A bubble has surface tension S. The ideal gas inside the bubble has ratio of specific heats . The bubble is exposed to the atmosphere and it always retains its spherical shape. When the atmospheric pressure is , the radius of the bubble is found to be r1 and the temperature of the enclosed gas is T1. When the atmospheric pressure is , 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 ?
Correct Answer :
If the surface of the bubble is a perfect heat conductor and the change in atmospheric temperature is negligible, then
If the surface of the bubble is perfect heat insulator, then
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
2. If the surface of the bubble is perfect heat insulator, then
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:
Therefore, the total absolute pressure P of the gas inside the bubble when the atmospheric pressure is Pa is:
For state 1:
For state 2:
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:
Substituting the volume of a sphere :
Simplifying the above equation:
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 , the relationship between pressure P and temperature T is given by:
Substituting into the equation:
Taking both sides to the power of gives:
Equating the initial and final states:
Rearranging terms to find the ratio of temperatures:
Thus, this statement is also correct.
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