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A soft plastic bottle, filled with water of density 1 gm/cc, carries an inverted glass test-tube with some air (ideal gas) trapped as shown in the figure. The test-tube has a mass of 5 gm, and it is made of a thick glass of density 2.5 gm/cc. Initially the bottle is sealed at atmospheric pressure $$p_0 = 10^5$$ Pa so that the volume of the trapped air is $$v_0 = 3.3 cc$$ When the bottle is squeezed from outside at constant temperature, the pressure inside rises and the volume of the trapped air reduces. It is found that the test tube begins to sink at pressure $$p_0 + \triangle p$$ without changing its orientation. At this pressure, the volume of the trapped air is $$v_0 - \triangle v$$.
Let $$\triangle v = X cc$$ and $$\triangle p = Y \times 10^3$$ Pa.

Question 27

The value of Y is ___ .


Correct Answer: 9.00 - 10.10

The mass of the glass test-tube is $$m = 5\,$$g and the density of glass is $$\rho_g = 2.5\,$$g cc-1.

Volume of the glass material:
$$V_g = \frac{m}{\rho_g} = \frac{5}{2.5} = 2 \text{ cc}$$

The trapped air initially occupies $$v_0 = 3.3 \text{ cc}$$. Therefore, at the start, the total volume that excludes water (and hence displaces water) is
$$V_{\text{disp,0}} = V_g + v_0 = 2 + 3.3 = 5.3 \text{ cc}$$

Bouyant force equals weight of the water of this displaced volume. When the test-tube just begins to sink, the buoyant force must equal its own weight (air’s mass is negligible):
$$\rho_{\text{water}} g (V_g + v_0 - \Delta v) = mg$$

Taking $$\rho_{\text{water}} = 1 \text{ g\,cc}^{-1}$$ and cancelling $$g$$ gives
$$V_g + v_0 - \Delta v = m \quad -(1)$$

Substituting the numbers into (1):
$$2 + 3.3 - \Delta v = 5$$
$$\Rightarrow \Delta v = 0.3 \text{ cc}$$

The trapped air behaves as an ideal gas at constant temperature, so
$$p_0 v_0 = (p_0 + \Delta p)\,(v_0 - \Delta v) \quad -(2)$$

With $$p_0 = 10^5 \text{ Pa}$$, $$v_0 = 3.3 \text{ cc}$$ and $$\Delta v = 0.3 \text{ cc}$$, equation (2) becomes
$$10^5 \times 3.3 = (10^5 + \Delta p)\times 3.0$$

Simplifying:
$$10^5 + \Delta p = 10^5 \times \frac{3.3}{3.0} = 1.1 \times 10^5$$
$$\Rightarrow \Delta p = (1.1 - 1.0)\times 10^5 = 0.1 \times 10^5 \text{ Pa} = 1.0 \times 10^4 \text{ Pa}$$

Since $$\Delta p = Y \times 10^3 \text{ Pa}$$,
$$Y = \frac{1.0 \times 10^4}{10^3} = 10$$

Hence, $$Y \approx 10$$, which lies in the accepted range 9.00 - 10.10.

Answer: 10

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