SAT Subject Physics Practice Question 320: Answer and Explanation
16. Two 1 liter containers contain 1 mole each of the same gas at the same temperature. Container A is allowed to expand until the temperature of gas A is reduced by half. Container B is compressed until the temperature of gas B doubles. Both gases are then mixed together in a 2 liter container. Which of the following statements about the gases is correct?
A. Both gases gain heat.
B. Both gases lose heat.
C. Gas A loses heat to gas B.
D. Gas B loses heat to gas A.
E. The equilibrium temperature of the gases is exactly one half the original starting temperature of the gases.
Correct Answer: D
The correct answer is (D). According to the first law of thermodynamics, the internal heat of the system remains the same unless work is done or heat is added to the system. Neither of those two things occurs, so the system of two objects, one hot and one cold, transfers heat between itself with the hot object losing to the cold object until thermal equilibrium is reached.