Physics quiz question: no-work pressurization

Physics 8A Quiz 12, Spring Semester 2007
Cuesta College, San Luis Obispo, CA

Physics 8A learning goal Q12.4

[3.0 points.] Consider a system with a diatomic ideal gas where no work is done on or by the gas.

(Cf. Young and Freeman, University Physics, 11/e, Problem 17.101(a).)

[Version 1]

Which one of the following choices best describes the heat flow during this process, if the pressure increases?
(A) Heat flows into the system.
(B) No heat is transferred into/out of this system.
(C) Heat flows out of the system.
(D) (None of the above choices (A)-(C), as not enough information is given.)

Correct answer: (A)
If no work is done, there there is no area under this process path on a pV-diagram, thus this must be an isochoric process. From the ideal gas law, for pressure to increase (while volume remains constant), the temperature must increase. Then the first law of thermodynamics states:

+Q_in - W_out = delta(U),

where W_out = 0, delta(U) > 0, and thus +Q_in > 0, and heat was put into the gas.

Student responses:
(A) : 11 students
(B) : 2 students
(C) : 1 student
(D) : 1 student

[Version 2]

Which one of the following choices best describes the heat flow during this process, if the pressure decreases?
(A) Heat flows into the system.
(B) No heat is transferred into/out of this system.
(C) Heat flows out of the system.
(D) (None of the above choices (A)-(C), as not enough information is given.)

Correct answer: (C)
If no work is done, there there is no area under this process path on a pV-diagram, thus this must be an isochoric process. From the ideal gas law, for pressure to decrease (while volume remains constant), the temperature must decrease. Then the first law of thermodynamics states:

+Q_in - W_out = delta(U),

where W_out = 0, delta(U) < 0, and thus +Q_in < 0, and heat was taken out of the gas.

Student responses:
(A) : 2 students
(B) : 2 students
(C) : 5 students
(D) : 0 students