Question Details

A bubble has surface tension S. The ideal gas inside the bubble has ratio of specific heats γ=53. The bubble is exposed to the atmosphere and it always retains its spherical shape. When the atmospheric pressure is Pa1, the radius of the bubble is found to be r1 and the temperature of the enclosed gas is T1. When the atmospheric pressure is Pa2, the radius of the bubble and the temperature of the enclosed gas are r2 and T2, respectively. Which of the following statement(s) is (are) correct ?

Options

A

If the surface of the bubble is a perfect heat insulator, then (r1r2)5=Pa2+2Sr2Pa1+2Sr1.

B

If the surface of the bubble is a perfect heat insulator, then the total internal energy of the bubble including its surface energy does not change with the external atmospheric pressure.

C

If the surface of the bubble is a perfect heat conductor and the change in atmospheric temperature is negligible, then (r1r2)3=Pa2+4Sr2Pa1+4Sr1

D

If the surface of the bubble is perfect heat insulator, then (T2T1)52=Pa2+4Sr2Pa1+4Sr1

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Correct Answer :

Option C

If the surface of the bubble is a perfect heat conductor and the change in atmospheric temperature is negligible, then (r1r2)3=Pa2+4Sr2Pa1+4Sr1

Option D

If the surface of the bubble is perfect heat insulator, then (T2T1)52=Pa2+4Sr2Pa1+4Sr1

Solution :

The correct statements are:
1. If the surface of the bubble is a perfect heat conductor and the change in atmospheric temperature is negligible, then (r1r2)3=Pa2+4Sr2Pa1+4Sr1
2. If the surface of the bubble is perfect heat insulator, then (T2T1)52=Pa2+4Sr2Pa1+4Sr1

Step 1: Pressure inside the bubble
A spherical bubble has two free surfaces (inner and outer), so the excess pressure inside the bubble due to surface tension S and radius r is given by:
ΔP=4Sr
Therefore, the total absolute pressure P of the gas inside the bubble when the atmospheric pressure is Pa is:
P=Pa+4Sr

For state 1:
P1=Pa1+4Sr1

For state 2:
P2=Pa2+4Sr2

Step 2: Analysis when the bubble surface is a perfect heat conductor (Isothermal Process)
If the surface is a perfect heat conductor and the ambient atmospheric temperature remains constant, heat exchange occurs instantaneously, keeping the temperature of the gas inside the bubble constant (T1 = T2).

Using Boyle's Law for an ideal gas at constant temperature:
P1V1=P2V2

Substituting the volume of a sphere V=43πr3:
(Pa1+4Sr1)43πr13=(Pa2+4Sr2)43πr23

Simplifying the above equation:
(r1r2)3=Pa2+4Sr2Pa1+4Sr1
Thus, this statement is correct.

Step 3: Analysis when the bubble surface is a perfect heat insulator (Adiabatic Process)
If the surface is a perfect heat insulator, no heat is exchanged between the enclosed gas and the environment (Q = 0). Thus, the process is adiabatic.

For an adiabatic process of an ideal gas with ratio of specific heats γ=53, the relationship between pressure P and temperature T is given by:
TγP1-γ=constant

Substituting γ=53 into the equation:
T53P1-53=T53P-23=constant

Taking both sides to the power of 32 gives:
T52P=constant

Equating the initial and final states:
T152P1=T252P2

Rearranging terms to find the ratio of temperatures:
(T2T1)52=P2P1=Pa2+4Sr2Pa1+4Sr1
Thus, this statement is also correct.

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