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JEE Advanced 2024 · Mechanical Properties of Fluids

A spherical soap bubble inside an air chamber at pressure P0=105 PaP_0 = 10^{5}\ \text{Pa} has a certain radius so that the excess pressure inside the bubble is ΔP=144 Pa\Delta P = 144\ \text{Pa}. Now, the chamber pressure is reduced to 8P0/278P_0/27 so that the bubble radius and its excess pressure change. In this process, all the temperatures remain unchanged. Assume air to be an ideal gas and the excess pressure ΔP\Delta P in both the cases to be much smaller than the chamber pressure. The new excess pressure ΔP\Delta P in Pa\text{Pa} is ______.

Answer: 9696

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Written solution

Step 1: Identify the given parameters.

The initial chamber pressure is P0=105 PaP_0 = 10^{5}\text{ Pa}, the initial excess pressure is ΔP1=144 Pa\Delta P_1 = 144\text{ Pa}, and the final chamber pressure is P0′=8P027P_0' = \frac{8P_0}{27}.

Step 2: Relate the excess pressure to the bubble radius.

For a spherical soap bubble with two surfaces, the excess pressure is determined by the surface tension σ\sigma and radius rr.

ΔP=4σr\Delta P = \frac{4\sigma}{r}

Step 3: Apply the ideal gas law at constant temperature.

Since the excess pressure is much smaller than the chamber pressure, the pressure inside the bubble is approximately equal to the chamber pressure.

P1V1=P2V2P_1 V_1 = P_2 V_2 P0V1=8P027V2P_0 V_1 = \frac{8P_0}{27} V_2

Step 4: Determine the ratio of the radii.

The volume of a spherical bubble is proportional to the cube of its radius.

V2V1=278\frac{V_2}{V_1} = \frac{27}{8} r2r1=(278)1/3=32\frac{r_2}{r_1} = \left(\frac{27}{8}\right)^{1/3} = \frac{3}{2}

Step 5: Calculate the new excess pressure.

The excess pressure is inversely proportional to the radius of the bubble.

ΔP2ΔP1=r1r2=23\frac{\Delta P_2}{\Delta P_1} = \frac{r_1}{r_2} = \frac{2}{3} ΔP2=144×23=96 Pa\Delta P_2 = 144 \times \frac{2}{3} = 96\text{ Pa}

Answer: 96

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