu_0 I}{2\pi L}\left(\frac{1}{\sqrt2}+\frac{1}{\sqrt2}\right)$$

$$= \frac{\mu_0 I}{\sqrt2\pi L}$$

Magnetic field due to all four sides :

$$B = 4B_1$$

$$= \frac{4\mu_0 I}{\sqrt2\pi L}$$

$$= \frac{2\sqrt2\mu_0 I}{\pi L}$$

Since \(L \gg R\), magnetic field over the small circular loop is nearly uniform.

Magnetic flux linked with circular loop :

$$\phi = BA$$

$$= \frac{2\sqrt2\mu_0 I}{\pi L}\times \pi R^2$$

$$= \frac{2\sqrt2\mu_0 IR^2}{L}$$

Mutual inductance :

$$M = \frac{\phi}{I}$$

$$= \frac{2\sqrt2\mu_0 R^2}{L}$$

Final Answer :

$$M = \frac{2\sqrt2\mu_0 R^2}{L}$$

Let us match each item in List-I with the appropriate item in List-II:

A. AC generator - An AC generator works on the principle of Electromagnetic Induction (II). When a coil rotates in a magnetic field, the changing magnetic flux induces an EMF.

B. Transformer - A transformer works on the principle of Mutual Inductance (IV). The changing current in the primary coil induces an EMF in the secondary coil through mutual induction.

C. Resonance phenomenon to occur - Resonance in an AC circuit occurs when both an inductor (L) and capacitor (C) are present, i.e., Presence of both L and C (I). At resonance, $$X_L = X_C$$.

D. Sharpness of resonance - The sharpness of resonance is measured by the Quality factor (III), defined as $$Q = \frac{\omega_0 L}{R} = \frac{1}{R}\sqrt{\frac{L}{C}}$$.

Therefore, the correct matching is: A-II, B-IV, C-I, D-III.

When the switch is closed, the steady current through the coil:

$$I = \frac{V}{R} = \frac{12}{6} = 2 \text{ A}$$

When the switch is opened, the current drops from 2 A to 0 in $$\Delta t = 1$$ ms.

The induced emf:

$$\varepsilon = L\frac{\Delta I}{\Delta t} = L \times \frac{2}{1 \times 10^{-3}}$$

Given $$\varepsilon = 20$$ V:

$$20 = L \times 2000$$

$$L = \frac{20}{2000} = 0.01 \text{ H} = 10 \text{ mH}$$

The inductance is $$\mathbf{10}$$ mH.

The induced emf in a wire moving perpendicular to a magnetic field is given by:

$$\varepsilon=Blv$$

Where:

• B=2 T
• l=1 m
• v=8 ms−1

Substituting:

$$\varepsilon=2\times1\times8$$

$$\varepsilon=16\text{ V}$$

Solution :

Magnetic flux through the circular loop is :

$$\phi = BA$$

$$= B\pi r^2$$

Induced emf :

$$\mathcal{E} = \left|\frac{d\phi}{dt}\right|$$

$$= \left|\frac{d}{dt}(B\pi r^2)\right|$$

Since \(B\) is constant,

$$\mathcal{E} = 2\pi Br\left|\frac{dr}{dt}\right|$$

Given :

$$B = 0.8\text{ T}$$

$$r = 10\text{ cm} = 0.1\text{ m}$$

$$\left|\frac{dr}{dt}\right| = 2\text{ cm s}^{-1} = 0.02\text{ m s}^{-1}$$

Substituting values :

$$\mathcal{E} = 2\pi \times 0.8 \times 0.1 \times 0.02$$

$$= 0.0032\pi\text{ V}$$

$$\approx 0.01\text{ V}$$

$$= 10\text{ mV}$$

Final Answer :

$$10$$

Given: $$l = 1$$ m, $$B = 0.15$$ T, $$R = 5$$ $$\Omega$$, $$v = 4$$ m/s.

EMF induced: $$\varepsilon = Blv = 0.15 \times 1 \times 4 = 0.6$$ V

Current: $$I = \frac{\varepsilon}{R} = \frac{0.6}{5} = 0.12$$ A

Force needed to maintain constant speed (against the magnetic braking force):

$$F = BIl = 0.15 \times 0.12 \times 1 = 0.018 \text{ N} = 18 \times 10^{-3} \text{ N}$$

The force needed is 18 $$\times 10^{-3}$$ N.

EMF developed when a rod rotates in a magnetic field:

$$\varepsilon = \frac{1}{2}B\omega L^2$$

Given: $$L = 20$$ cm $$= 0.2$$ m, $$B = 0.2$$ T, $$N = 210$$ rpm.

$$\omega = \frac{2\pi N}{60} = \frac{2\pi \times 210}{60} = 7\pi$$ rad/s

$$\varepsilon = \frac{1}{2} \times 0.2 \times 7\pi \times (0.2)^2 = \frac{1}{2} \times 0.2 \times 7\pi \times 0.04$$

$$= 0.028\pi \text{ V} = 0.028 \times \frac{22}{7} = 0.028 \times 3.1428... \approx 0.088 \text{ V} = 88 \text{ mV}$$

Using $$\pi = 22/7$$: $$\varepsilon = 0.028 \times 22/7 = 0.088$$ V $$= 88$$ mV.

The EMF developed is $$\mathbf{88}$$ mV.

A metallic cube of side 15 cm moves along the y-axis at 2 m/s in a magnetic field of 0.5 T along the z-axis. We need to find the potential difference between the faces.

To begin,

When a conductor moves through a magnetic field, the free charges inside experience a Lorentz force. The velocity is along $$\hat{j}$$ (y-axis) and the magnetic field is along $$\hat{k}$$ (z-axis). The magnetic force on a positive charge is:

$$ \vec{F} = q\vec{v} \times \vec{B} = q(v\hat{j}) \times (B\hat{k}) $$

Using $$\hat{j} \times \hat{k} = \hat{i}$$:

$$ \vec{F} = qvB\hat{i} $$

So the force on positive charges is in the $$+x$$ direction. This causes charge separation along the x-axis.

Next,

The charges separate until the electric field from the separation balances the magnetic force. The induced EMF across the length $$l$$ in the x-direction is:

$$ \mathcal{E} = Bvl $$

where $$l = 15$$ cm = 0.15 m is the side of the cube (the dimension along which the potential difference develops, i.e., the x-direction).

$$ \mathcal{E} = 0.5 \times 2 \times 0.15 = 0.15\;\text{V} = 150\;\text{mV} $$

The potential difference between the faces is 150 mV.

$$n = 50/\text{cm} = 5000/\text{m}$$, area of loop = $$(0.02)^2 = 4 \times 10^{-4}$$ m².

Flux: $$\Phi = \mu_0 n I A = 4\pi \times 10^{-7} \times 5000 \times 2.5\sin(700t) \times 4 \times 10^{-4}$$

EMF amplitude = $$\mu_0 n I_0 A \omega = 4\pi \times 10^{-7} \times 5000 \times 2.5 \times 4 \times 10^{-4} \times 700$$

$$= 4 \times 22/7 \times 10^{-7} \times 5000 \times 2.5 \times 4 \times 10^{-4} \times 700$$

$$= 88/7 \times 10^{-7} \times 5000 \times 700 \times 10^{-3}$$

$$= 88/7 \times 3500 \times 10^{-7} = 44000 \times 10^{-7} = 44 \times 10^{-4}$$ V.

The answer is 44.

A square-shaped coil of area 70 cm$$^2$$ with 600 turns rotates in a magnetic field of 0.4 Wb/m$$^2$$. The coil completes 500 revolutions per minute. Find the instantaneous emf when the plane of the coil is inclined at 60° with the field.

The coil has $$N = 600$$ turns, area $$A = 70$$ cm$$^2 = 70 \times 10^{-4}$$ m$$^2$$, and it rotates at $$f = \frac{500}{60}$$ rev/s in a magnetic field of strength $$B = 0.4$$ Wb/m$$^2$$. Here $$\pi = \frac{22}{7}$$.

The angular velocity is given by $$\omega = 2\pi f = 2 \times \frac{22}{7} \times \frac{500}{60} = \frac{22000}{420} = \frac{1100}{21}$$ rad/s.

The peak emf is $$\varepsilon_0 = NBA\omega = 600 \times 0.4 \times 70 \times 10^{-4} \times \frac{1100}{21}$$
$$= 600 \times 0.4 \times 0.007 \times \frac{1100}{21}$$
$$= 1.68 \times \frac{1100}{21} = 1.68 \times 52.381 = 88$$ V.

The instantaneous emf is $$\varepsilon = \varepsilon_0 \sin(\omega t)$$. When the plane of the coil is inclined at 60° with the field, the angle between the area vector and the field is $$90° - 60° = 30°$$. Thus, $$\varepsilon = \varepsilon_0 \sin(30°) = 88 \times \frac{1}{2} = 44$$ V.

The answer is $$44$$ V.

We have a transformer with primary voltage $$V_p = 12\;\text{kV} = 12000\;\text{V}$$, secondary voltage $$V_s = 120\;\text{V}$$, and average power consumption $$P = 60\;\text{kW} = 60000\;\text{W}$$.

For an ideal transformer, power is conserved. The current in the secondary circuit is

Now the resistive load in the secondary circuit is

Hence, the value of the resistive load is $$240$$ m$$\Omega$$. So, the answer is $$240$$.

$$|\epsilon| = \left| \frac{d\Phi}{dt} \right|$$

$$\Phi = B \times A$$

$$|\epsilon| = A \times \left| \frac{dB}{dt} \right|$$

$$\frac{dB}{dt} = \frac{B_2 - B_1}{t_2 - t_1} = \frac{8\text{ mT} - 4\text{ mT}}{4\text{ s} - 2\text{ s}}$$

$$\frac{dB}{dt} = \frac{4\text{ mT}}{2\text{ s}} = 2\text{ mT/s}$$

$$|\epsilon| = 4\text{ m}^2 \times 2\text{ mT/s}$$

$$|\epsilon| = 8\text{ mV}$$

At equilibrium, the current through the coil has reached its steady-state value where the inductor acts as a short circuit.

$$I_0 = \frac{V}{R} = \frac{10}{4} = 2.5 \text{ A}$$

The energy stored in the magnetic field:

$$U = \frac{1}{2}LI_0^2 = \frac{1}{2} \times 2 \times (2.5)^2 = \frac{1}{2} \times 2 \times 6.25 = 6.25 \text{ J}$$

$$= 625 \times 10^{-2} \text{ J}$$

We need to match the devices in List-I with their descriptions in List-II.

(A) AC Generator:

An AC generator converts mechanical energy (rotation of the coil in a magnetic field) into electrical energy (alternating current). This matches (II).

(B) Galvanometer:

A galvanometer is a device used to detect the presence and direction of current in a circuit. This matches (I).

(C) Transformer:

A transformer is used to step up or step down an alternating voltage, i.e., it changes an alternating voltage to a smaller or greater value. This matches (IV).

(D) Metal Detector:

A metal detector works on the principle of resonance in an AC circuit. When a metal object is brought near the coil, it changes the impedance and disturbs the resonance condition. This matches (III).

Therefore the correct matching is: A - II, B - I, C - IV, D - III.

The correct answer is Option A: A - II, B - I, C - IV, D - III.

We need to determine the mutual inductance ($$M$$) of a system consisting of a small square loop of side $$l$$ placed inside a large square loop of side $$L$$ ($$L \gg l$$) such that they are coplanar and concentric.

1. Find the Magnetic Field at the Center due to the Large Loop

Let a current $$I$$ flow through the large square loop. The loop consists of four identical linear segments of length $$L$$. The distance from each side to the common center $$O$$ is:

$$d = \frac{L}{2}$$

The magnetic field ($$B_1$$) produced at the center by a single straight wire segment of length $$L$$ carrying current $$I$$ is given by Biot-Savart Law:

$$B_1 = \frac{\mu_0 I}{4\pi d} (\sin \theta_1 + \sin \theta_2)$$

For a square loop, the lines connecting the center to the corners form angles of $$\theta_1 = \theta_2 = 45^\circ$$ with the normal. Substituting these along with $$d = \frac{L}{2}$$:

$$B_1 = \frac{\mu_0 I}{4\pi \left(\frac{L}{2}\right)} \left(\sin 45^\circ + \sin 45^\circ\right) = \frac{\mu_0 I}{2\pi L} \left(\frac{1}{\sqrt{2}} + \frac{1}{\sqrt{2}}\right) = \frac{\mu_0 I}{2\pi L} \left(\frac{2}{\sqrt{2}}\right) = \frac{\sqrt{2}\mu_0 I}{\pi L}$$

Since all four sides of the large loop contribute equally and in the same direction, the total magnetic field ($$B$$) at the center $$O$$ is:

$$B = 4 \times B_1 = 4 \times \frac{\sqrt{2}\mu_0 I}{\pi L} = \frac{4\sqrt{2}\mu_0 I}{\pi L}$$

2. Calculate the Magnetic Flux through the Small Loop

Since the inner loop is extremely small ($$L \gg l$$), we can safely assume that the magnetic field $$B$$ is practically uniform across its entire inner area ($$A = l^2$$).

The magnetic flux ($$\Phi$$) linked with the small loop is:

$$\Phi = B \cdot A = \left(\frac{4\sqrt{2}\mu_0 I}{\pi L}\right) \cdot l^2 = \frac{4\sqrt{2}\mu_0 l^2 I}{\pi L}$$

3. Establish the Mutual Inductance ($$M$$)

By definition, the magnetic flux through the secondary loop is proportional to the current in the primary loop:

$$\Phi = M \cdot I$$

Equating the two expressions for $$\Phi$$ and canceling out the current ($$I$$):

$$M = \frac{4\sqrt{2}\mu_0 l^2}{\pi L}$$

Conclusion

The mutual inductance of the system is $$\frac{4\sqrt{2}\mu_0 l^2}{\pi L}$$.

We have an LR circuit with inductance $$L = 1$$ H, resistance $$R = 100$$ $$\Omega$$, and battery voltage $$V = 6$$ V.

The time constant of the LR circuit is $$\tau = \frac{L}{R} = \frac{1}{100} = 0.01$$ s $$= 10$$ ms. The steady-state current is $$I_0 = \frac{V}{R} = \frac{6}{100} = 0.06$$ A.

(a) Time to reach half the steady-state current: The current in an LR circuit grows as $$I = I_0(1 - e^{-t/\tau})$$. Setting $$I = \frac{I_0}{2}$$: $$\frac{1}{2} = 1 - e^{-t/\tau}$$, so $$e^{-t/\tau} = \frac{1}{2}$$, giving $$t = \tau\ln 2 = 0.01 \times 0.693 = 0.00693$$ s $$\approx 7$$ ms.

(b) Energy stored at $$t = 15$$ ms: At $$t = 15$$ ms $$= 0.015$$ s, we have $$\frac{t}{\tau} = \frac{0.015}{0.01} = 1.5$$. The current at this instant is $$I = I_0(1 - e^{-1.5}) = 0.06(1 - e^{-3/2})$$. Using the given value $$e^{-3/2} = 0.25$$: $$I = 0.06(1 - 0.25) = 0.06 \times 0.75 = 0.045$$ A.

The energy stored in the inductor is $$U = \frac{1}{2}LI^2 = \frac{1}{2}(1)(0.045)^2 = \frac{1}{2}(0.002025) \approx 0.001$$ J $$= 1$$ mJ.

Hence, the correct answer is Option C.

Given:
$$Two\ inductors\ of\ self\ induc\tan ce\ L_1\ ​and\ L_2\ connected\ in\ series\ with\ mutual\ induc\tan ce\ M.$$

Concept:
For series combination of coupled inductors:

• $$If\ currents\ are\ in\ same\ direction\ (aiding):\ L=L1+L2+2M$$
• $$If\ currents\ are\ in\ opposite\ direction\ (oppo\sin g):L=L_1+L_2−2M$$

Here:
Currents are in opposite direction ⇒ opposing case

Result:

$$Leq=L_1+L_2−2M$$

A metal detector raises an alarm when a metal object passes through it. We need to identify the underlying principle.

Understand how a metal detector works.

A metal detector uses a coil that is part of an AC circuit tuned to a specific resonant frequency. When a metallic object comes near the coil, it changes the inductance and impedance of the circuit, shifting the resonant frequency and altering the current amplitude.

Identify the correct principle.

The detector works by monitoring the resonance condition of the AC circuit. When metal enters the field, the change in impedance disturbs the resonance, which triggers the alarm. This is the principle of resonance in AC circuits.

The correct answer is Option B: Resonance in ac circuits.

Given:
$$Magnetic\ field\ B=0.2\times\ 10^{-4},\ length\ l=1m,\ angular\ velocity\ ω=5rad/s$$

Formula (emf in rotating rod):

$$E=\ \frac{\ 1}{2}Bωl^2$$

Calculation:

$$E=1/2​\times0.2\times5\times(10^{-4})\times\ 1^2=0.5\times10−4V$$

$$E=50\times10^{-6}V=50μV$$

E=50×10−6 V=50 μV$$\mathcal{E} = 50 \times 10^{-6}\,$$

$$\text{V} = 50\,\mu\text{V}E=50×10−6V=50μV$$

Final Answer:

50 μV

A circular coil of $$1000$$ turns, each with area $$1$$ m$$^2$$, is rotated about its vertical diameter at the rate of one revolution per second in a uniform horizontal magnetic field of $$0.07$$ T. We need to find the maximum voltage generated.

Recall that the instantaneous EMF induced in a rotating coil is given by

$$\varepsilon = NBA\omega \sin(\omega t)$$

Since the maximum value occurs when $$\sin(\omega t) = 1$$, this leads to

$$\varepsilon_{max} = NBA\omega$$

As the coil completes one revolution per second, its angular frequency is

$$\omega = 2\pi f = 2\pi \times 1 = 2\pi \text{ rad/s}$$

Substituting $$N = 1000$$, $$B = 0.07$$ T, $$A = 1$$ m$$^2$$ and $$\omega = 2\pi$$ rad/s into the expression for $$\varepsilon_{max}$$ gives

$$\varepsilon_{max} = 1000 \times 0.07 \times 1 \times 2\pi$$

$$\varepsilon_{max} = 70 \times 2\pi = 140\pi$$

Evaluating this numerically yields

$$\varepsilon_{max} = 140 \times \frac{22}{7} = 140 \times 3.1429 = 440 \text{ V}$$

Therefore, the maximum voltage generated is 440 V.

Given the resistance of the coil $$R = 8 \, \Omega$$ and the magnetic flux $$\phi = \frac{2}{3}(9 - t^2)$$, we first determine the induced EMF.

Since the induced EMF is $$\text{EMF} = -\frac{d\phi}{dt} = -\frac{2}{3}(-2t) = \frac{4t}{3}$$, we next find the moment when the flux becomes zero.

When the magnetic flux vanishes, $$\frac{2}{3}(9 - t^2) = 0 \implies t^2 = 9 \implies t = 3 \text{ s}$$.

Substituting the expression for EMF into Ohm’s law yields the current in the coil as $$i = \frac{\text{EMF}}{R} = \frac{4t}{3 \times 8} = \frac{t}{6}$$.

Therefore, the total heat produced in the coil over the interval from 0 to 3 s is given by $$H = \int_0^3 i^2 R \, dt = \int_0^3 \left(\frac{t}{6}\right)^2 \times 8 \, dt$$. This reduces to $$= \int_0^3 \frac{t^2}{36} \times 8 \, dt = \frac{8}{36} \int_0^3 t^2 \, dt = \frac{2}{9} \left[\frac{t^3}{3}\right]_0^3 = \frac{2}{9} \times \frac{27}{3} = \frac{2}{9} \times 9 = 2 \text{ J}$$.

The total heat produced in the coil is $$\textbf{2}$$ J.

Given: $$\phi = 8t^2 - 9t + 5$$ (in weber), and resistance $$R = 20 \text{ }\Omega$$.

The induced EMF is:

$$\varepsilon = -\dfrac{d\phi}{dt} = -(16t - 9)$$

At $$t = 0.25 \text{ s}$$:

$$\varepsilon = -(16 \times 0.25 - 9) = -(4 - 9) = -(-5) = 5 \text{ V}$$

The magnitude of the induced current:

$$I = \dfrac{|\varepsilon|}{R} = \dfrac{5}{20} = 0.25 \text{ A} = 250 \text{ mA}$$

Therefore, the magnitude of the induced current is $$\boxed{250}$$ mA.

Given:
l=20 cm=0.2 m,  v=20 m/s

Step 1: Vertical component of Earth’s field

$$B_V=B_H​\tanθ=B_H​(\sin ce\ θ=45∘)$$

Step 2: Induced emf

$$\varepsilon=(\vec{v}\times\vec{B})\cdot\vec{l}\ =B_Vvl$$

Step 3: Calculation

V$$\varepsilon=(4\times10^{-3})(20)(0.2)$$

= $$16\times10^{-3}V$$

= $$=16mV$$

Final Answer:

16 mV

We are given a conducting circular loop of radius $$r = 1$$ m placed in the X-Y plane, and the magnetic field is:

$$\vec{B} = 3t^3\hat{j} + 3t^2\hat{k}$$

Since the circular loop lies in the X-Y plane, its area vector is along the $$\hat{k}$$ direction (perpendicular to the X-Y plane), only the $$\hat{k}$$ component of $$\vec{B}$$ contributes to the magnetic flux through the loop because the $$\hat{j}$$ component is parallel to the plane and does not contribute.

Substituting this into the definition of magnetic flux gives

$$\Phi = \vec{B} \cdot \vec{A} = (3t^2) \times (\pi r^2) = 3t^2 \times \pi \times (1)^2 = 3\pi t^2$$

From the above flux expression, using Faraday’s law, the magnitude of the induced EMF is

$$|\mathcal{E}| = \left|\frac{d\Phi}{dt}\right| = \frac{d}{dt}(3\pi t^2) = 6\pi t$$

Evaluating this expression at $$t = 2\text{ s}$$ gives

$$|\mathcal{E}| = 6\pi \times 2 = 12\pi \text{ V}$$

Since the induced EMF is $$n\pi$$ V, we have

$$n = 12$$

The answer is 12.

The self-inductance L = 2.0 H and the current is given by i = 2 sin(t²). We need to find the energy spent during the period when the current changes from 0 to 2 A.

The energy stored in an inductor carrying current $$i$$ is given by $$U = \frac{1}{2}Li^2.$$

For $$i = 0$$, the energy stored is $$U_1 = \frac{1}{2} \times 2 \times 0^2 = 0 \text{ J},$$ and for $$i = 2$$ A, $$U_2 = \frac{1}{2} \times 2 \times 2^2 = 4 \text{ J}.$$

The energy spent equals the change in energy stored, $$\Delta U = U_2 - U_1 = 4 - 0 = 4 \text{ J}.$$ Thus the amount of energy spent is 4 J.

The magnetic field is $$\vec{B} = B_0\left(\frac{x}{a}\right)\hat{k}$$, which varies linearly with $$x$$. The square loop of side $$d$$ has its edges along the $$x$$ and $$y$$ axes, and it moves with velocity $$\vec{v} = v_0\hat{i}$$.

Let the left edge of the loop be at position $$x_0$$ at time $$t$$, so the right edge is at $$x_0 + d$$. The magnetic flux through the loop is $$\Phi = \int_0^d \int_{x_0}^{x_0+d} B_0\frac{x}{a}\,dx\,dy = d \cdot \frac{B_0}{a}\int_{x_0}^{x_0+d} x\,dx$$.

Evaluating the integral: $$\int_{x_0}^{x_0+d} x\,dx = \frac{(x_0+d)^2 - x_0^2}{2} = \frac{2x_0 d + d^2}{2} = x_0 d + \frac{d^2}{2}$$.

So $$\Phi = \frac{B_0 d}{a}\left(x_0 d + \frac{d^2}{2}\right)$$. Since $$x_0 = v_0 t + \text{const}$$, we have $$\frac{dx_0}{dt} = v_0$$.

The induced EMF is $$\varepsilon = -\frac{d\Phi}{dt} = -\frac{B_0 d}{a} \cdot d \cdot v_0 = -\frac{B_0 v_0 d^2}{a}$$.

The magnitude of the induced EMF is $$|\varepsilon| = \frac{B_0 v_0 d^2}{a}$$.

We have a large square loop of side $$b$$ carrying a current $$I$$. The small square loop of side $$a$$ lies at the centre, in the same plane and with only one turn. To obtain the mutual inductance, we must first calculate the magnetic flux linked with the small loop when the current $$I$$ flows in the large one, and then divide that flux by the current.

The magnetic flux through the small loop is $$\Phi = BA$$ because the field is essentially uniform over the tiny area (remember that $$b \gg a$$). Here $$B$$ is the magnetic field at the centre of the big loop and $$A = a^{2}$$ is the area of the small loop. So we need $$B$$ at the centre of a square loop.

For one straight, finite conductor carrying current $$I$$, the magnetic field at a point lying a perpendicular distance $$r$$ from its midpoint is given by the Biot-Savart result

$$B = \frac{\mu_{0} I}{4\pi r}\left(\sin \theta_{1} + \sin \theta_{2}\right),$$

where $$\theta_{1}$$ and $$\theta_{2}$$ are the angles that the lines joining the point to the two ends of the wire make with the wire.

Now consider one side of the square loop. The centre of the square is at a perpendicular distance $$r = \dfrac{b}{2}$$ from this side, and the ends of the side subtend equal angles. Each angle is clearly $$\theta_{1} = \theta_{2} = 45^{\circ}$$ because the half-length of the side is also $$\dfrac{b}{2}$$, forming an isosceles right triangle with the centre.

Hence for one side,

$$\sin \theta_{1} = \sin \theta_{2} = \sin 45^{\circ} = \frac{1}{\sqrt{2}}.$$

Substituting these values into the Biot-Savart expression, we get for a single side

$$\begin{aligned} B_{\text{one side}} &= \frac{\mu_{0} I}{4\pi \left(\dfrac{b}{2}\right)} \left(\frac{1}{\sqrt{2}} + \frac{1}{\sqrt{2}}\right) \\ &= \frac{\mu_{0} I}{4\pi \left(\dfrac{b}{2}\right)}\left(\frac{2}{\sqrt{2}}\right) \\ &= \frac{\mu_{0} I}{4\pi}\left(\frac{2}{b}\right)\left(\sqrt{2}\right) \\ &= \frac{\mu_{0} I}{4\pi}\frac{2\sqrt{2}}{b}. \end{aligned}$$

The square loop has four identical sides, and the directions of the fields from all sides at the centre are the same (into or out of the page depending on the current direction). Therefore the total magnetic field at the centre is

$$\begin{aligned} B_{\text{total}} &= 4 \times B_{\text{one side}} \\ &= 4 \times \frac{\mu_{0} I}{4\pi}\frac{2\sqrt{2}}{b} \\ &= \frac{\mu_{0} I}{4\pi}\frac{8\sqrt{2}}{b}. \end{aligned}$$

Now we turn to the flux through the small loop. Its area is $$A = a^{2}$$, so

$$\Phi = B_{\text{total}} \, A = \left(\frac{\mu_{0} I}{4\pi}\frac{8\sqrt{2}}{b}\right)(a^{2}) = \frac{\mu_{0} I}{4\pi}\frac{8\sqrt{2} \, a^{2}}{b}.$$

The mutual inductance $$M$$ is defined by $$\Phi = M I$$ (one turn only), so dividing the flux by the current we obtain

$$\begin{aligned} M &= \frac{\Phi}{I} \\ &= \frac{\mu_{0}}{4\pi}\frac{8\sqrt{2} \, a^{2}}{b}. \end{aligned}$$

This expression exactly matches Option A.

Hence, the correct answer is Option A.

We need to determine the directions of the induced currents $$I_1$$ and $$I_2$$ flowing through the resistors $$R_1$$ and $$R_2$$ when a conducting bar is pulled to the left at a constant speed $$v$$ in a uniform magnetic field pointing into the page.

We can solve this problem using two well-established physics approaches: Lenz's Law and the Motional EMF (Right-Hand Rule) method.

Method 1: Using Lenz's Law (Magnetic Flux Approach)

Lenz's Law states that the direction of an induced current will always oppose the change in magnetic flux that produces it.

1. Left Loop containing Resistor $$R_1$$:

• As the conducting bar moves to the left, the area of the left closed loop decreases.
• Since the area decreases, the inward magnetic flux ($\otimes$) passing through this loop decreases.
• To oppose this decrease, the induced current $$I_1$$ must create its own magnetic field pointing into the page ($\otimes$) to reinforce the original field.
• According to the right-hand grip rule, a magnetic field pointing into the page corresponds to a clockwise current direction.

2. Right Loop containing Resistor $$R_2$$:

• As the conducting bar moves to the left, the area of the right closed loop increases.
• Since the area increases, the inward magnetic flux ($\otimes$) passing through this loop increases.
• To oppose this increase, the induced current $$I_2$$ must create its own magnetic field pointing out of the page ($\odot$) to cancel out the excess field.
• According to the right-hand grip rule, a magnetic field pointing out of the page corresponds to an anticlockwise current direction.

Method 2: Using Motional EMF ($\vec{v} \times \vec{B}$)

When the conducting rod moves through the magnetic field, the free charges inside the rod experience a magnetic Lorentz force given by $$\vec{F} = q(\vec{v} \times \vec{B})$$.

• The velocity vector $$\vec{v}$$ points to the left ($\leftarrow$).
• The magnetic field vector $$\vec{B}$$ points into the page ($\otimes$).
• Using the right-hand rule for the cross product $$\vec{v} \times \vec{B}$$, pointing your fingers to the left and curling them into the page causes your thumb to point downwards ($\downarrow$).
• This means positive charge carriers are driven to the bottom end of the bar, making the bottom end positive and the top end negative. The moving bar behaves exactly like a battery with its positive terminal at the bottom.

Now, tracing how this "battery" drives current through each parallel loop:

• For the left loop ($$R_1$$), the current leaves the positive bottom terminal, moves left through the bottom rail, goes up through $$R_1$$, and returns right to the negative top terminal. This forms a clockwise loop.
• For the right loop ($$R_2$$), the current leaves the positive bottom terminal, moves right through the bottom rail, goes up through $$R_2$$, and returns left to the negative top terminal. This forms an anticlockwise loop.

Conclusion

Both methods yield the same consistent result:

• $$I_1$$ is in the clockwise direction.
• $$I_2$$ is in the anticlockwise direction.

Therefore, the correct answer is Option C: $$I_1$$ is in clockwise direction and $$I_2$$ is in anticlockwise direction.

The aeroplane flies horizontally with its wings spread $$l = 10$$ m at speed $$v = 180$$ km/h $$= 50$$ m/s. The total intensity of earth's magnetic field is $$B = 2.5 \times 10^{-4}$$ Wb/m$$^2$$ and the angle of dip is $$\delta = 60°$$.

When the plane flies horizontally, the EMF is induced due to the vertical component of the earth's magnetic field, which is $$B_v = B\sin\delta = 2.5 \times 10^{-4} \times \sin 60° = 2.5 \times 10^{-4} \times \frac{\sqrt{3}}{2}$$.

The induced EMF between the wing tips is $$\varepsilon = B_v \times l \times v = 2.5 \times 10^{-4} \times \frac{\sqrt{3}}{2} \times 10 \times 50$$.

Calculating: $$\varepsilon = 2.5 \times 10^{-4} \times 0.866 \times 500 = 2.5 \times 10^{-4} \times 433 = 0.10825$$ V $$= 108.25$$ mV.

We know that the magnetic energy stored in an inductor is given by the formula $$U=\dfrac12\,L\,I^{2}$$, where $$U$$ is the energy, $$L$$ the inductance and $$I$$ the current through the inductor.

Here the stored energy is $$U=64\ \text{J}$$ and the steady current is $$I=8\ \text{A}$$. Substituting these values, we get

$$64=\dfrac12\,L\,(8)^{2}$$

First square the current: $$(8)^{2}=64.$$ So the equation becomes

$$64=\dfrac12\,L\,\times 64$$

Multiply both sides by $$2$$ to remove the fraction:

$$128=L\times 64$$

Now divide by $$64$$ to isolate $$L$$:

$$L=\dfrac{128}{64}=2\ \text{H}$$

Thus the inductance of the coil is $$L=2\ \text{H}.$$

Next, the rate at which electrical energy is dissipated as heat in the resistance of the coil is the electric power $$P$$, and for a resistor this power is given by the formula $$P=I^{2}R,$$ where $$R$$ is the resistance.

The problem states that the coil dissipates energy at the rate $$P=640\ \text{W}$$ when the same current $$I=8\ \text{A}$$ flows. Substituting these values, we have

$$640=(8)^{2}\,R$$

Again $$ (8)^{2}=64 $$, so

$$640=64\,R$$

Dividing both sides by $$64$$ gives

$$R=\dfrac{640}{64}=10\ \Omega$$

Hence the resistance of the coil is $$R=10\ \Omega.$$

When this coil is connected across an ideal battery, the circuit behaves as an $$LR$$ circuit. The time constant $$\tau$$ of such a circuit is defined by the relation $$\tau=\dfrac{L}{R}.$$ Stating the formula explicitly: the time constant is the ratio of the inductance to the resistance.

Substituting $$L=2\ \text{H}$$ and $$R=10\ \Omega$$, we obtain

$$\tau=\dfrac{2\ \text{H}}{10\ \Omega}=0.2\ \text{s}$$

Therefore, the time constant of the circuit is $$0.2\ \text{s}.$$

Hence, the correct answer is Option B.

When a solenoid of inductance $$L$$ and resistance $$R$$ is connected to a battery, the current grows as $$i = i_0\left(1 - e^{-Rt/L}\right)$$, where $$i_0$$ is the maximum (steady-state) current.

The magnetic energy stored in the inductor is $$U = \frac{1}{2}Li^2 = \frac{1}{2}Li_0^2\left(1 - e^{-Rt/L}\right)^2$$. The maximum energy is $$U_{\max} = \frac{1}{2}Li_0^2$$.

Setting $$U = \frac{1}{4}U_{\max}$$ gives $$\left(1 - e^{-Rt/L}\right)^2 = \frac{1}{4}$$. Taking the positive square root: $$1 - e^{-Rt/L} = \frac{1}{2}$$, so $$e^{-Rt/L} = \frac{1}{2}$$.

Taking the natural logarithm: $$-\frac{Rt}{L} = -\ln 2$$, giving $$t = \frac{L}{R}\ln 2$$.

We need to determine the constant speed $$v_0$$ of a square loop so that a steady current of $$2\text{ mA}$$ flows through it as it moves through a magnetic field and interacts with an external resistor network.

1. Analyze the External Resistor Network

From the layout described on the page, the loop is connected to an infinite or balanced grid network of resistors where each resistor has a value of $$R_0 = 4\ \Omega$$.

• For the standard infinite grid of square resistors, the equivalent resistance ($$R_{\text{net}}$$) measured across two adjacent terminals where the loop connects simplifies cleanly to:

  $$R_{\text{net}} = R_0 = 4\ \Omega$$

• The square loop itself has its own internal resistance:

  $$R_{\text{loop}} = 1\ \Omega$$

• Therefore, the total equivalent resistance ($$R_{\text{total}}$$) of the entire circuit is the sum of the loop resistance and the network resistance:

  $$R_{\text{total}} = R_{\text{loop}} + R_{\text{net}} = 1\ \Omega + 4\ \Omega = 5\ \Omega$$

2. Calculate the Induced Electromotive Force (emf)

As the square loop moves to the right with speed $$v_0$$, its right arm (of length $$l = 20\text{ cm} = 0.2\text{ m}$$) cuts through a uniform magnetic field ($$B = 5\text{ T}$$) going into the page. This produces a motional electromotive force ($$e$$) given by:

$$e = B \cdot l \cdot v_0$$

Substitute the given numerical values into the expression:

$$e = 5 \times 0.2 \times v_0 = 1 \cdot v_0 = v_0$$

3. Relate emf to the Steady Current

According to Ohm's law, the steady current ($$I$$) flowing through the circuit is equal to the induced emf divided by the total resistance:

$$I = \frac{e}{R_{\text{total}}}$$

We are given that the required steady current is $$I = 2\text{ mA} = 2 \times 10^{-3}\text{ A}$$. Substitute this and our resistance values into the formula:

$$2 \times 10^{-3} = \frac{v_0}{5}$$

$$v_0 = 5 \times 2 \times 10^{-3} = 10 \times 10^{-3} = 0.01\text{ m s}^{-1}$$

Converting the velocity from meters per second ($$\text{m s}^{-1}$$) to centimeters per second ($$\text{cm s}^{-1}$$):

$$v_0 = 0.01 \times 100 = 1\text{ cm s}^{-1}$$

Conclusion

The speed $$v_0$$ of the loop should be $$1\text{ cm s}^{-1}$$, which matches Option C.

We have a circular coil with $$N = 20$$ turns and radius $$R = 8.0\text{ cm}$$. First, we convert the radius into SI units because the magnetic field $$B$$ is given in tesla.

$$R = 8.0\text{ cm} = 8.0 \times 10^{-2}\text{ m} = 0.08\text{ m}$$

The angular speed of rotation is $$\omega = 50\ \text{rad s}^{-1}$$ and the magnetic field is uniform with magnitude $$B = 3.0 \times 10^{-2}\ \text{T}$$.

For a coil rotating in a uniform magnetic field, the maximum (peak) emf induced is given by the well-known generator formula

$$\mathcal{E}_{\max} = N\,B\,A\,\omega,$$

where $$A$$ is the area of one turn of the coil.

Now, we calculate the area. The coil is circular, so

$$A = \pi R^{2}.$$

Substituting $$R = 0.08\ \text{m},$$ we get

$$A = \pi \,(0.08)^{2} = \pi \times 0.0064 = 0.0064\pi\ \text{m}^{2}.$$

Using $$\pi \approx 3.1416,$$

$$A \approx 0.0064 \times 3.1416 = 0.02010624\ \text{m}^{2}.$$

Now we substitute all known values into the emf formula:

$$\mathcal{E}_{\max} = N\,B\,A\,\omega$$

$$\mathcal{E}_{\max} = (20)\,(3.0 \times 10^{-2})\,(0.02010624)\,(50).$$

First, multiply the number of turns with the magnetic field:

$$20 \times 3.0 \times 10^{-2} = 0.6.$$

Next, multiply this result by the area:

$$0.6 \times 0.02010624 = 0.012063744.$$

Finally, multiply by the angular speed:

$$0.012063744 \times 50 = 0.6031872\ \text{V}.$$

The question expresses the answer in units of $$\times 10^{-2}$$ volt. Dividing by $$10^{-2}$$ (that is, multiplying by 100) converts our result:

$$0.6031872\ \text{V} = \frac{0.6031872}{0.01} = 60.31872 \times 10^{-2}\ \text{V}.$$

Rounding this to the nearest integer gives $$60.$$

So, the answer is $$60$$.

We begin with the given data. The circular coil has radius $$r = 1\ \text{m}$$, so its area is obtained from the usual formula for the area of a circle, $$A = \pi r^{2}$$. Substituting the radius we get $$A = \pi \times (1)^{2} = \pi\ \text{m}^{2}.$$

The resistance of the coil is stated to be $$R = 2\ \mu\Omega = 2 \times 10^{-6}\ \Omega.$$

The magnetic field perpendicular to the plane of the coil is turned off slowly according to the time-dependent expression $$B(t) = \frac{4}{\pi}\times 10^{-3}\ \text{T}\,\Bigl(1 - \frac{t}{100}\Bigr).$$ The switch-off process lasts until $$t = 100\ \text{s},$$ at which instant the field becomes zero.

The magnetic flux $$\Phi$$ through the single-turn coil is given by the definition $$\Phi = B\,A.$$ Hence, $$\Phi(t) = \bigl[\tfrac{4}{\pi}\times 10^{-3}\,(1 - \tfrac{t}{100})\bigr]\;(\pi).$$

We now compute the induced emf using Faraday’s law. First we need the rate of change of magnetic field. Differentiating $$B(t)$$ with respect to time,

$$\frac{dB}{dt} = \frac{4}{\pi}\times 10^{-3} \,\Bigl(-\frac{1}{100}\Bigr) = -\,\frac{4}{\pi}\times 10^{-5}\ \text{T s}^{-1}.$$

The negative sign merely indicates the direction of change; for the magnitude we note

$$\left|\frac{dB}{dt}\right| = \frac{4}{\pi}\times 10^{-5}\ \text{T s}^{-1}.$$

Faraday’s law for a single loop is $$\mathcal{E} = -\,\frac{d\Phi}{dt} = -\,A\,\frac{dB}{dt}.$$ Taking the magnitude,

$$\mathcal{E} = A\,\left|\frac{dB}{dt}\right| = (\pi)\,\Bigl(\frac{4}{\pi}\times 10^{-5}\Bigr) = 4 \times 10^{-5}\ \text{V}.$$

Because $$\frac{dB}{dt}$$ is a constant, the induced emf $$\mathcal{E}$$ is also constant over the entire interval from $$t = 0$$ to $$t = 100\ \text{s}.$$

Ohm’s law gives the induced current: $$I = \frac{\mathcal{E}}{R}.$$ Substituting the numerical values,

$$I = \frac{4 \times 10^{-5}}{2 \times 10^{-6}} = 20\ \text{A}.$$

This current too remains constant during the 100-second interval. The electrical power dissipated as heat in the coil is given by the Joule-heating relation $$P = I^{2}R.$$ Hence,

$$P = (20)^{2}\,(2 \times 10^{-6}) = 400 \times 2 \times 10^{-6} = 8 \times 10^{-4}\ \text{W}.$$

Finally, the total energy $$E$$ dissipated in the entire interval is the product of power and time: $$E = P\,\Delta t$$ with $$\Delta t = 100\ \text{s}.$$

$$E = (8 \times 10^{-4})\,(100) = 8 \times 10^{-2}\ \text{J}.$$

Because $$1\ \text{J} = 1000\ \text{mJ},$$ we convert:

$$E = 8 \times 10^{-2}\ \text{J} = 0.08\ \text{J} = 80\ \text{mJ}.$$

Hence, the correct answer is Option C.

A coil of inductance $$L = 2\,\text{H}$$ with negligible resistance is connected to a voltage source $$V = 3t$$ volts. The voltage across an inductor is $$V = L\frac{dI}{dt}$$.

Substituting: $$3t = 2\frac{dI}{dt}$$, which gives $$\frac{dI}{dt} = \frac{3t}{2}$$.

Integrating from $$t = 0$$ (where $$I = 0$$): $$I = \frac{3t^2}{4}$$.

At $$t = 4\,\text{s}$$: $$I = \frac{3 \times 16}{4} = 12\,\text{A}$$.

The energy stored in the inductor is $$U = \frac{1}{2}LI^2 = \frac{1}{2} \times 2 \times (12)^2 = 144\,\text{J}$$.

Therefore, the energy stored in the coil after 4 seconds is $$144$$ J.

For an R-L circuit we first recall the basic facts. When a constant emf is suddenly applied, the current grows according to

$$I(t)=I_{0}\left(1-e^{-t/\tau}\right),$$

and when the source is removed after reaching the steady value $$I_{0},$$ the current decays as

$$I(t)=I_{0}\,e^{-t/\tau}.$$

Here $$\tau$$ is the time constant of the circuit, given by the well-known formula

$$\tau=\frac{L}{R},$$

where $$L$$ is the self-inductance and $$R$$ is the resistance.

Now let us insert the numerical data. We have

$$L = 10\ \text{mH}=10\times10^{-3}\ \text{H},\qquad R = 10\ \text{k}\Omega = 10\times10^{3}\ \Omega.$$

Therefore

$$\tau = \frac{L}{R} = \frac{10\times10^{-3}}{10\times10^{3}} = \frac{10}{10}\times\frac{10^{-3}}{10^{3}} = 1\times10^{-6}\ \text{s} = 1\ \mu\text{s}.$$

Before the switch is opened, the current has reached its maximum steady value. Ohm’s law for the dc source gives this value as

$$I_{0}=\frac{V}{R} =\frac{20\ \text{V}}{10\times10^{3}\ \Omega} =\frac{20}{10}\times10^{-3}\ \text{A} =2\times10^{-3}\ \text{A} =2\ \text{mA}.$$

Immediately after the battery is disconnected, the current starts to fall. After a time $$t$$ the current is given by the exponential decay formula

$$I(t)=I_{0}\,e^{-t/\tau}.$$

We are asked to find the current at $$t = 1\ \mu\text{s}.$$ Substituting $$t=1\ \mu\text{s}$$ and $$\tau = 1\ \mu\text{s},$$ we get

$$I(1\ \mu\text{s}) = I_{0}\,e^{-1} = 2\ \text{mA}\times e^{-1}.$$

The problem statement provides the numerical value $$e^{-1}=0.37.$$ Hence

$$I(1\ \mu\text{s}) = 2\ \text{mA}\times 0.37 = 0.74\ \text{mA}.$$

We can express $$0.74\ \text{mA}$$ as a fraction of a milliampere:

$$0.74\ \text{mA} = \frac{74}{100}\ \text{mA}.$$

By comparison with the form given in the question, $$\displaystyle I=\frac{x}{100}\ \text{mA},$$ we identify

$$x = 74.$$

So, the answer is $$74$$.

We have a very long solenoid of radius $$R$$ carrying a current that varies with time as

$$I(t)=I_0\,t(1-t), \qquad 0 \le t \le 1.$$

First recall the formula for the magnetic field at the centre of a long solenoid. For $$n$$ turns per unit length, the field is

$$B(t)=\mu_0 n I(t).$$

This field exists almost uniformly throughout the circular cross-section of the solenoid (radius $$R$$) and is negligible outside it. A coaxial conducting ring of radius $$2R$$ surrounds the solenoid near its middle. Though the ring is larger, only the flux linked with the area of the solenoid itself (radius $$R$$) matters because the field outside the solenoid is practically zero. Hence the magnetic flux through the ring is

$$\Phi(t)=B(t)\,\times\,(\text{area of solenoid cross-section})$$

$$\phantom{\Phi(t)}=\mu_0 n I(t)\;(\pi R^2).$$

Substituting the given current we get

$$\Phi(t)=\mu_0 n \pi R^2\,I_0\,t(1-t) =\bigl(\mu_0 n \pi R^2 I_0\bigr)\,\bigl(t-t^2\bigr).$$

Faraday’s law states that the induced emf is the negative time derivative of the flux,

$$V_R(t)=-\dfrac{d\Phi}{dt}.$$

Differentiating explicitly,

$$\dfrac{d\Phi}{dt}=\bigl(\mu_0 n \pi R^2 I_0\bigr)\,\dfrac{d}{dt}(t-t^2) =\bigl(\mu_0 n \pi R^2 I_0\bigr)\,(1-2t).$$

Therefore

$$V_R(t)=-\bigl(\mu_0 n \pi R^2 I_0\bigr)\,(1-2t),$$

and the magnitude of the emf is

$$|V_R(t)|=\bigl(\mu_0 n \pi R^2 I_0\bigr)\,|1-2t|.$$

The induced current $$I_R(t)$$ in the ring depends on the emf and its direction follows Lenz’s law. The sign of $$V_R(t)$$ (or equally the sign of $$1-2t$$) tells us the direction:

• For $$0 \le t < 0.5$$, $$1-2t>0$$, so $$V_R(t)<0$$ and the induced current has one fixed direction.
• For $$t > 0.5$$, $$1-2t<0$$, so $$V_R(t)>0$$ and the induced current reverses direction.

Exactly at $$t=0.5$$ we have $$1-2t=0$$. Thus

$$V_R(0.5)=0,$$

and at this instant the induced current changes its sense (because the emf passes through zero and changes sign).

For completeness, observe the magnitude behaviour:

$$|V_R(t)|=\bigl(\mu_0 n \pi R^2 I_0\bigr)\,|1-2t|$$

is maximum at the end points $$t=0$$ and $$t=1$$ (value = the coefficient itself) and falls linearly to zero at $$t=0.5$$. So the emf is certainly not maximum at $$t=0.25$$ or $$t=0.5$$; it is actually zero at $$t=0.5$$.

Putting all observations together:

• The induced current $$I_R$$ keeps the same direction from $$t=0$$ up to just before $$t=0.5$$.
• At $$t=0.5$$ the emf becomes zero and immediately after that the current reverses direction.

Hence, the correct answer is Option D.

We recall Faraday’s law of electromagnetic induction, which states that the magnitude of induced emf in a loop is

$$\varepsilon = \left|\,\dfrac{d\Phi}{dt}\,\right|,$$

where $$\Phi$$ is the magnetic flux through the loop.

If a loop of area $$A$$ is placed in a uniform magnetic field of magnitude $$B$$ and the angle between the field and the normal (area vector) to the plane of the loop is $$\theta$$, then the flux is

$$\Phi = BA\cos\theta.$$

At $$t = 0$$ the plane of the loop is perpendicular to the magnetic field, so the normal is parallel to the field. Hence

$$\theta(0)=0^{\circ}\quad\Longrightarrow\quad\Phi(0)=BA\cos0^{\circ}=BA.$$

The loop rotates with a period $$T = 10\,\text{s}$$ about an axis lying in its own plane. That axis is perpendicular to the normal, so the normal vector performs a simple rotation about the field direction. The angular frequency is

$$\omega = \dfrac{2\pi}{T} = \dfrac{2\pi}{10} = \dfrac{\pi}{5}\,\text{rad s}^{-1}.$$

Because at $$t=0$$ the normal is along the field, the instantaneous angle between the normal and the field after time $$t$$ is

$$\theta(t)=\omega t.$$

Therefore the flux as a function of time is

$$\Phi(t)=BA\cos(\omega t).$$

We now differentiate to find the induced emf:

$$\varepsilon(t)=\left|\,\dfrac{d}{dt}\bigl[BA\cos(\omega t)\bigr]\,\right| = \left|\,BA\bigl(-\omega\sin(\omega t)\bigr)\,\right| = BA\omega\,|\sin(\omega t)|.$$

The magnitude $$\varepsilon(t)$$ is maximum when

$$|\sin(\omega t)| = 1\quad\Longrightarrow\quad \sin(\omega t)=\pm1.$$

This occurs at

$$\omega t = \dfrac{\pi}{2},\,\dfrac{3\pi}{2},\,\dfrac{5\pi}{2},\ldots$$

Taking the first positive time, put $$\omega t = \dfrac{\pi}{2}$$:

$$t_{\text{max}}= \dfrac{\pi/2}{\omega}= \dfrac{\pi/2}{\pi/5}= \dfrac{5}{2}=2.5\,\text{s}.$$

The magnitude $$\varepsilon(t)$$ is minimum (in fact zero) when

$$|\sin(\omega t)| = 0\quad\Longrightarrow\quad \sin(\omega t)=0,$$

which happens for

$$\omega t = 0,\,\pi,\,2\pi,\ldots$$

Choosing the first non-zero positive time, set $$\omega t = \pi$$:

$$t_{\text{min}}= \dfrac{\pi}{\omega}= \dfrac{\pi}{\pi/5}=5\,\text{s}.$$

Thus, the induced emf attains its first maximum at $$2.5\ \text{s}$$ and its next minimum at $$5.0\ \text{s}$$.

Comparing with the given options, this corresponds to Option B.

Hence, the correct answer is Option B.

We are given a metal wire of total length $$L = 30\ \text{cm} = 0.30\ \text{m}$$ which is bent to form a square loop. A uniform magnetic field $$B$$ is perpendicular to the plane of the loop and is changing steadily at the rate $$\dfrac{dB}{dt}=0.032\ {\rm T\,s^{-1}}$$. The resistivity of the metal is $$\rho = 1.23 \times 10^{-8}\ \Omega\text{m}$$ and the diameter of the wire is $$d = 4\ \text{mm} = 4 \times 10^{-3}\ \text{m}$$.

First we determine the length of one side of the square. Because the wire forms a square, its four sides have equal length, so

$$4\,\ell = L \;\;\Longrightarrow\;\; \ell = \frac{L}{4} = \frac{0.30\ \text{m}}{4} = 0.075\ \text{m} = 7.5\ \text{cm}.$$

Now we find the area $$A$$ enclosed by the square loop:

$$A = \ell^{2} = (0.075\ \text{m})^{2} = 0.005625\ \text{m}^{2}.$$

The changing magnetic field produces an emf according to Faraday's law. We first state Faraday’s law in magnitude form:

$$\mathcal{E}= \left|\frac{d\Phi}{dt}\right|,$$

where the flux $$\Phi$$ through the loop is $$\Phi = BA$$. Because the field is perpendicular to the plane, the cosine factor is unity, and we can write

$$\frac{d\Phi}{dt} = A\,\frac{dB}{dt}.$$

Substituting the given numbers,

$$\frac{d\Phi}{dt} = (0.005625\ \text{m}^{2})\,(0.032\ \text{T\,s}^{-1}) = 1.8 \times 10^{-4}\ \text{Wb\,s}^{-1} = 1.8 \times 10^{-4}\ \text{V}.$$

Thus the induced emf is

$$\mathcal{E} = 1.8 \times 10^{-4}\ \text{V}.$$

Next we calculate the electrical resistance of the square loop. The cross-sectional area $$S$$ of the wire is obtained from its radius $$r$$. The radius is half the diameter:

$$r = \frac{d}{2} = \frac{4 \times 10^{-3}\ \text{m}}{2} = 2 \times 10^{-3}\ \text{m}.$$

So

$$S = \pi r^{2} = \pi\,(2 \times 10^{-3}\ \text{m})^{2} = \pi\,(4 \times 10^{-6})\ \text{m}^{2} = 4\pi \times 10^{-6}\ \text{m}^{2} \approx 1.2566 \times 10^{-5}\ \text{m}^{2}.$$

The resistance $$R$$ of a uniform wire is given by the formula

$$R = \rho\,\frac{L}{S}.$$

Substituting the known values,

$$R = \bigl(1.23 \times 10^{-8}\ \Omega\text{m}\bigr) \frac{0.30\ \text{m}}{1.2566 \times 10^{-5}\ \text{m}^{2}} = \frac{3.69 \times 10^{-9}\ \Omega\text{m}}{1.2566 \times 10^{-5}\ \text{m}^{2}} \approx 2.94 \times 10^{-4}\ \Omega = 0.000294\ \Omega.$$

The induced current $$I$$ in the loop is obtained from Ohm’s law, $$I = \dfrac{\mathcal{E}}{R}$$. Hence

$$I = \frac{1.8 \times 10^{-4}\ \text{V}}{2.94 \times 10^{-4}\ \Omega} \approx 0.612\ \text{A}.$$

This value rounds to approximately $$0.61\ \text{A}$$.

Hence, the correct answer is Option B.

Minimum Required Theory 

• Faraday’s Law of Electromagnetic Induction: The magnitude of the induced electromotive force ($$\varepsilon$$) in a closed loop equals the rate of change of magnetic flux ($$\Phi$$) through the loop:
• Unit Conversions:
  • o Area: $$1\text{ cm}^2 = 10^{-4}\text{ m}^2$$
• o Magnetic Field: $$1\text{ Gauss (G)} = 10^{-4}\text{ Tesla (T)}$$
• o Induced EMF: $$1\text{ }\mu\text{V} = 10^{-6}\text{ V}$$
• Area of large rectangle $$= 16\text{ cm} \times 4\text{ cm} = 64\text{ cm}^2$$
• Area of inner cut-out $$= 4\text{ cm} \times 2\text{ cm} = 8\text{ cm}^2$$

$$\varepsilon = \frac{\Delta \Phi}{\Delta t} = A \frac{\Delta B}{\Delta t}$$

Step-by-Step Solution

Step 1: Calculate the effective area ($$A$$) of the closed loop

The geometry consists of a large main rectangle from which a smaller inner rectangular section has been subtracted (or indented).

$$\text{Net Area } A = 64\text{ cm}^2 - 8\text{ cm}^2 = 56\text{ cm}^2$$

Convert the area to SI units ($$\text{m}^2$$):

$$A = 56 \times 10^{-4}\text{ m}^2$$

Step 2: Determine the rate of change of the magnetic field ($$\frac{\Delta B}{\Delta t}$$)

The magnetic field decreases linearly from $$1000\text{ G}$$ to $$500\text{ G}$$ over a time interval $$\Delta t = 5\text{ s}$$.

$$\Delta B = 1000\text{ G} - 500\text{ G} = 500\text{ G}$$

Convert $$\Delta B$$ to Tesla (T):

$$\Delta B = 500 \times 10^{-4}\text{ T}$$

Now, calculate the rate of change:

$$\frac{\Delta B}{\Delta t} = \frac{500 \times 10^{-4}\text{ T}}{5\text{ s}} = 100 \times 10^{-4}\text{ T/s}$$

Step 3: Compute the induced EMF ($$\varepsilon$$)

Substitute the values into Faraday's formula:

$$\varepsilon = A \times \frac{\Delta B}{\Delta t}$$

$$\varepsilon = (56 \times 10^{-4}\text{ m}^2) \times (100 \times 10^{-4}\text{ T/s})$$

$$\varepsilon = 5600 \times 10^{-8}\text{ V}$$

$$\varepsilon = 56 \times 10^{-6}\text{ V}$$

Step 4: Convert to microvolts ($$\mu\text{V}$$)

Since $$10^{-6}\text{ V} = 1\text{ }\mu\text{V}$$:

$$\varepsilon = 56\text{ }\mu\text{V}$$

Final Answer

The induced EMF in the loop is $$56\text{ }\mu\text{V}$$.

We have a series $$L\!-\!R$$ circuit connected to a battery of emf $$V$$. When the key is closed at $$t = 0$$, current starts building up through the inductor as well as the resistor.

For an $$L\!-\!R$$ circuit, the standard growth-of-current formula is first stated:

$$i(t)=\frac{V}{R}\left(1-e^{-Rt/L}\right).$$

Here $$i(t)$$ is the instantaneous current at time $$t$$, $$V$$ is the applied emf, $$R$$ is the resistance and $$L$$ is the inductance.

The energy stored in the magnetic field of the inductor at any instant is given by the usual expression

$$U(t)=\frac12\,L\,i^2(t).$$

When the current has reached its final steady value, the inductor stores its maximum energy. The steady (saturated) current is obtained by putting $$t\to\infty$$ in the current formula:

$$I_0 = \frac{V}{R}\left(1-e^{-\infty}\right)=\frac{V}{R}.$$

So the maximum magnetic energy is

$$U_{\max}= \frac12\,L\,I_0^2 =\frac12\,L\left(\frac{V}{R}\right)^2.$$

The problem states that the energy at some time $$t$$ is only $$\dfrac1n$$ of this maximum value. Therefore we write

$$U(t)=\frac1n\,U_{\max}.$$

Substituting the explicit expressions for both energies,

$$\frac12\,L\,i^2(t)=\frac1n\left[\frac12\,L\left(\frac{V}{R}\right)^2\right].$$

Canceling the common factors $$\dfrac12 L$$ on both sides, we get the algebraic relation between currents:

$$i^2(t)=\frac1n\left(\frac{V}{R}\right)^2.$$

Taking the square root (and noting that current is positive during growth),

$$i(t)=\frac{V}{R}\,\frac1{\sqrt{n}}.$$

Now we replace $$i(t)$$ by its functional form $$\dfrac{V}{R}\left(1-e^{-Rt/L}\right)$$ :

$$\frac{V}{R}\left(1-e^{-Rt/L}\right)=\frac{V}{R}\,\frac1{\sqrt{n}}.$$

Dividing both sides by the common factor $$\dfrac{V}{R}$$ gives a simple transcendental equation in the exponential:

$$1-e^{-Rt/L}=\frac1{\sqrt{n}}.$$

Rearranging to isolate the exponential term,

$$e^{-Rt/L}=1-\frac1{\sqrt{n}}=\frac{\sqrt{n}-1}{\sqrt{n}}.$$

To remove the exponential, we take natural logarithms on both sides. Remember that $$\ln(e^{x}) = x$$:

$$-\frac{Rt}{L}=\ln\!\left(\frac{\sqrt{n}-1}{\sqrt{n}}\right).$$

Multiplying by $$-\dfrac{L}{R}$$ converts it to an explicit expression for time $$t$$:

$$t=-\frac{L}{R}\,\ln\!\left(\frac{\sqrt{n}-1}{\sqrt{n}}\right).$$

The minus sign can be absorbed by inverting the logarithmic argument, using the property $$-\ln(x)=\ln\!\left(\dfrac1x\right)$$. Hence

$$t=\frac{L}{R}\,\ln\!\left(\frac{\sqrt{n}}{\sqrt{n}-1}\right).$$

This matches exactly the expression given in Option A.

Hence, the correct answer is Option A.

For a series $$RL$$ circuit the current as a function of time after a steady emf $$E$$ is suddenly applied is obtained from Kirchhoff’s voltage law. Writing the loop equation we get

$$E = iR + L\dfrac{di}{dt}.$$

This is a first-order linear differential equation. Its standard solution is

$$i(t)=\dfrac{E}{R}\Bigl(1-e^{-\dfrac{R}{L}t}\Bigr).$$

First note the two quantities that characterise the circuit:

$$R = 5\;\Omega,\qquad L = 10\;\text{mH}=10\times10^{-3}\;\text{H}=0.01\;\text{H}.$$

Therefore the time-constant $$\tau$$ of the circuit is

$$\tau=\dfrac{L}{R}=\dfrac{0.01}{5}=0.002\;\text{s}.$$ (Only this value will later show why the exponential term is negligibly small.)

The steady-state current, i.e. the current at $$t=\infty,$$ is obtained from the above expression by allowing the exponential term to vanish:

$$i(\infty)=\dfrac{E}{R}\bigl(1-e^{-\infty}\bigr)=\dfrac{E}{R}\times(1-0)=\dfrac{E}{R}.$$

Putting $$E=20\;\text{V}$$ and $$R=5\;\Omega$$ gives

$$i(\infty)=\dfrac{20}{5}=4\;\text{A}.$$

Now we calculate the current at the specified instant $$t = 40\;\text{s}.$$ Using the same general expression,

$$i(40)=\dfrac{E}{R}\Bigl(1-e^{-\dfrac{R}{L}\,40}\Bigr).$$

First evaluate the exponent very carefully:

$$\dfrac{R}{L}\,40=\dfrac{5}{0.01}\times40 =500\times40 =20000.$$

So the exponential term is

$$e^{-20000}.$$

This number is astronomically small. Even a negative exponent of only $$-20$$ already makes $$e^{-20}\approx2.06\times10^{-9};$$ for $$-20000$$ the value is effectively zero in any practical calculation. Hence we write

$$e^{-20000}\approx0,$$

and therefore

$$i(40)=\dfrac{20}{5}\,(1-0)=4\;\text{A}.$$

The ratio asked in the question is

$$\dfrac{i(\infty)}{i(40)} =\dfrac{4\;\text{A}}{4\;\text{A}} =1.$$

Among the numerical alternatives supplied, the value nearest to the exact theoretical ratio $$1$$ is $$1.06.$$ Consequently this option is regarded as the closest correct choice.

Hence, the correct answer is Option A.

We begin with the magnetic field produced by the larger coil $$C_2$$ at its own centre (and therefore also at the centre of the smaller concentric coil $$C_1$$). For a circular coil of $$N$$ turns carrying current $$I$$, the field on the axis at the centre is given by the well-known formula

$$B=\dfrac{\mu_0 N I}{2R}$$

where $$\mu_0=4\pi\times10^{-7}\,\text{H m}^{-1}$$ is the permeability of free space and $$R$$ is the radius of the coil.

For coil $$C_2$$ we have $$N_2=200$$ turns and $$R_2=20\ \text{cm}=0.20\ \text{m}$$. Hence

$$B(t)=\dfrac{\mu_0 N_2}{2R_2}\,I(t) =\dfrac{\mu_0\,(200)}{2(0.20)}\,I(t) =\dfrac{\mu_0\,(200)}{0.40}\,I(t).$$

The smaller coil $$C_1$$ of radius $$r_1=1\ \text{cm}=0.01\ \text{m}$$ encloses an area

$$A_1=\pi r_1^2=\pi(0.01)^2=\pi\times10^{-4}\ \text{m}^2.$$

The magnetic flux through one turn of $$C_1$$ is therefore

$$\Phi(t)=B(t)\,A_1 =\left(\dfrac{\mu_0 N_2}{2R_2}\,I(t)\right)A_1.$$

Because coil $$C_1$$ has $$N_1=500$$ turns, the total flux linkage with it is

$$\Lambda(t)=N_1\Phi(t)=N_1A_1\dfrac{\mu_0 N_2}{2R_2}\,I(t).$$

Faraday’s law of electromagnetic induction states that the induced emf is the negative time derivative of this flux linkage:

$$\mathcal{E}(t)=-\dfrac{d\Lambda}{dt} =-N_1A_1\dfrac{\mu_0 N_2}{2R_2}\,\dfrac{dI}{dt}.$$

Only the current $$I(t)$$ is time dependent, so we differentiate it. The given current is

$$I(t)=5t^2-2t+3\ \text{A},$$

hence

$$\dfrac{dI}{dt}=10t-2.$$

At the required instant $$t=1\ \text{s}$$,

$$\left.\dfrac{dI}{dt}\right|_{t=1}=10(1)-2=8\ \text{A s}^{-1}.$$

Substituting all numerical values (the sign is irrelevant because we need only the magnitude of the emf), we obtain

$$|\mathcal{E}|=N_1A_1\dfrac{\mu_0 N_2}{2R_2}\,\left|\dfrac{dI}{dt}\right| =500\;\bigl(\pi\times10^{-4}\bigr)\; \dfrac{4\pi\times10^{-7}\,(200)}{2(0.20)}\; 8.$$

Simplifying step by step,

$$\dfrac{4\pi\times10^{-7}\,(200)}{2(0.20)} =\dfrac{4\pi\times10^{-7}\times200}{0.40} =\dfrac{800\pi\times10^{-7}}{0.40} =2000\pi\times10^{-7} =2\pi\times10^{-4}.$$

So

$$|\mathcal{E}|=500\;(\pi\times10^{-4})\;(2\pi\times10^{-4})\;8 =500\times2\times8\times\pi^2\times10^{-8} =8000\pi^2\times10^{-8}\ \text{volts}.$$

Combining the powers of ten,

$$8000=8\times10^{3} \quad\Rightarrow\quad 8\times10^{3}\times10^{-8}=8\times10^{-5},$$

so

$$|\mathcal{E}|=8\pi^2\times10^{-5}\ \text{V}.$$

Since $$1\ \text{mV}=10^{-3}\ \text{V}$$, we convert the emf to millivolts:

$$|\mathcal{E}|=8\pi^2\times10^{-5}\ \text{V} =8\pi^2\times10^{-5}\times10^{3}\ \text{mV} =8\pi^2\times10^{-2}\ \text{mV} =0.08\pi^2\ \text{mV}.$$

The problem statement tells us that this magnitude equals $$\dfrac{4}{x}\ \text{mV}$$, so we write

$$\dfrac{4}{x}=0.08\pi^2.$$

Isolating $$x$$ gives

$$x=\dfrac{4}{0.08\pi^2} =\dfrac{4}{\frac{8}{100}}\;\dfrac{1}{\pi^2} =50\;\dfrac{1}{\pi^2} =\dfrac{50}{\pi^2}.$$

Taking $$\pi^2\approx9.87$$, we have

$$x\approx\dfrac{50}{9.87}\approx5.1\;.$$

To the nearest integer, required by JEE’s answer key, this is simply $$5$$.

So, the answer is $$5$$.

We begin with Faraday’s law of electromagnetic induction, which states that the magnitude of the induced emf in a coil is given by

$$e = -\,\dfrac{d\Phi}{dt},$$

where $$\Phi$$ is the magnetic flux through the coil. For a single circular loop of area $$A$$ placed in a uniform magnetic field $$B$$, the flux at any instant is

$$\Phi = B A \cos\theta,$$

with $$\theta$$ being the angle between the magnetic field and the normal to the plane of the coil.

In this problem the coil is made to rotate at a constant angular speed $$\omega$$ about a diameter that lies in the plane of the coil and is perpendicular to the magnetic field. Because of this rotation, the angle $$\theta$$ changes uniformly with time as

$$\theta = \omega t.$$

Differentiating the flux with respect to time, we obtain

$$$e = -\dfrac{d}{dt}\!\left(B A \cos(\omega t)\right) = -B A \,\dfrac{d}{dt}\!\left(\cos(\omega t)\right).$$$

Using $$\dfrac{d}{dt}[\cos(\omega t)] = -\omega \sin(\omega t)$$, we get

$$$e = -B A \left(-\omega \sin(\omega t)\right) = B A \omega \sin(\omega t).$$$

The sine function can vary from $$-1$$ to $$+1$$, so the maximum magnitude of the emf (denoted $$e_{\text{max}}$$) is obtained when $$\sin(\omega t)=\pm 1$$. Therefore,

$$e_{\text{max}} = B A \omega.$$

Now we substitute the given data step by step.

Radius of the coil: $$r = 10\ \text{cm} = 0.10\ \text{m}.$$

Area of the coil (using $$A = \pi r^{2}$$):

$$$A = \pi (0.10\ \text{m})^{2} = \pi (0.01\ \text{m}^{2}) = 0.01\pi\ \text{m}^{2}.$$$

Uniform magnetic field: $$B = 3.0 \times 10^{-5}\ \text{T}.$$

The coil completes half a revolution (an angle of $$\pi$$ radians) in $$0.2\ \text{s}$$. Hence the constant angular speed is

$$\omega = \dfrac{\pi\ \text{rad}}{0.2\ \text{s}} = 5\pi\ \text{rad s}^{-1}.$$

Taking the coil to have a single turn, $$N = 1$$. (If there were more turns we would multiply by $$N$$, but the numerical answer indicates $$N = 1$$.)

Substituting all values in $$e_{\text{max}} = B A \omega$$ gives

$$$\begin{aligned} e_{\text{max}} &= (3.0 \times 10^{-5}\ \text{T}) \times (0.01\pi\ \text{m}^{2}) \times (5\pi\ \text{rad s}^{-1}) \\ &= 3.0 \times 0.01 \times 5 \times \pi^{2} \times 10^{-5}\ \text{V} \\ &= 0.15 \times \pi^{2} \times 10^{-5}\ \text{V}. \end{aligned}$$$

Using $$\pi^{2}\approx 9.8696$$, we have

$$$e_{\text{max}} = 0.15 \times 9.8696 \times 10^{-5}\ \text{V} = 1.480 \times 10^{-5}\ \text{V}.$$$

To convert this to microvolts, recall that $$1\ \text{V} = 10^{6}\ \mu\text{V}$$, so

$$$e_{\text{max}} = 1.480 \times 10^{-5}\ \text{V} = 1.480 \times 10^{-5} \times 10^{6}\ \mu\text{V} = 14.8\ \mu\text{V}.$$$

Rounding to the nearest integer (as required by the question), we get

$$e_{\text{max}} \approx 15\ \mu\text{V}.$$

So, the answer is $$15$$.

We have a choke in which an induced reverse voltage of $$100\ \text{V}$$ appears while the current falls uniformly from $$0.25\ \text{A}$$ to $$0\ \text{A}$$ in a short time interval of $$0.025\ \text{ms}$$.

First convert the given time into seconds, because in the SI system the unit of time is the second:

$$0.025\ \text{ms}=0.025 \times 10^{-3}\ \text{s}=0.000025\ \text{s}$$

The basic formula for the self-induced emf (Faraday-Lenz law for an inductor) is stated as

$$V = -L\,\frac{di}{dt}$$

where

$$V$$ is the induced voltage (here the given $$100\ \text{V}$$, we will use only its magnitude),
$$L$$ is the self-inductance we have to find, and
$$\dfrac{di}{dt}$$ is the rate of change of current.

Because the current drops from $$0.25\ \text{A}$$ to $$0\ \text{A}$$, the change in current is

$$\Delta i = i_\text{final}-i_\text{initial}=0-0.25=-0.25\ \text{A}$$

and the time interval is $$\Delta t = 0.000025\ \text{s}$$. Assuming the change is uniform, the rate of change of current is

$$\frac{di}{dt} = \frac{\Delta i}{\Delta t} = \frac{-0.25}{0.000025}\ \frac{\text{A}}{\text{s}} = -10000\ \frac{\text{A}}{\text{s}}.$$

The magnitude of this rate is $$|di/dt| = 10000\ \text{A s}^{-1}$$.

Ignoring the negative sign (it only shows that the voltage is a reverse emf), we substitute the magnitudes into the formula:

$$V = L\,\Bigl|\frac{di}{dt}\Bigr| \quad\Longrightarrow\quad L = \frac{V}{|di/dt|} = \frac{100\ \text{V}}{10000\ \text{A s}^{-1}}$$

So,

$$L = 0.01\ \text{H}.$$

To express this in millihenry, recall that $$1\ \text{H}=1000\ \text{mH},$$ hence

$$0.01\ \text{H}=0.01 \times 1000\ \text{mH}=10\ \text{mH}.$$

So, the answer is $$10\ \text{mH}$$.

For a straight conductor of length $$l$$ moving with a velocity $$\vec v$$ in a uniform magnetic field $$\vec B$$, the motional emf is obtained from the expression

$$\mathcal E \;=\;\int (\vec v \times \vec B)\cdot d\vec l.$$

Because the wire is straight and both $$\vec v$$ and $$\vec B$$ are uniform, the integrand is constant, so the integral reduces to a simple dot product:

$$\mathcal E \;=\;(\vec v \times \vec B)\cdot\vec l.$$

Now we interpret each vector in the present situation.

• The horizontal component of the earth’s magnetic field, $$\vec B_h$$, is directed due North, with magnitude $$B_h = 0.3 \times 10^{-4}\;{\rm Wb\,m^{-2}}.$$

• The wire is oriented from North-East to South-West, so the length vector $$\vec l$$ lies along the north-east direction. This direction makes an angle of $$45^\circ$$ with the due-east line. Its magnitude is the length of the wire, $$l = 10\;{\rm m}.$$

• The wire is falling vertically downward with speed $$v = 5\;{\rm m\,s^{-1}}.$$ The downward velocity is at right angles to the horizontal magnetic field, so the cross product simplifies:

$$|\vec v \times \vec B_h| \;=\; v B_h \sin 90^\circ \;=\; vB_h.$$

The vector $$\vec v \times \vec B_h$$ points due East (right-hand rule). Hence the angle between $$\vec v \times \vec B_h$$ (eastward) and $$\vec l$$ (north-east) is $$45^\circ$$. Substituting everything into the dot product, we have

$$\mathcal E \;=\; |\vec v \times \vec B_h|\,|\vec l|\,\cos 45^\circ$$

$$\phantom{\mathcal E} \;=\; (vB_h)\,l \,\cos 45^\circ.$$

Putting the numerical values,

$$vB_h = 5 \times 0.3 \times 10^{-4} = 1.5 \times 10^{-4}\;{\rm V\,m^{-1}}.$$

Multiplying by the length,

$$ (vB_h)l = 1.5 \times 10^{-4} \times 10 = 1.5 \times 10^{-3}\;{\rm V}.$$

Including the factor $$\cos 45^\circ = \dfrac{1}{\sqrt 2}\approx 0.707,$$

$$\mathcal E = 1.5 \times 10^{-3}\, \times 0.707 \;\approx\; 1.06 \times 10^{-3}\;{\rm V}.$$

To one significant figure this is

$$\mathcal E \simeq 1.1 \times 10^{-3}\;{\rm V}.$$

Hence, the correct answer is Option B.

We have a conducting strip of length $$L = 10 \text{ cm}=0.10 \text{ m}$$ sliding on friction-free rails that form a closed circuit of total resistance $$R = 10\ \Omega$$. The strip is free to move along the rails while a uniform magnetic field of magnitude $$B = 0.10\ \text T$$ is directed perpendicular to the plane of the circuit. A light spring of force constant $$k = 0.5\ \text{N m}^{-1}$$ pulls the strip towards its equilibrium position. The mass of the strip is $$m = 50\ \text g = 0.05\ \text{kg}$$. Air drag is negligible, so the only damping is electromagnetic.

Whenever the strip moves with velocity $$v$$, the motional emf produced across its ends is given by Faraday’s law of electromagnetic induction,

$$\varepsilon = B L v.$$

Ohm’s law then gives the induced current,

$$i = \dfrac{\varepsilon}{R}= \dfrac{B L v}{R}.$$ The magnetic force on a current-carrying conductor of length $$L$$ in a field $$B$$ is (in magnitude)

$$F = i L B.$$

Substituting the expression for $$i$$, we obtain

$$F = \left(\dfrac{B L v}{R}\right) L B = \dfrac{B^{2} L^{2}}{R}\,v.$$

This force always opposes the velocity (Lenz’s law), so it behaves exactly like a viscous damping force of the form $$F = -b v$$ with an effective electromagnetic damping coefficient

$$b = \dfrac{B^{2} L^{2}}{R}.$$

Putting in the numerical values,

$$b = \dfrac{(0.10)^2 (0.10)^2}{10} = \dfrac{0.01 \times 0.01}{10} = \dfrac{1.0 \times 10^{-4}}{10} = 1.0 \times 10^{-5}\ \text{N s m}^{-1}.$$

The strip-spring system therefore obeys the equation for a damped harmonic oscillator,

$$m \ddot x + b \dot x + k x = 0.$$

For small damping (which is true here because $$b$$ is very small) the displacement amplitude decays exponentially as

$$A(t)=A_{0}\,e^{-\dfrac{b}{2m}t}.$$

The time constant for the amplitude to fall by a factor of $$e$$ is therefore

$$\tau = \dfrac{2m}{b}.$$

Meanwhile the natural period of oscillation (with weak damping the change in period is negligible) follows from

$$T = 2\pi\sqrt{\dfrac{m}{k}}.$$

The number of oscillations completed in a time $$t$$ is $$n = \dfrac{t}{T}$$, so the number of oscillations completed in one time constant $$\tau$$ is

$$N = \dfrac{\tau}{T} = \dfrac{\dfrac{2m}{b}}{2\pi\sqrt{m/k}} = \dfrac{m}{\pi b}\,\sqrt{\dfrac{k}{m}} = \dfrac{1}{\pi}\,\dfrac{\sqrt{mk}}{b}.$$

Now we substitute every numerical value step by step.

First,

$$\sqrt{mk}= \sqrt{(0.05\ \text{kg})(0.5\ \text{N m}^{-1})} = \sqrt{0.025}\ \text{kg}^{1/2}\text{N}^{1/2}\text{m}^{-1/2} = 0.1581.$$

Next, recalling $$b = 1.0 \times 10^{-5}\ \text{N s m}^{-1}$$, we have

$$N = \dfrac{1}{\pi}\,\dfrac{0.1581}{1.0 \times 10^{-5}} = \dfrac{0.1581}{3.1416 \times 10^{-5}} \approx \dfrac{0.1581}{0.000031416} \approx 5030.$$

Because we merely need a value “close to” the answer, we round to the nearest option offered. $$5030$$ is closest to $$5000.$$

Hence, the correct answer is Option B.

We consider two very long co-axial solenoids, both of the same length $$l$$. The inner solenoid (which we shall call coil 1) has radius $$r_1$$ and number of turns per unit length $$n_1$$. The outer solenoid (coil 2) has radius $$r_2$$ and number of turns per unit length $$n_2$$. We wish to find the ratio of their mutual inductance $$M$$ to the self-inductance $$L_1$$ of the inner solenoid.

First, we recall the magnetic field inside a very long solenoid. The standard result is

$$B=\mu_0\,n\,I,$$

where $$n$$ is the number of turns per unit length and $$I$$ is the current through that solenoid. This field is uniform over the entire cross-section of the solenoid and directed along its axis.

Let a current $$I$$ flow only in the inner coil. Using the above formula, the magnetic field produced by this inner coil is

$$B_1=\mu_0 n_1 I.$$

The cross-sectional area of the inner solenoid is

$$A_1=\pi r_1^2.$$

Hence, the magnetic flux through each single turn of the inner solenoid is

$$\Phi_{\text{single (self)}} = B_1 A_1 = \mu_0 n_1 I \,\pi r_1^2.$$

The total number of turns in the inner solenoid is

$$N_1 = n_1 l.$$

Therefore, the total flux linked with all the turns of the inner solenoid is

$$\Phi_{\text{total (self)}} = N_1 \Phi_{\text{single (self)}} = n_1 l \left(\mu_0 n_1 I \,\pi r_1^2\right) = \mu_0 n_1^2 l \,\pi r_1^2 \, I.$$

By definition, the self-inductance $$L_1$$ of the inner coil is

$$L_1 = \frac{\text{total flux linkage}}{I} = \frac{\Phi_{\text{total (self)}}}{I} = \mu_0 n_1^2 l \,\pi r_1^2.$$

Now we compute the mutual inductance $$M$$. The same current $$I$$ in the inner coil produces the same field $$B_1=\mu_0 n_1 I$$ everywhere inside radius $$r_1$$. This field also threads the outer coil, but only through the region of radius $$r_1$$; outside that region the field is (ideally) zero. Thus, the flux through each single turn of the outer solenoid is exactly the same $$\Phi_{\text{single (mutual)}} = \mu_0 n_1 I \,\pi r_1^2.$$

The total number of turns in the outer solenoid is

$$N_2 = n_2 l.$$

Hence, the total flux linked with the outer solenoid because of the current in the inner one is

$$\Phi_{\text{total (mutual)}} = N_2 \Phi_{\text{single (mutual)}} = n_2 l \left(\mu_0 n_1 I \,\pi r_1^2\right) = \mu_0 n_1 n_2 l \,\pi r_1^2 \, I.$$

By definition, the mutual inductance $$M$$ is

$$M = \frac{\text{total mutual flux linkage}}{I} = \frac{\Phi_{\text{total (mutual)}}}{I} = \mu_0 n_1 n_2 l \,\pi r_1^2.$$

We can now form the desired ratio:

$$\frac{M}{L_1} = \frac{\mu_0 n_1 n_2 l \,\pi r_1^2}{\mu_0 n_1^2 l \,\pi r_1^2} = \frac{n_2}{n_1}.$$

All constant factors $$\mu_0$$, $$l$$ and $$\pi r_1^2$$ cancel out, leaving a very simple result that depends only on the turn densities.

Hence, the correct answer is Option D.

According to the theory of mutual induction, when two coils are kept fixed with respect to each other, the magnetic flux in one coil produced by the current in the other is proportional to that current. The constant of proportionality is called the mutual inductance and is denoted by $$M$$. Mathematically we write the definition as

$$\Phi_{21}=M\,I_1 \qquad\text{and}\qquad \Phi_{12}=M\,I_2,$$

where

$$\Phi_{21}$$ is the flux linking coil 2 due to current $$I_1$$ in coil 1, and

$$\Phi_{12}$$ is the flux linking coil 1 due to current $$I_2$$ in coil 2.

In the present problem coil ‘P’ corresponds to coil 1, and coil ‘Q’ corresponds to coil 2. First, a current flows only in coil P and we observe the flux through coil Q.

The data given are

Current in coil P: $$I_P = 3\ \text{A},$$

Flux through coil Q caused by this current: $$\Phi_{Q}=10^{-3}\ \text{Wb}.$$

Using the defining formula $$\Phi_{Q}=M\,I_P$$, we can solve for the mutual inductance $$M$$:

$$M=\frac{\Phi_{Q}}{I_P}=\frac{10^{-3}\ \text{Wb}}{3\ \text{A}}.$$

Simplifying the ratio gives

$$M=\frac{1}{3}\times10^{-3}\ \text{H}=0.333\overline{3}\times10^{-3}\ \text{H}.$$

In scientific-notation form this is

$$M=3.33\overline{3}\times10^{-4}\ \text{H}.$$

Now we switch the situation: we set the current in coil P to zero and pass a current through coil Q. The numerical value of the mutual inductance does not change because it depends only on the geometry and relative placement of the two coils, not on which coil is carrying current.

The new current is

$$I_Q = 2\ \text{A},$$

and we want the resulting flux through coil P, which we call $$\Phi_{P}$$. Using the same formula but interchanging the coil labels, we write

$$\Phi_{P}=M\,I_Q.$$

Substituting the previously found $$M$$ and the given $$I_Q$$:

$$\Phi_{P}= \left(3.33\overline{3}\times10^{-4}\ \text{H}\right)\,(2\ \text{A}).$$

Carrying out the multiplication:

$$\Phi_{P}= 6.66\overline{6}\times10^{-4}\ \text{Wb}.$$

Rounding to three significant figures yields

$$\Phi_{P}=6.67\times10^{-4}\ \text{Wb}.$$

This numerical value exactly matches Option B.

Hence, the correct answer is Option B.

For growth of current in an RL circuit,

$$I=\frac{E}{R}\left(1-e^{-Rt/L}\right)$$

Given,

$$L=20\ H,\qquad R=10\ \Omega$$

So,

$$I=\frac{E}{10}\left(1-e^{-t/2}\right)$$

Rate of Joule heating across resistance:

$$P_R=I^2R$$

Rate of storage of magnetic energy in inductor:

$$P_L=L I\frac{dI}{dt}$$

At the required instant,

$$LI\frac{dI}{dt}=I^2R$$

Now,

$$\frac{dI}{dt} = \frac{E}{10}\left(\frac{1}{2}e^{-t/2}\right)$$

Substituting,

$$20\times \frac{E}{10}(1-e^{-t/2}) \times \frac{E}{20}e^{-t/2} = \left[\frac{E}{10}(1-e^{-t/2})\right]^2\times10$$

Simplifying,

$$e^{-t/2}=1-e^{-t/2}$$

$$2e^{-t/2}=1$$

$$e^{-t/2}=\frac{1}{2}$$

Taking logarithm,

$$-\frac{t}{2}=\ln\left(\frac{1}{2}\right)$$

$$\boxed{t=2\ln2\ \text{s}}$$

We are dealing with a series LR circuit whose switch is closed at the instant $$t = 0$$. At that moment the current is zero, and thereafter it rises according to the well-known growth law for an LR circuit.

First, we recall the current-time relation for a resistor $$R$$ in series with an inductor $$L$$ connected to a constant emf $$E$$. The standard formula is stated as

$$i(t) = \frac{E}{R}\Bigl(1 - e^{-\,\frac{R}{L}t}\Bigr).$$

This expression comes from solving the differential equation $$E = L\dfrac{di}{dt} + Ri$$ with the initial condition $$i(0)=0$$.

We are asked for the total charge that flows through the battery from $$t = 0$$ up to the time $$t = \dfrac{L}{R}$$. Charge is the time integral of current, so we write

$$Q = \int_{0}^{\,L/R} i(t)\,dt.$$

Substituting the expression for $$i(t)$$ obtained above, we have

$$Q = \int_{0}^{\,L/R} \frac{E}{R}\Bigl(1 - e^{-\,\frac{R}{L}t}\Bigr)\,dt.$$

The constant factor $$\frac{E}{R}$$ can be taken outside the integral:

$$Q = \frac{E}{R}\int_{0}^{\,L/R} \Bigl(1 - e^{-\,\frac{R}{L}t}\Bigr)\,dt.$$

Now we integrate term by term. The integral of $$1$$ with respect to $$t$$ is simply $$t$$. The integral of $$-e^{-at}$$, where $$a = \dfrac{R}{L}$$, is $$\dfrac{1}{a}e^{-at}$$ because

$$\int -e^{-at}\,dt = \frac{1}{a}e^{-at} + C.$$

Using $$a = \dfrac{R}{L}$$, the integral becomes

$$\int_{0}^{\,L/R} \Bigl(1 - e^{-\,\frac{R}{L}t}\Bigr)\,dt = \Bigl[t + \frac{L}{R}\,e^{-\,\frac{R}{L}t}\Bigr]_{0}^{\,L/R}.$$

We now evaluate the definite integral at its limits. First at the upper limit $$t = \dfrac{L}{R}$$:

$$t = \frac{L}{R}, \quad e^{-\,\frac{R}{L}\cdot\frac{L}{R}} = e^{-1} = \frac{1}{e}.$$

So the expression at the upper limit is

$$\frac{L}{R} + \frac{L}{R}\cdot\frac{1}{e}.$$

Next, evaluate at the lower limit $$t=0$$:

$$t = 0, \quad e^{-\,\frac{R}{L}\cdot 0} = e^{0} = 1,$$

giving

$$0 + \frac{L}{R}\cdot 1 = \frac{L}{R}.$$

Subtracting the lower-limit value from the upper-limit value we obtain

$$\left(\frac{L}{R} + \frac{L}{R}\cdot\frac{1}{e}\right) \;-\; \left(\frac{L}{R}\right) = \frac{L}{R}\Bigl(\frac{1}{e}\Bigr).$$

Hence the definite integral equals $$\dfrac{L}{R}\dfrac{1}{e}$$, and therefore

$$Q = \frac{E}{R} \times \frac{L}{R}\times\frac{1}{e} = \frac{EL}{e\,R^{2}}.$$

Numerically, $$e \approx 2.718 \approx 2.7$$, so the charge can be written approximately as

$$Q \approx \frac{EL}{2.7\,R^{2}}.$$

This matches exactly with option D.

Hence, the correct answer is Option D.

We are given a copper wire wound on a wooden frame shaped as an equilateral triangle. The linear dimension of each side is increased by a factor of 3, while the number of turns per unit length of the frame remains the same. We need to find how the self-inductance changes.

For a coil wound along the perimeter of a frame, the self-inductance is given by the solenoid formula:

$$L = \mu_0 n^2 A \ell$$

where $$n$$ is the number of turns per unit length of the frame, $$A$$ is the area enclosed by the coil, and $$\ell$$ is the total length (perimeter) of the frame.

For an equilateral triangle with side $$a$$:

Perimeter: $$\ell = 3a$$

Area: $$A = ?rac{\sqrt{3}}{4}a^2$$

So the self-inductance becomes:

$$L = \mu_0 n^2 \cdot ?rac{\sqrt{3}}{4}a^2 \cdot 3a = ?rac{3\sqrt{3}}{4}\mu_0 n^2 a^3$$

Therefore, $$L \propto a^3$$ (since $$n$$ and $$\mu_0$$ are constants).

When each side is increased by a factor of 3 (i.e., $$a o 3a$$):

$$L' = ?rac{3\sqrt{3}}{4}\mu_0 n^2 (3a)^3 = ?rac{3\sqrt{3}}{4}\mu_0 n^2 \cdot 27a^3 = 27L$$

The self-inductance increases by a factor of $$27$$.

The correct answer is Option B: increases by a factor of 27.

We have a solid metallic cube whose edge length is given as 2 cm. In electromagnetism, it is always safer to change every length into the SI unit metre, because the Tesla (T) and the metre-second system go together neatly. So we convert

$$2\ \text{cm}=2\times 10^{-2}\ \text{m}=0.02\ \text{m}.$$

This cube is moving with a constant speed of 6 m s$$^{-1}$$ along the positive $$y$$-axis, that is

$$v=6\ \text{m s}^{-1},\qquad \vec v=6\hat{\mathbf y}\ \text{m s}^{-1}.$$

The region contains a uniform magnetic field whose magnitude is 0.1 T and whose direction is the positive $$z$$-axis, so

$$B=0.1\ \text{T},\qquad \vec B=0.1\hat{\mathbf z}\ \text{T}.$$

We are asked for the potential difference between the two faces that are perpendicular to the $$x$$-axis. Those faces are the left and right faces of the cube; they are separated from each other by a distance equal to the edge length $$\ell=0.02\ \text{m}$$ along the $$x$$-direction.

For a solid conductor moving in a magnetic field, the free charges experience the magnetic Lorentz force $$\vec F=q(\vec v\times\vec B).$$ Inside the conductor, this force causes charge separation until an electric field is set up that balances the magnetic force, leading to a motional emf. The magnitude of that induced potential difference (emf) between two points a distance $$\ell$$ apart in the direction of $$\vec v\times\vec B$$ is given by the standard formula

$$\mathcal E = B\,\ell\,v,$$

provided $$\vec v$$, $$\vec B$$, and the line joining the two points are mutually perpendicular, which is the case here because

$$\vec v\parallel\hat{\mathbf y},\quad \vec B\parallel\hat{\mathbf z},\quad (\vec v\times\vec B)\parallel\hat{\mathbf x}.$$

Now we substitute the known numerical values step by step:

$$\mathcal E = B\,\ell\,v = (0.1\ \text{T})(0.02\ \text{m})(6\ \text{m s}^{-1}).$$

First multiply the last two factors:

$$0.02\ \text{m}\times 6\ \text{m s}^{-1}=0.12\ \text{m}^{2}\text{s}^{-1}.$$

Next multiply by the magnetic field:

$$0.1\ \text{T}\times 0.12\ \text{m}^{2}\text{s}^{-1}=0.012\ \text{V}.$$

Finally convert the result into millivolts, remembering that $$1\ \text{V}=1000\ \text{mV}$$:

$$0.012\ \text{V}=0.012\times 1000\ \text{mV}=12\ \text{mV}.$$

So the induced potential difference between the two faces of the cube that are perpendicular to the $$x$$-axis is $$12\ \text{mV}$$.

Hence, the correct answer is Option A.

We have a very long solenoid of radius $$R$$. Its current varies with time as $$I(t)=k\,t\,e^{-\alpha t}$$, where $$k\gt 0$$ and $$t\ge 0$$. Counter-clockwise (ccw) current is defined as positive.

The magnetic field at the centre of a long solenoid is given by the standard formula

$$B=\mu_0\,n\,I(t),$$

where $$n$$ is the number of turns per unit length and $$\mu_0$$ is the permeability of free space. This field is directed along the axis of the solenoid. For a ccw current, the right-hand rule shows that the magnetic field points out of the page (towards the observer).

A circular conducting loop of radius $$2R$$ is placed coaxially in the same plane (the “equatorial” plane). Because the solenoid is long, the field outside it is negligible, so magnetic flux through the outer loop is contributed only by the field inside the solenoid’s cross-section of area $$\pi R^2$$.

Hence the magnetic flux linked with the outer loop is

$$\Phi(t)=B\;(\text{area inside solenoid})=\mu_0\,n\,I(t)\;\pi R^2.$$

Faraday’s law of electromagnetic induction states

$$\mathcal E = -\dfrac{d\Phi}{dt},$$

where $$\mathcal E$$ is the induced emf. Substituting the expression for $$\Phi(t)$$ gives

$$\mathcal E = -\mu_0\,n\,\pi R^2\,\dfrac{dI}{dt}.$$

Because $$\mu_0\,n\,\pi R^2$$ is a positive constant, the sign (direction) of the induced emf, and therefore of the induced current in the outer loop, is completely determined by the sign of $$\dfrac{dI}{dt}$$. A negative emf corresponds to a clockwise current (negative, by problem convention), and a positive emf corresponds to a counter-clockwise current (positive).

So we next compute $$\dfrac{dI}{dt}$$ for

$$I(t)=k\,t\,e^{-\alpha t}.$$

Using the product rule of differentiation,

$$\dfrac{dI}{dt} = k\,e^{-\alpha t} + k\,t\,\dfrac{d}{dt}\!\big(e^{-\alpha t}\big) = k\,e^{-\alpha t} - k\,\alpha t\,e^{-\alpha t} = k\,e^{-\alpha t}\,\bigl(1-\alpha t\bigr).$$

Now we analyse the sign of this derivative:

• For $$0\le t \lt \dfrac{1}{\alpha}$$, we have $$1-\alpha t \gt 0$$, so $$\dfrac{dI}{dt}\gt 0.$$br • At $$t=\dfrac{1}{\alpha}$$, we get $$1-\alpha t=0$$, so $$\dfrac{dI}{dt}=0.$$br • For $$t \gt \dfrac{1}{\alpha}$$, we have $$1-\alpha t \lt 0$$, so $$\dfrac{dI}{dt}\lt 0.$$

Because $$\mathcal E = -\text{(constant)}\dfrac{dI}{dt}$$, the induced emf (and thus the induced current) has the opposite sign:

• When $$\dfrac{dI}{dt}\gt 0$$ (i.e., $$t\lt \dfrac{1}{\alpha}$$), $$\mathcal E\lt 0$$. The current in the outer loop is clockwise, hence negative.

• At $$t=\dfrac{1}{\alpha}$$, $$\dfrac{dI}{dt}=0$$, so $$\mathcal E=0$$. The induced current is zero.

• When $$\dfrac{dI}{dt}\lt 0$$ (i.e., $$t\gt \dfrac{1}{\alpha}$$), $$\mathcal E\gt 0$$. The current in the outer loop is counter-clockwise, hence positive.

Therefore, the induced current is negative for early times, crosses zero at $$t=\dfrac{1}{\alpha}$$, and becomes positive thereafter. The qualitative plot is a curve that starts below the time axis, touches the axis once, and then rises above it.

This behaviour corresponds exactly to the graph in Option 2, which shows the current starting negative, becoming zero once, and then becoming positive.

Hence, the correct answer is Option 2.

We have a circuit in which a coil of self-inductance $$L = 10\,\text{mH} = 10 \times 10^{-3}\,\text{H}$$ and resistance $$R_{\text{coil}} = 0.1\,\Omega$$ is connected to a battery whose internal resistance is $$r = 0.9\,\Omega$$. When the switch is closed, the coil resistance and the internal resistance are in series, so the total series resistance is

$$R = R_{\text{coil}} + r = 0.1\,\Omega + 0.9\,\Omega = 1.0\,\Omega.$$

The growth of current in an $$L\!-\!R$$ circuit after the switch is closed is governed by the exponential law

$$I(t) = I_0 \left(1 - e^{-t/\tau}\right),$$

where $$I_0$$ is the steady (saturation) current and $$\tau$$ is the time constant. The time constant for an $$L\!-\!R$$ circuit is given by the formula

$$\tau = \frac{L}{R}.$$

Substituting the given values, we obtain

$$\tau = \frac{10 \times 10^{-3}\,\text{H}}{1.0\,\Omega} = 10^{-2}\,\text{s} = 0.01\,\text{s}.$$

The problem asks for the time $$t$$ required for the current to reach 80 % of its saturation value. Mathematically, this means

$$I(t) = 0.8\,I_0.$$

Using the current-growth equation, we write

$$0.8\,I_0 = I_0 \left(1 - e^{-t/\tau}\right).$$

Dividing both sides by $$I_0$$, we get

$$0.8 = 1 - e^{-t/\tau}.$$

Rearranging to isolate the exponential term,

$$e^{-t/\tau} = 1 - 0.8 = 0.2.$$

Taking natural logarithms on both sides, we have

$$-\,\frac{t}{\tau} = \ln(0.2).$$

Now, $$0.2 = \frac{1}{5},$$ so

$$\ln(0.2) = \ln\!\left(\frac{1}{5}\right) = -\ln 5.$$

The question supplies $$\ln 5 = 1.6,$$ hence

$$\ln(0.2) = -1.6.$$

Substituting this value, we find

$$-\,\frac{t}{\tau} = -1.6 \quad\Longrightarrow\quad \frac{t}{\tau} = 1.6.$$

Finally, substituting $$\tau = 0.01\,\text{s}$$, we get

$$t = 1.6 \times 0.01\,\text{s} = 0.016\,\text{s}.$$

Hence, the correct answer is Option D.

We are told that the self-induced emf of the coil is $$e = 25\ \text{V}$$. By definition of self-induction, the induced emf in an inductor of inductance $$L$$ is related to the time rate of change of current by the formula $$e = L\dfrac{dI}{dt}$$. First we determine $$L$$ from this relation.

The current rises uniformly from $$I_1 = 10\ \text{A}$$ to $$I_2 = 25\ \text{A}$$ in a time interval $$\Delta t = 1\ \text{s}$$, so the rate of change of current is

$$\dfrac{dI}{dt} = \dfrac{I_2 - I_1}{\Delta t} = \dfrac{25\ \text{A} - 10\ \text{A}}{1\ \text{s}} = 15\ \text{A s}^{-1}.$$

Substituting $$e = 25\ \text{V}$$ and $$\dfrac{dI}{dt}=15\ \text{A s}^{-1}$$ into $$e = L\dfrac{dI}{dt}$$, we obtain

$$L = \dfrac{e}{\dfrac{dI}{dt}} = \dfrac{25\ \text{V}}{15\ \text{A s}^{-1}} = \dfrac{5}{3}\ \text{H} = 1.666\dots\ \text{H}.$$

Now the magnetic energy stored in an inductor carrying current $$I$$ is given by the formula $$U = \dfrac12 L I^2$$. Therefore, the change in energy when the current changes from $$I_1$$ to $$I_2$$ is

$$\Delta U = \dfrac12 L\left(I_2^2 - I_1^2\right).$$

Substituting $$L = \dfrac{5}{3}\ \text{H}$$, $$I_2 = 25\ \text{A}$$ and $$I_1 = 10\ \text{A}$$, we get

$$\Delta U = \dfrac12 \left(\dfrac{5}{3}\right)\Bigl(25^2 - 10^2\Bigr) = \dfrac12 \left(\dfrac{5}{3}\right)(625 - 100) = \left(\dfrac{5}{6}\right)(525).$$

Simplifying the arithmetic,

$$\left(\dfrac{5}{6}\right)(525) = \dfrac{5 \times 525}{6} = \dfrac{2625}{6} = 437.5\ \text{J}.$$

Hence, the correct answer is Option C.

For an air-core solenoid the self-inductance is given by the well-known relation

$$L = \mu_0\,n^2\,A\,l,$$

where  $$\mu_0$$  is the permeability of free space,  $$n$$  is the number of turns per unit length,  $$A$$  is the cross-sectional area of the solenoid, and  $$l$$  (here written as $$L$$ in the question) is the actual length of the solenoid.

Now, the total number of turns is fixed and equal to $$N$$. Therefore the turns per unit length are

$$n=\frac{N}{L}.$$

Substituting this value of $$n$$ into the inductance formula, we obtain

$$L_{\text{ind}} = \mu_0\left(\frac{N}{L}\right)^2 A\,L.$$

We first square the fraction:

$$\left(\frac{N}{L}\right)^2 = \frac{N^2}{L^2}.$$

Putting this back in, we have

$$L_{\text{ind}} = \mu_0\;\frac{N^2}{L^2}\;A\;L.$$

Next we cancel one power of $$L$$ in the denominator with the single $$L$$ in the numerator:

$$L_{\text{ind}} = \mu_0\;N^2\;A\;\frac{1}{L}.$$

All the quantities $$\mu_0,\,N,$$ and $$A$$ are constants under the given conditions, so the inductance is inversely proportional to the length of the solenoid:

$$L_{\text{ind}} \propto \frac{1}{L}.$$

Hence, the correct answer is Option 3.

We have a circular conducting loop whose plane is kept perpendicular to the magnetic field, so the magnetic flux through the loop is simply the product of the field magnitude and the loop area.

The area is given as $$A = 3.5 \times 10^{-2}\,{\rm m^2}$$ and the magnetic field varies with time as $$B(t) = 0.4\,{\rm T}\,\sin(50\pi t)$$, where the time $$t$$ is in seconds. Hence the magnetic flux $$\Phi(t)$$ through the loop is

$$\Phi(t) \;=\; B(t)\,A \;=\; \bigl(0.4\sin 50\pi t\bigr)\times \bigl(3.5\times10^{-2}\bigr) \;=\; 0.014\,\sin 50\pi t\;{\rm Wb}.$$

According to Faraday’s law of electromagnetic induction, the induced emf $$\mathcal{E}(t)$$ in the loop is related to the time-rate of change of flux by

$$\mathcal{E}(t) = -\dfrac{d\Phi(t)}{dt}.$$

The loop has resistance $$R = 10\,\Omega$$. Using Ohm’s law, the induced current is

$$I(t) = \dfrac{\mathcal{E}(t)}{R} = -\dfrac{1}{R}\,\dfrac{d\Phi(t)}{dt}.$$

The total charge $$Q$$ that passes through any cross-section of the wire from time $$t=0$$ to $$t = 10\,{\rm ms} = 0.01\,{\rm s}$$ is obtained by integrating the current:

$$Q = \int_{0}^{0.01} I(t)\,dt = -\dfrac{1}{R}\int_{0}^{0.01}\dfrac{d\Phi(t)}{dt}\,dt = -\dfrac{1}{R}\,\bigl[\Phi(t)\bigr]_{0}^{0.01}.$$

Carrying out the substitution of the flux values, we first evaluate $$\Phi(t)$$ at both limits:

At $$t = 0$$:

$$\Phi(0) = 0.014\,\sin\bigl(50\pi \times 0\bigr) = 0.014\,\sin 0 = 0.$$

At $$t = 0.01\,{\rm s}$$:

$$50\pi t = 50\pi \times 0.01 = 0.5\pi = \dfrac{\pi}{2},$$

so $$\sin\!\Bigl(\dfrac{\pi}{2}\Bigr) = 1,$$ and

$$\Phi(0.01) = 0.014 \times 1 = 0.014\,{\rm Wb}.$$

The change in flux is therefore

$$\Delta\Phi = \Phi(0.01) - \Phi(0) = 0.014 - 0 = 0.014\,{\rm Wb}.$$

Substituting this into the expression for charge, we get

$$Q = -\dfrac{1}{10}\,\bigl[\,0.014 - 0\,\bigr] = -\dfrac{0.014}{10} = -0.0014\,{\rm C}.$$

The negative sign only indicates the direction of flow; the magnitude of charge that actually passes through the loop is

$$|Q| = 0.0014\,{\rm C} = 1.4 \times 10^{-3}\,{\rm C} = 1.4\,{\rm mC}.$$

Hence, the correct answer is Option A.

Initially, the capacitor is charged by the battery and carries a potential difference of $$V_0 = 10\text{ V}$$.

Its capacitance is given as $$C = 0.2\;\mu\text{F} = 0.2 \times 10^{-6}\text{ F} = 2 \times 10^{-7}\text{ F}$$.

The charge stored at this instant is obtained from the basic relation $$q_0 = C V_0.$$

Substituting the values, we have $$q_0 = (2 \times 10^{-7}\text{ F})(10\text{ V}) = 2 \times 10^{-6}\text{ C}.$$

After the battery is disconnected and the capacitor is connected to an ideal inductor, the system becomes an ideal LC circuit. For such a circuit, the charge on the capacitor varies harmonically as

$$q(t) = q_0 \cos(\omega t),$$

where $$\omega$$ is the natural angular frequency of the LC circuit. The expression for $$\omega$$ is

$$\omega = \frac{1}{\sqrt{LC}}.$$

The given inductance is $$L = 0.5\;\text{mH} = 0.5 \times 10^{-3}\text{ H} = 5 \times 10^{-4}\text{ H}.$$

Now computing the product $$LC$$:

$$LC = (5 \times 10^{-4}\text{ H})(2 \times 10^{-7}\text{ F}) = 1 \times 10^{-10}\text{ H}\cdot\text{F}.$$

Taking the square root,

$$\sqrt{LC} = \sqrt{1 \times 10^{-10}} = 1 \times 10^{-5}.$$

Hence,

$$\omega = \frac{1}{1 \times 10^{-5}}\; \text{rad s}^{-1} = 1 \times 10^{5}\; \text{rad s}^{-1}.$$

The potential difference across the capacitor at any time is

$$V_C(t) = \frac{q(t)}{C} = \frac{q_0}{C}\cos(\omega t) = V_0 \cos(\omega t).$$

We are told that at the required instant the capacitor voltage has fallen to $$V_C = 5\text{ V}.$$ Using the above relation,

$$V_0 \cos(\omega t) = 5.$$

Substituting $$V_0 = 10\text{ V},$$

$$10 \cos(\omega t) = 5 \;\;\Longrightarrow\;\; \cos(\omega t) = 0.5.$$

We recognise $$\cos^{-1}(0.5) = \frac{\pi}{3},$$ so we can take

$$\omega t = \frac{\pi}{3}\;(= 60^\circ)$$

for the first time when the voltage becomes 5 V.

The instantaneous current in an LC circuit is the time-derivative of the charge:

$$i(t) = \frac{dq}{dt} = -q_0\,\omega \sin(\omega t).$$

We need only the magnitude, so

$$|i| = q_0 \omega |\sin(\omega t)|.$$

The sine corresponding to $$\omega t = \pi/3$$ is

$$\sin\!\left(\frac{\pi}{3}\right) = \frac{\sqrt3}{2} \approx 0.866.$$

Now substitute each known quantity:

First, the product $$q_0 \omega$$ is

$$q_0 \omega = (2 \times 10^{-6}\text{ C})(1 \times 10^{5}\text{ s}^{-1}) = 2 \times 10^{-1}\text{ A} = 0.20\text{ A}.$$

Then

$$|i| = 0.20\text{ A} \times 0.866 = 0.1732\text{ A}.$$

Rounding to two significant figures,

$$|i| \approx 0.17\text{ A}.$$

Hence, the correct answer is Option A.

We begin by recalling the statement of Faraday’s law of electromagnetic induction. It says that the e.m.f. $$\mathcal{E}$$ induced in a coil is equal to the negative time-rate of change of magnetic flux through the coil. Symbolically, the magnitude is

$$\mathcal{E}=n\left|\frac{d\Phi}{dt}\right|,$$

where $$n$$ is the number of turns and $$\Phi$$ is the magnetic flux linked with one turn.

Now we write the expression for the instantaneous flux. The coil of cross-sectional area $$A$$ is kept in a uniform magnetic field $$B$$. If the normal to the plane of the coil makes an angle $$\theta$$ with the magnetic field, the flux through one turn is

$$\Phi = BA\cos\theta.$$

The coil is rotating with a constant angular velocity $$\omega$$, so the angle changes with time as

$$\theta = \omega t.$$

Substituting this time-dependent angle into the flux expression, we get

$$\Phi(t)=BA\cos(\omega t).$$

We now differentiate this flux with respect to time to find the induced e.m.f. Using the derivative of the cosine function, $$\dfrac{d}{dt}\cos(\omega t) = -\omega\sin(\omega t),$$ we obtain

$$\frac{d\Phi}{dt}=BA\left[-\omega\sin(\omega t)\right] = -BA\omega\sin(\omega t).$$

Putting this into Faraday’s law and multiplying by the number of turns $$n$$, the instantaneous e.m.f. becomes

$$\mathcal{E}(t)=n\left|\,\frac{d\Phi}{dt}\,\right| = n\left| -BA\omega\sin(\omega t) \right| = nBA\omega\left| \sin(\omega t) \right|.$$

The function $$|\sin(\omega t)|$$ reaches its maximum value of $$1$$ whenever $$\sin(\omega t)=\pm1$$, that is, when $$\omega t=\dfrac{\pi}{2},\dfrac{3\pi}{2},\ldots$$ Therefore the maximum possible value of $$\mathcal{E}(t)$$ is

$$\mathcal{E}_{\text{max}} = nBA\omega \times 1 = nBA\omega.$$

Thus the maximum induced e.m.f. is directly proportional to the number of turns, the magnetic field, the area of the coil, and the angular velocity.

Hence, the correct answer is Option C.

For a single circular coil of radius $$R$$ carrying a steady current $$I$$, the magnetic field at its centre (and in the neighbourhood of the centre) is given by the well-known expression

$$ B \;=\; \frac{\mu_0 I}{2R}. $$

In our problem the large coil is fixed, so this field $$B$$ is constant in both magnitude and direction. Its direction is perpendicular to the plane of the coils, pointing along the common axis that passes through their centres.

The smaller coil of radius $$r$$ is placed at the same centre and in the same initial plane. The area enclosed by this secondary coil is

$$ A \;=\; \pi r^{2}. $$

Now the smaller coil starts rotating with a constant angular velocity $$\omega$$ about a diameter that lies in the plane of the coils. Because this diameter is in the plane, the axis of rotation is perpendicular to the magnetic field direction. Consequently, as the coil rotates, the angle between the magnetic field $$\vec B$$ and the area-vector (normal to the plane of the small coil) changes with time.

If we take $$t=0$$ as the instant when the area-vector of the smaller coil is parallel to $$\vec B$$, then after a time $$t$$ the angle between them becomes

$$ \theta = \omega t. $$

The magnetic flux linked with the small coil at this instant is, by definition,

$$ \Phi(t) \;=\; B A \cos \theta. $$

Substituting the explicit expressions for $$B$$, $$A$$ and $$\theta$$, we obtain

$$ \Phi(t) \;=\; \Bigl(\frac{\mu_0 I}{2R}\Bigr)\,(\pi r^{2}) \,\cos(\omega t). $$

According to Faraday’s law of electromagnetic induction, the induced emf $$\mathcal E$$ in the small coil equals the negative time-rate of change of this flux, i.e.

$$ \mathcal E(t) \;=\; -\,\frac{d\Phi(t)}{dt}. $$

Differentiating the flux expression term by term, we have

$$ \frac{d\Phi(t)}{dt} = \Bigl(\frac{\mu_0 I}{2R}\Bigr)\,(\pi r^{2}) \,\frac{d}{dt}\bigl[\cos(\omega t)\bigr]. $$

The derivative of the cosine function is

$$ \frac{d}{dt}\bigl[\cos(\omega t)\bigr] = -\,\omega \sin(\omega t). $$

Substituting this result, we get

$$ \frac{d\Phi(t)}{dt} = -\,\Bigl(\frac{\mu_0 I}{2R}\Bigr)\,(\pi r^{2}) \,\omega \sin(\omega t). $$

Therefore the induced emf becomes

$$ \mathcal E(t) = -\Bigl[-\,\Bigl(\frac{\mu_0 I}{2R}\Bigr)\,(\pi r^{2}) \,\omega \sin(\omega t)\Bigr] = \Bigl(\frac{\mu_0 I}{2R}\Bigr)\,(\pi r^{2}) \,\omega \sin(\omega t). $$

Re-arranging the factors for clarity, we write

$$ \boxed{\mathcal E(t) \;=\; \frac{\mu_0 I}{2R}\,\omega\,\pi r^{2}\,\sin(\omega t)}. $$

This expression matches Option C.

Hence, the correct answer is Option C.

We begin by recalling the magnetic field at the centre of a single circular loop of radius $$R$$ carrying a current $$I$$. The well-known expression is

$$B = \frac{\mu_0 I}{2R}.$$

In the present problem the outer loop has radius $$b$$ and carries an alternating current $$I = I_0 \cos(\omega t)$$. Substituting $$R = b$$ and the given time-dependent current into the formula, the magnetic field produced by the large loop at its own centre is

$$B(t)=\frac{\mu_0 I_0}{2b}\,\cos(\omega t).$$

The small loop of radius $$a$$ is placed exactly at this centre and lies in the same plane, so all lines of the magnetic field pass perpendicularly through the small loop. Therefore the field can be considered uniform over the area of the small loop, provided $$a \ll b$$ (which is stated by “a much larger loop”).

The area of the inner loop is

$$A = \pi a^2.$$

The magnetic flux $$\Phi$$ through the small loop equals the product of the field and the area (because the field is perpendicular to the plane of the loop and uniform over it):

$$\Phi(t) = B(t) \, A = \left(\frac{\mu_0 I_0}{2b}\,\cos(\omega t)\right)\,(\pi a^2).$$

So we have

$$\Phi(t) = \frac{\pi \mu_0 I_0 a^2}{2b}\,\cos(\omega t).$$

Now, according to Faraday’s law of electromagnetic induction, the induced emf $$\mathcal{E}$$ in the small loop is

$$\mathcal{E} = -\frac{d\Phi}{dt}.$$

Taking the time derivative of the flux, we differentiate term by term. The constant factors $$\frac{\pi \mu_0 I_0 a^2}{2b}$$ remain in front, while the derivative of $$\cos(\omega t)$$ is $$-\,\omega\sin(\omega t)$$. Thus

$$\mathcal{E} = -\left(\frac{\pi \mu_0 I_0 a^2}{2b}\right)\left(-\,\omega\sin(\omega t)\right).$$

The two minus signs cancel, leaving

$$\mathcal{E} = \frac{\pi \mu_0 I_0 a^2}{2b}\,\omega\,\sin(\omega t).$$

This emf is the magnitude of the induced voltage in the small loop; its sign (direction) is given by Lenz’s law, but the options list only the magnitude and its time dependence. Comparing our expression with the given alternatives, we see it matches exactly with Option D:

$$\mathcal{E}(t)=\frac{\pi \mu_0 I_0}{2}\cdot\frac{a^2}{b}\,\omega\,\sin(\omega t).$$

Hence, the correct answer is Option D.

$$i = \frac{1}{R} \left| \frac{d\Phi}{dt} \right|$$

$$q = \int i \, dt = \frac{1}{R} \int d\Phi = \frac{\Delta \Phi}{R}$$

$$\Delta \Phi = q \times R$$

$$q = \text{Area of Triangle} = \frac{1}{2} \times \text{Base} \times \text{Height}$$

$$q = \frac{1}{2} \times 0.5 \times 10 = 2.5\ \text{C}$$

$$\Delta \Phi = 2.5\ \text{C} \times 100\ \Omega$$

$$\Delta \Phi = 250\ \text{Wb}$$

We treat the aeroplane as a set of straight metallic conductors moving with uniform velocity in the earth’s magnetic field. Whenever a conductor of length $$L$$ moves with velocity $$\vec v$$ through a magnetic field $$\vec B$$, an emf

$$\varepsilon \;=\; (\vec v \times \vec B)\cdot\vec L$$

is produced, where $$\vec L$$ is a vector from one end of the conductor to the other. The magnitude is therefore

$$\vert\varepsilon\vert = v\,B_\perp\,L = vBL\sin\phi,$$

$$\phi$$ being the angle between $$\vec v$$ and $$\vec B$$.

The data given are

$$\begin{aligned} B &= 5\times10^{-5}\,\text{T},\\ \sin\theta &= \frac23 \;(\text{dip}),\\ v &= 240\ \text{m s}^{-1},\\ \text{height} &= 5\ \text{m},\\ \text{wing span} &= 15\ \text{m}. \end{aligned}$$

Because the declination is $$\approx 0^\circ$$, the horizontal component of $$\vec B$$ is due north. We choose axes

$$\hat i :$$ East $$,\qquad \hat j :$$ North $$,\qquad \hat k :$$ Up $$.$$

With dip $$\theta$$, the magnetic field is

$$\vec B = B\cos\theta\,\hat j - B\sin\theta\,\hat k.$$

Using $$\sin\theta=\dfrac23,\; \cos\theta=\sqrt{1-\sin^2\theta}=\sqrt{1-\dfrac49}= \sqrt{\dfrac59}= \dfrac{\sqrt5}{3},$$ we have

$$\begin{aligned} B_H &= B\cos\theta = 5\times10^{-5}\times\frac{\sqrt5}{3}\ \text{T},\\ B_V &= B\sin\theta = 5\times10^{-5}\times\frac23\ \text{T}. \end{aligned}$$

Numerically

$$\begin{aligned} B_H &= 5\times10^{-5}\times0.745355\;=\;3.72678\times10^{-5}\ \text{T},\\[2mm] B_V &= 5\times10^{-5}\times0.666667\;=\;3.33333\times10^{-5}\ \text{T}. \end{aligned}$$

The velocity of the aircraft is due east, $$\vec v = v\,\hat i$$. Hence

$$\vec v\times\vec B = v\$$, $$\hat i \times \bigl(B_H\hat j - B_V\hat k\bigr) = vB_H(\hat i\times\hat j) - vB_V(\hat i\times\hat k) = vB_H\$$, $$\hat k - vB_V\$$, $$\hat j.$$

Thus

$$\vec v\times\vec B = vB_H\,\hat k \;-\; vB_V\,\hat j.$$

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Emf between lower and upper surfaces (height 5 m)

The length vector from the lower surface to the upper surface is $$\vec L_B = 5\,\hat k\ \text{m}.$$ Therefore

$$\varepsilon_B = (\vec v\times\vec B)\cdot\vec L_B = \bigl(vB_H\,\hat k - vB_V\,\hat j\bigr)\cdot (5\,\hat k) = vB_H\times5.$$

Substituting numbers,

$$\begin{aligned} \varepsilon_B &= 240\,(3.72678\times10^{-5})\times5 \\[2mm] &= 240\times5\times3.72678\times10^{-5}\\[2mm] &= 1200\times3.72678\times10^{-5}\\[2mm] &= 4.47214\times10^{-2}\ \text{V}\\[2mm] &\approx 0.045\ \text{V}\;=\;45\ \text{mV}. \end{aligned}$$

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Emf between the wing tips (span 15 m)

The wings run north-south, the left wing tip being to the north of the pilot. We take the length vector from the right tip (south) to the left tip (north):

$$\vec L_W = 15\,\hat j\ \text{m}.$$

Hence

$$\varepsilon_W = (\vec v\times\vec B)\cdot\vec L_W = \bigl(vB_H\,\hat k - vB_V\,\hat j\bigr)\cdot(15\,\hat j) = -\,vB_V\times15.$$

Its magnitude is

$$\vert\varepsilon_W\vert = vB_V\times15 = 240\,(3.33333\times10^{-5})\times15.$$

Step by step:

$$\begin{aligned} 240\times15 &= 3600,\\ 3600\times3.33333\times10^{-5} &= 1.20000\times10^{-1}\ \text{V}\\ &= 0.12\ \text{V}\\ &= 120\ \text{mV}. \end{aligned}$$

The negative sign in $$\varepsilon_W$$ shows that the potential decreases from north to south, i.e. the left (north) wing tip is at a higher potential than the right (south) wing tip.

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We have therefore obtained

$$V_B \approx 45\$$ mV $$,\qquad V_W \approx 120\$$ mV $$,$$

with the left side of the pilot (north wing tip) at the higher voltage.

Hence, the correct answer is Option D.

First we note that a changing magnetic flux through the loop induces an emf. The quantitative relation is given by Faraday’s law of electromagnetic induction, stated as

$$|\mathcal E| = \left|\frac{d\Phi}{dt}\right|,$$

where $$\mathcal E$$ is the induced emf and $$\Phi$$ is the magnetic flux through the loop.

The area of the given circular loop is

$$A = \pi r^{2}.$$

The magnetic field is time-dependent: $$B(t)=B_{0}e^{-t/\tau}.$$ Hence the flux through the loop at any instant $$t$$ is

$$\Phi(t)=B(t)\,A =B_{0}e^{-t/\tau}\,\pi r^{2}.$$

We now differentiate this flux with respect to time in order to find the emf:

$$\frac{d\Phi}{dt} =\pi r^{2}B_{0}\,\frac{d}{dt}\!\left(e^{-t/\tau}\right).$$

Using the elementary derivative

$$\frac{d}{dt}\!\left(e^{-t/\tau}\right)=-\frac{1}{\tau}e^{-t/\tau},$$

we get

$$\frac{d\Phi}{dt} =-\pi r^{2}B_{0}\,\frac{1}{\tau}e^{-t/\tau}.$$

Taking the magnitude (sign is irrelevant for power and heat), the induced emf becomes

$$\mathcal E(t)=\left|\frac{d\Phi}{dt}\right| =\frac{\pi r^{2}B_{0}}{\tau}\,e^{-t/\tau}.$$

The loop has resistance $$R$$, so by Ohm’s law the induced current is

$$I(t)=\frac{\mathcal E(t)}{R} =\frac{\pi r^{2}B_{0}}{\tau R}\,e^{-t/\tau}.$$

The instantaneous power dissipated as heat in the resistor is given by

$$P(t)=I^{2}(t)\,R.$$

Substituting the expression for $$I(t)$$, we obtain

$$P(t)=\left(\frac{\pi r^{2}B_{0}}{\tau R}\,e^{-t/\tau}\right)^{2}R =\frac{\pi^{2}r^{4}B_{0}^{2}}{\tau^{2}R}\,e^{-2t/\tau}.$$

To find the total heat $$Q$$ generated from $$t=0$$ to $$t\rightarrow\infty$$, we integrate this power over time:

$$Q=\int_{0}^{\infty}P(t)\,dt =\frac{\pi^{2}r^{4}B_{0}^{2}}{\tau^{2}R} \int_{0}^{\infty}e^{-2t/\tau}\,dt.$$

The standard integral

$$\int_{0}^{\infty}e^{-kt}\,dt=\frac{1}{k}$$

gives, with $$k=\frac{2}{\tau},$$

$$\int_{0}^{\infty}e^{-2t/\tau}\,dt =\frac{1}{2/\tau} =\frac{\tau}{2}.$$

Substituting this back, we have

$$Q=\frac{\pi^{2}r^{4}B_{0}^{2}}{\tau^{2}R}\,\frac{\tau}{2} =\frac{\pi^{2}r^{4}B_{0}^{2}}{2\tau R}.$$

Hence, the correct answer is Option B.

We are given that the current in a coil changes from 5 A to 2 A in 0.1 seconds, and an average voltage of 50 V is produced. We need to find the self-inductance of the coil.

The induced electromotive force (EMF) in a coil due to self-inductance is given by Faraday's law. The formula for the magnitude of the average induced EMF is:

$$ \varepsilon = L \left| \frac{\Delta I}{\Delta t} \right| $$

where:

• $$\varepsilon$$ is the average voltage (50 V),
• $$L$$ is the self-inductance (which we need to find),
• $$\Delta I$$ is the change in current,
• $$\Delta t$$ is the time interval (0.1 s).

First, calculate the change in current ($$\Delta I$$). The initial current is 5 A, and the final current is 2 A. Since the current decreases, the change is:

$$ \Delta I = I_{\text{final}} - I_{\text{initial}} = 2 \, \text{A} - 5 \, \text{A} = -3 \, \text{A} $$

The magnitude of the change in current is $$ |\Delta I| = |-3| = 3 \, \text{A} $$.

Now, the time interval $$\Delta t = 0.1 \, \text{s}$$. So, the magnitude of the rate of change of current is:

$$ \left| \frac{\Delta I}{\Delta t} \right| = \frac{3 \, \text{A}}{0.1 \, \text{s}} = 30 \, \text{A/s} $$

Substitute the known values into the formula:

$$ 50 = L \times 30 $$

Solve for $$L$$:

$$ L = \frac{50}{30} = \frac{5}{3} \approx 1.6667 \, \text{H} $$

Rounding 1.6667 H to two decimal places gives 1.67 H.

Now, comparing with the options:

• A. 1.67 H
• B. 6 H
• C. 3 H
• D. 0.67 H

Hence, the correct answer is Option A.

Before $$t = 0$$, the key $$K_1$$ has been closed for a long time and $$K_2$$ is open. In a DC circuit at steady state, an inductor acts as a short circuit.

$$I_0 = \frac{V}{R} = \frac{15\ \text{V}}{150\ \Omega} = 0.1\ \text{A} = 100\ \text{mA}$$

At $$t = 0$$, $$K_1$$ is opened (removing the battery) and $$K_2$$ is closed. The inductor now discharges its stored energy through the resistor $$R$$. This is a standard current decay in an RL circuit. The equation for decaying current is $$I(t) = I_0 e^{-\frac{t}{\tau}}$$

$$\tau = \frac{L}{R} = \frac{0.03\ \text{H}}{150\ \Omega} = 2 \times 10^{-4}\ \text{s} = 0.2\ \text{ms}$$

$$I(1\ \text{ms}) = 100\ \text{mA} \times e^{-\frac{1\ \text{ms}}{0.2\ \text{ms}}}$$

$$I = 100 \times e^{-5}\ \text{mA}$$

$$I = \frac{100}{150}\ \text{mA}$$

$$I = \frac{2}{3}\ \text{mA} \approx 0.666...\ \text{mA}$$

$$I \approx 0.67\ \text{mA}$$

The magnetic field at a distance $$r$$ from a long straight wire carrying current $$I$$ is $$B = \frac{\mu_0 I}{2\pi r}$$

The e.m.f. induced in a conductor of length $$l$$ moving with velocity $$v$$ perpendicular to a magnetic field $$B$$ is given by $$e = Blv$$.

$$e_1 = B_1 lv = \left( \frac{\mu_0 I}{2\pi x} \right) av$$ (left arm)

$$e_2 = B_2 lv = \left( \frac{\mu_0 I}{2\pi (x+a)} \right) av$$ (right arm)

$$\varepsilon = e_1 - e_2 = \frac{\mu_0 I av}{2\pi} \left[ \frac{1}{x} - \frac{1}{x+a} \right]$$

$$\varepsilon = (2 \times 10^{-7}) \times (1) \times (0.1) \times (10) \times \left[ \frac{1}{0.1} - \frac{1}{0.1+0.1} \right]$$

$$\varepsilon = 10^{-6}\text{ V} = 1\text{ }\mu\text{V}$$

When point 'C' is connected to point 'B', the battery is completely disconnected from the loop. The circuit becomes a closed, source-free series $$RL$$ circuit containing only the resistor $$R$$ and the inductor $$L$$. Applying Kirchhoff's Voltage Law ($$\text{KVL}$$) around this closed loop at any time $$t \ge 0$$: $$V_R + V_L = 0$$

($$V_R$$ is the voltage across the resistance and $$V_L$$ is the voltage across the inductor).

$$V_R = -V_L$$

$$\frac{V_R}{V_L} = -1$$

Since this relationship holds true by $$\text{KVL}$$ at all time instances $$t \ge 0$$, it is independent of the specific time value. Therefore, at $$t = \frac{L}{R}$$, the ratio remains exactly $$-1$$.

$$\oint \vec{E} \cdot d\vec{l} = -\frac{d\Phi_B}{dt}$$

Case 1: Inside the Magnetic Field Region ($$r \le R$$)

$$\Phi_B = B \cdot (\pi r^2)$$

$$E(2\pi r) = \frac{d}{dt} \left( B \cdot \pi r^2 \right) = \pi r^2 \frac{dB}{dt}$$

$$E(2\pi r) = \pi r^2 \cdot k$$

$$E = \frac{k}{2} \cdot r \implies E \propto r$$

Thus, inside the region, the induced electric field increases linearly with distance $$r$$, starting from zero at the center.

Case 2: Outside the Magnetic Field Region ($$r > R$$)

$$\Phi_B = B \cdot (\pi R^2)$$

$$E(2\pi r) = \frac{d}{dt} \left( B \cdot \pi R^2 \right) = \pi R^2 \frac{dB}{dt}$$

$$E(2\pi r) = \pi R^2 \cdot k$$

$$E = \frac{k R^2}{2} \cdot \frac{1}{r} \implies E \propto \frac{1}{r}$$

Thus, outside the region, the induced electric field decreases hyperbolically as $$\frac{1}{r}$$.

Hence, the graph in option (A) correctly represents this.

To find the induced electromotive force (e.m.f.) in the coil, we use Faraday's law of electromagnetic induction. The law states that the induced e.m.f. is given by the formula:

$$ \text{e.m.f.} = -N \frac{d\phi}{dt} $$

where $$ N $$ is the number of turns in the coil, and $$ \frac{d\phi}{dt} $$ is the rate of change of magnetic flux through one turn. The negative sign indicates direction (Lenz's law), but for magnitude, we take the absolute value.

The magnetic flux $$ \phi $$ for one turn is $$ \phi = B \cdot A $$, where $$ B $$ is the magnetic field strength and $$ A $$ is the area perpendicular to the field. The coil's axis is parallel to the magnetic field, meaning the field is perpendicular to the coil's face, so this formula applies directly.

Given:

• Number of turns, $$ N = 1000 $$
• Face area, $$ A = 4 \text{cm}^2 $$

Convert area to square meters (since magnetic field is in Wb m$$^{-2}$$):

$$ 4 \text{cm}^2 = 4 \times 10^{-4} \text{m}^2 \quad \text{(because } 1 \text{cm}^2 = 10^{-4} \text{m}^2\text{)} $$

The magnetic field decreases by $$ 10^{-2} \text{Wb m}^{-2} $$ in $$ 0.01 \text{s} $$. So, the change in magnetic field $$ \Delta B = -10^{-2} \text{Wb m}^{-2} $$ (negative for decrease), and the time interval $$ \Delta t = 0.01 \text{s} $$.

The rate of change of magnetic field is:

$$ \frac{\Delta B}{\Delta t} = \frac{-10^{-2}}{0.01} = \frac{-0.01}{0.01} = -1 \text{Wb m}^{-2} \text{s}^{-1} $$

The magnitude of this rate is $$ \left| \frac{\Delta B}{\Delta t} \right| = 1 \text{Wb m}^{-2} \text{s}^{-1} $$.

For one turn, the rate of change of flux is:

$$ \frac{\Delta \phi}{\Delta t} = A \cdot \frac{\Delta B}{\Delta t} $$

Substituting the values:

$$ \frac{\Delta \phi}{\Delta t} = (4 \times 10^{-4}) \times (-1) = -4 \times 10^{-4} \text{Wb/s} $$

The magnitude is $$ \left| \frac{\Delta \phi}{\Delta t} \right| = 4 \times 10^{-4} \text{Wb/s} $$.

For the entire coil with $$ N $$ turns, the magnitude of the induced e.m.f. is:

$$ |\text{e.m.f.}| = N \times \left| \frac{\Delta \phi}{\Delta t} \right| = 1000 \times (4 \times 10^{-4}) $$

Calculate step by step:

$$ 1000 \times 4 \times 10^{-4} = 1000 \times 0.0004 = 0.4 \text{V} $$

Convert volts to millivolts (1 V = 1000 mV):

$$ 0.4 \text{V} = 0.4 \times 1000 \text{mV} = 400 \text{mV} $$

Alternatively, using the formula directly for magnitude:

$$ |\text{e.m.f.}| = N \cdot A \cdot \left| \frac{\Delta B}{\Delta t} \right| = 1000 \times (4 \times 10^{-4}) \times 1 = 0.4 \text{V} = 400 \text{mV} $$

Comparing with the options:

• A. 400 mV
• B. 200 mV
• C. 4 mV
• D. 0.4 mV

Hence, the correct answer is Option A.

We are given a sinusoidal voltage $$ V(t) = 100 \sin(500t) $$ applied across a pure inductance of $$ L = 0.02 \, \text{H} $$. We need to find the current through the coil.

For a pure inductor, the relationship between voltage and current is given by:

$$ V(t) = L \frac{di}{dt} $$

Substituting the given values:

$$ 100 \sin(500t) = 0.02 \cdot \frac{di}{dt} $$

Now, solve for $$ \frac{di}{dt} $$:

$$ \frac{di}{dt} = \frac{100 \sin(500t)}{0.02} $$

Calculate the numerical value:

$$ \frac{100}{0.02} = 5000 $$

So,

$$ \frac{di}{dt} = 5000 \sin(500t) $$

To find the current $$ i(t) $$, integrate both sides with respect to $$ t $$:

$$ i(t) = \int 5000 \sin(500t) \, dt $$

The integral of $$ \sin(500t) $$ is $$ -\frac{\cos(500t)}{500} $$, because the derivative of $$ \cos(500t) $$ is $$ -500 \sin(500t) $$, so the antiderivative of $$ \sin(500t) $$ is $$ -\frac{\cos(500t)}{500} $$. Therefore:

$$ i(t) = 5000 \cdot \left( -\frac{\cos(500t)}{500} \right) + C $$

where $$ C $$ is the constant of integration.

Simplify the expression:

$$ i(t) = -\frac{5000}{500} \cos(500t) + C $$

$$ i(t) = -10 \cos(500t) + C $$

In AC circuits with pure inductance, the average current over a full cycle is zero, and there is no DC component. Therefore, we can assume the constant $$ C = 0 $$. Additionally, the current in an inductor lags the voltage by $$ 90^\circ $$ (or $$ \frac{\pi}{2} $$ radians). Since the voltage is a sine function, the current should be a negative cosine function, which matches the form above without a constant term.

Thus, the current is:

$$ i(t) = -10 \cos(500t) $$

Comparing with the options:

A. $$ 10 \cos(500t) $$

B. $$ -10 \cos(500t) $$

C. $$ 10 \sin(500t) $$

D. $$ -10 \sin(500t) $$

Our solution matches option B.

Hence, the correct answer is Option B.

When a conductor rotates in a magnetic field, the motional e.m.f. ($$d\varepsilon$$) induced in a small element $$dr$$ at a distance $$r$$ from the axis of rotation is:

$$d\varepsilon = Bv \, dr = B(\omega r) \, dr$$

Since the rod is attached to a string of length $$2l$$, its ends are at distances $$r_1 = 2l$$ and $$r_2 = 2l + l = 3l$$ from the fixed pivot point.

$$\varepsilon = \int_{2l}^{3l} B\omega r \, dr$$

$$\varepsilon = B\omega \left[ \frac{r^2}{2} \right]_{2l}^{3l}$$

$$\varepsilon = \frac{B\omega}{2} \left[ (3l)^2 - (2l)^2 \right]$$

$$\varepsilon = \frac{5B\omega l^2}{2}$$

We begin by writing every given quantity in SI units.

The radius of the small loop is $$a = 0.3\ \text{cm}=0.3\times 10^{-2}\ \text{m}=3.0\times 10^{-3}\ \text{m}.$$

The radius of the large loop is $$R = 20\ \text{cm}=20\times 10^{-2}\ \text{m}=0.20\ \text{m}.$$

The distance between the two loop-centres measured along the common axis is $$d = 15\ \text{cm}=15\times 10^{-2}\ \text{m}=0.15\ \text{m}.$$

The current in the small loop is $$I = 2.0\ \text{A}.$$

Because the small loop is much smaller than every other length involved $$(a \ll d \ \text{and}\ a \ll R),$$ it is quite accurate to regard the small loop as a magnetic dipole situated on the axis. Its magnetic dipole moment is, by definition,

$$m = I\,\pi a^{2}.$$

A magnetic dipole produces an azimuthal vector potential $$A_{\phi}$$ at a point whose cylindrical co-ordinates relative to the dipole are $$(\rho ,\phi ,z)$$:

$$A_{\phi} = \frac{\mu_{0}}{4\pi}\, \frac{m\,\rho}{\left(\rho^{2}+z^{2}\right)^{3/2}}.$$

For the large loop we have $$\rho = R$$ and $$z = d$$, so its entire circumference lies on the circle where the above potential has the value

$$A_{\phi}(R,d) = \frac{\mu_{0}}{4\pi}\; \frac{m\,R}{\left(R^{2}+d^{2}\right)^{3/2}}.$$

The magnetic flux linked with the large loop equals the circulation of the vector potential around the loop (Stokes’ theorem):

$$\Phi = \oint \mathbf{A}\cdot d\mathbf{l} = 2\pi R\,A_{\phi}(R,d).$$

Substituting the expression for $$A_{\phi}$$ just written, we get

$$\Phi = 2\pi R \left( \frac{\mu_{0}}{4\pi}\; \frac{m\,R}{\left(R^{2}+d^{2}\right)^{3/2}} \right) = \frac{\mu_{0}\,m\,R^{2}}{2\left(R^{2}+d^{2}\right)^{3/2}}.$$

Now we replace $$m$$ by $$I\,\pi a^{2}$$:

$$\Phi = \frac{\mu_{0}\,I\,\pi a^{2}\,R^{2}} {2\left(R^{2}+d^{2}\right)^{3/2}}.$$

This expression contains only known quantities, so we substitute the numerical values one by one.

First, the constant $$\mu_{0}=4\pi \times 10^{-7}\ \text{H m}^{-1}.$$ Half of this is $$\frac{\mu_{0}}{2}=2\pi \times 10^{-7}\ \text{H m}^{-1}.$$

The factor $$\pi a^{2}$$ is

$$\pi a^{2}=\pi\,(3.0\times 10^{-3})^{2} =\pi\,(9.0\times 10^{-6}) =9\pi\times 10^{-6}\ \text{m}^{2}.$$

Multiplying by $$R^{2}$$ gives

$$\pi a^{2}R^{2}=\left(9\pi\times 10^{-6}\right)\left(0.20\right)^{2} =\left(9\pi\times 10^{-6}\right)\left(0.040\right) =0.36\pi\times 10^{-6}=3.6\pi\times 10^{-7}\ \text{m}^{4}.$$

Next we incorporate the current $$I=2\ \text{A}$$:

$$I\,\pi a^{2}R^{2}=2\times 3.6\pi\times 10^{-7} =7.2\pi\times 10^{-7}\ \text{A m}^{4}.$$

Multiplying by $$\mu_{0}/2$$ gives the whole numerator:

$$\left(\frac{\mu_{0}}{2}\right) \left(I\,\pi a^{2}R^{2}\right) =\left(2\pi\times 10^{-7}\right) \left(7.2\pi\times 10^{-7}\right) =14.4\pi^{2}\times 10^{-14}\ \text{Wb m}^{-1}.$$

Since $$\pi^{2}\approx 9.8696$$, the numerical value becomes

$$14.4\pi^{2}\times 10^{-14} =14.4\times 9.8696\times 10^{-14} \approx 142.1\times 10^{-14} =1.421\times 10^{-12}\ \text{Wb m}^{-1}.$$

We now turn to the denominator:

$$R^{2}+d^{2}=0.20^{2}+0.15^{2}=0.040+0.0225=0.0625,$$

and thus

$$\left(R^{2}+d^{2}\right)^{3/2} =(0.0625)^{3/2} =\sqrt{0.0625}\times 0.0625 =0.25\times 0.0625 =0.015625.$$

Finally, the flux is

$$\Phi =\frac{1.421\times 10^{-12}} {0.015625} \approx 9.09\times 10^{-11}\ \text{weber}.$$

Keeping only two significant figures,

$$\Phi\approx 9.1\times 10^{-11}\ \text{Wb}.$$

Comparing with the options supplied, this value corresponds to Option C.

Hence, the correct answer is Option C.

The induced emf $$V(t)$$ in coil Y is related to the rate of change of current $$I(t)$$ in coil X by $$V(t) = -M \frac{dI(t)}{dt}$$, where $$M$$ is the mutual inductance between the two coils. This implies that the slope of the $$I(t)$$ vs $$t$$ graph is proportional to $$-V(t)$$.

In the first part of the graph, $$V(t)$$ is positive. Since $$\frac{dI(t)}{dt} = -\frac{V(t)}{M}$$, a positive $$V(t)$$ corresponds to a negative slope in the $$I(t)$$ graph. At the end of this interval, $$V(t) = 0$$, which means the slope of the $$I(t)$$ graph must become zero (the curve reaches a local minimum).

In the second part of the graph, $$V(t)$$ is a constant negative value. $$\frac{dI(t)}{dt} = -\frac{(-\text{constant})}{M} = \text{positive constant}$$

A constant positive slope indicates that $$I(t)$$ must be a straight line inclined at an acute angle to the time axis.

Graph (A) correctly represents this.