A2L Item 089

• Description: Identify the correct assertion regarding the change of pressure in an expanding gas.
• Goal: Reasoning about adiabatic expansion.
• Source: UMPERG
• Keywords: Ideal Gas, Pressure, Thermodynamics

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The question for students:

An ideal gas is allowed to expand slowly. The system is thermally isolated.

Which statement regarding the final pressure is true?

1. P’ < P
2. P’ = P
3. P’ > P
4. Not enough information

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Commentary for teachers:

Answer

(1) For adiabatic expansion, pV^γ is constant. Since the volume increases, the pressure must decrease.

This result can also be reasoned by realizing that the gas won’t expand unless the external pressure on the piston is reduced. The gas expands because the piston moves to equalize the internal and external pressure.

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