Chapter 4: Exploring Magnets

When two magnetic poles are brought close to each other they behave in only two possible ways:

• Like poles (N–N or S–S)  ⟶  repel one another.
• Unlike poles (N–S or S–N)  ⟶  attract one another.

We now fill each row of the table using this single rule.

1. N – N
   Same poles ⇒ Repulsion.
2. N – _____ must give attraction, so the second pole has to be the opposite pole, i.e. S.
3. S – N
   Unlike poles ⇒ Attraction.
4. _____ – S must give repulsion, so the first pole must again be S.

The completely filled table is therefore:

Step 1 – Recall the property of a bar magnet
A bar magnet has two poles, one at each end. The magnetic force (ability to attract iron/steel objects) is maximum at the two ends and minimum around the middle (called the magnetic equator).

Step 2 – Interpret the three positions
From Fig. 4.15 in the textbook the three labelled spots are along the length of the magnet:

• Position A : one end (a pole)
• Position B : roughly the middle
• Position C : the other end (the opposite pole)

Step 3 – Predict the number of U-clips
Because attraction is strongest at the poles, $$\text{clips at A (pole)} \approx \text{clips at C (pole)} \gt \text{clips at B (middle)}.$$ Hence the correct observation must show large, roughly equal numbers at A and C and a much smaller number at B.

Step 4 – Match with the options in Table 4.3

Only Option (i) satisfies $$10\,(A)=10\,(C)\;>\;2\,(B).$$

Step 5 – Conclusion
Atharv is most likely to get the distribution shown in Option (i).

Let Reshma label the three identical bars as A, B and C.

1. Keep bar A fixed in one hand. Bring one end of bar B close to one end of bar A and note what happens.

   • If the two ends repel, both A and B must possess like poles — therefore both are magnets. The remaining bar C is the ordinary iron bar and the test is over.
   • If they only attract or show no effect, nothing can yet be concluded, because attraction is possible between a magnet and iron as well as between unlike poles of two magnets.

2. Still holding bar A, repeat the test with bar C.

   • If bar A and bar C repel, then A and C are the two magnets and bar B is iron.
   • If bar A repels neither B nor C, bar A itself is not a magnet; it is the iron bar.

3. To confirm the last case, bring bars B and C close to each other. Since A is iron, the two remaining bars must now repel each other, proving that B and C are the magnets.

Reasoning: Repulsion is a sure test of magnetism. Two unlike objects (magnet & iron or opposite poles of two magnets) always attract, but only two like poles of magnets can repel. Therefore the pair of bars that shows repulsion are the two magnets.

Fact to recall: like magnetic poles repel each other and unlike poles attract each other. Symbolically, we can write the rule as $$ \text{N--N or S--S} \;\longrightarrow\; \text{repulsion}, \qquad \text{N--S} \;\longrightarrow\; \text{attraction}. $$ This single property will help us decide which end of the unknown magnet is North (N) and which is South (S).

1. Collect the materials
   
   • One bar magnet whose poles are already marked (say N and S at the two ends).
   • The bar magnet whose poles are not marked.
   • Thin, non-magnetic thread (to hang the unknown magnet freely).
2. Hang the unmarked magnet
   Tie the thread exactly at the centre of the unmarked magnet and suspend it so that it can rotate without touching anything. Wait until it comes to rest; now both its ends are easy to approach with the test magnet.
3. Test one end using a known pole
   Bring the North pole of the marked magnet very slowly towards one end of the hanging magnet, call this end A. Keep a small gap; do not let the magnets touch.
4. Observe the interaction

5. Mark the poles
   If step 4 shows repulsion, write “N” on end A and “S” on the opposite end B. If step 4 shows attraction, write “S” on end A and “N” on end B.
6. (Optional) Confirm with the other pole
   Repeat the test with the South pole of the marked magnet: it must repel the end you have just labelled “S” and attract the end labelled “N”. This double-check removes any possible doubt.

Thus, by using the repulsion test with a magnet of known polarity, the North and South poles of an otherwise unmarked magnet can be located and permanently labelled.

Step 1 – Make the magnet free to rotate
Find a thin piece of cotton thread and tie it exactly at the middle of the bar magnet. Hang this thread from a stand, a nail, or the branch of a tree so that the magnet can spin in a horizontal plane without touching anything.

Step 2 – Let it settle
Give the hanging magnet a slight twist and then let it come to rest. Because the magnet is free of all other influences except Earth’s magnetic field, it will always finally stop in one definite direction.

Step 3 – Identify the geographical directions
Use any ordinary method you already know to decide which way is the Earth’s geographical north. For example, stand facing the rising Sun in the morning; straight ahead is East, your left-hand side is North. (A compass would do the same job, but the question forbids another magnet.)

Step 4 – Locate the North-seeking end of the bar magnet
The end of the bar magnet that points towards the Earth’s geographical north is called the north-seeking pole (or simply the North pole) of that magnet.
Therefore mark that end with ink or tie a small piece of coloured thread to remember it.

Why does this work? A freely suspended magnet always aligns itself along the Earth’s magnetic field lines: its North pole is attracted towards the Earth’s magnetic South pole, which lies near the geographical North. Thus just observing the resting direction tells us which end is which—without using a second magnet.

Step 1 · Recall what a compass tells us
A magnetic compass is just a small magnet that can turn freely. The end that always settles towards the geographical North is called the north-seeking pole (marked N). The opposite end is the south-seeking pole (marked S).

Step 2 · Recall the rule of attraction and repulsion
Unlike poles attract, like poles repel, so

$$N \;\text{of one magnet}\; \longrightarrow \; S \;\text{of another magnet}$$
$$S \;\text{of one magnet}\; \longrightarrow \; N \;\text{of another magnet}$$

Step 3 · Apply the rule to Earth
The N–marked tip of the compass is really a magnetic north pole. Since that end is pulled towards the geographical North of Earth, the place we call the “geographical North Pole” must be attracting a north pole. Therefore, in magnetic language it must be a south pole.

Similarly, the geographical South Pole must be a magnetic north pole because it attracts the S-marked end of the compass.

Step 4 · Give the final identification

• Geographical North Pole → magnetic south pole of Earth.
• Geographical South Pole → magnetic north pole of Earth.

Thus, by simply watching which way a compass points, we can guess and name the hidden magnetic poles of our planet.

Problem
The mechanic is using an ordinary steel screw-driver. Because the tool is not magnetic, any small steel screw that he tries to place on its tip keeps slipping off and drops to the ground.

Relevant concept from Chapter 4 – “Exploring Magnets”
• Steel (or iron) objects can be turned into temporary magnets.
• One common method is the single-stroke method: rub (stroke) the steel object with one pole of a permanent magnet repeatedly in the same direction. This lines up the tiny magnetic domains inside the steel and the object itself behaves like a magnet.

Step-by-step solution

1. Choose a magnet
   Take a bar magnet whose poles are marked $$N$$ and $$S$$.
2. Place the magnet
   Touch, say, the north pole $$N$$ of the bar magnet to the tip of the screw-driver.
3. Stroke in one direction
   Move the magnet from the tip towards the handle in one continuous motion.
4. Lift and repeat
   After reaching the handle, lift the magnet away, bring it back to the tip and stroke again in the same direction. Repeat the stroking 20 – 30 times.
5. Test the magnetised tip
   Bring the treated tip near a loose steel screw. The screw is attracted and sticks to the tip, showing that the screw-driver has become a temporary magnet.

Why it works
Every stroke forces many tiny magnetic domains inside the steel to point the same way. When most domains line up, the whole screw-driver produces a magnetic field strong enough to attract small iron objects.

Result for the mechanic
The magnetised screw-driver now holds screws firmly while he positions or tightens them, so the screws no longer fall. If the magnetism fades after some time, the stroking process can be repeated.

Step 1 : Recall the basic property of magnets
Like poles (N–N or S–S) repel each other, whereas unlike poles (N–S) attract each other.

Step 2 : Identify the poles that are facing each other
In Fig. 4.16 the lower face of ring X and the upper face of ring Y are held one above the other and the upper ring does not slide down. This can happen only if those two facing faces are like poles. Because :

• If they were unlike poles, there would be attraction and X would stick to Y.
• Since X remains suspended, the two facing faces must repel.

Step 3 : Explain why ring X stays where it is
Let the weight of ring X be $$W$$ (acting vertically downward).
The repulsive magnetic force exerted by ring Y on ring X is $$F_{\text{rep}}$$ (acting upward).
Because the rings are at rest, the two forces balance :

\[F_{\text{rep}} = W\]

Thus the upward magnetic repulsion exactly cancels the downward weight, so X does not move further down.

Step 4 : How to make the two magnets touch without pushing

• Remove either ring from the stand, turn it upside-down and place it back. Now the facing poles are unlike.
• Unlike poles attract, so the magnetic force becomes downward instead of upward.
  Therefore ring X slides down the plastic rod and sticks to ring Y without anyone having to push the magnets.

Alternative (if you do not want to lift a ring) : Bring a third strong bar magnet near the upper face of ring X with its opposite pole facing X. The bar magnet will attract X downward, overcoming the repulsion, and the two rings will come together.

Given information

• There are three identical bar magnets.
• Their free ends are numbered 1, 2, 3, 4, 5 and 6 as in Fig. 4.17 of the textbook (top magnet – ends 1 & 2; right-side vertical magnet – ends 5 & 6; bottom magnet – ends 3 & 4).
• The arrangement forms a ∟-shaped chain: end 2 touches end 5, and end 3 touches end 6.
• Polarity of end 5 is given – it is a $$S$$ (south) pole.

Facts about magnets that we shall use

1. Every bar magnet has exactly two poles: one $$N$$ (north) and the other $$S$$ (south).
2. Like poles repel, unlike poles attract. Therefore, if two ends are sticking together, they must be unlike (one $$N$$, the other $$S$$).

Step 1 – Find the pole at end 2 (touches 5)

End 2 is in direct contact with end 5. Since end 5 is already known to be a $$S$$ pole and the two ends attract each other, end 2 must be the opposite pole, i.e. a $$N$$ pole.

Step 2 – Find the pole at end 1 (other end of the same magnet as 2)

Ends 1 and 2 are the two opposite ends of the same bar magnet (the top one). Hence if end 2 is $$N$$, the far end 1 must be the opposite pole $$S$$.

Step 3 – Find the pole at end 6 (opposite end of the magnet having 5)

Ends 5 and 6 are the two ends of the right-side vertical magnet. Therefore, if end 5 is $$S$$, the opposite end 6 must be $$N$$.

Step 4 – Find the pole at end 3 (touches 6)

End 3 is sticking to end 6. Because end 6 has just been found to be $$N$$, the touching end 3 must be $$S$$ (unlike poles attract).

Step 5 – Find the pole at end 4 (other end of the same magnet as 3)

Ends 3 and 4 belong to the same bottom bar magnet. Since end 3 is $$S$$, the opposite end 4 must be $$N$$.

Summary of the polarities

Thus all unknown poles have been identified consistently using the two basic rules of magnetism.