Entropy and the Second Law of Thermodynamics AP Physics 2 MCQ

Entropy and the Second Law of Thermodynamics AP Physics 2 MCQ – Exam Style Questions etc.

Entropy and the Second Law of Thermodynamics AP Physics 2 MCQ

Unit 9: Thermodynamics

Weightage : 15–18%

AP Physics 2 Exam Style Questions – All Topics

Question

The figure shows a sample of an ideal gas enclosed within a cylinder that has been fitted with a movable piston. The piston and the sides of the cylinder are thermally insulated. The bottom of the cylinder is in contact with a thermal reservoir. The gas is compressed isothermally while thermal equilibrium is maintained with the reservoir. The piston, gas, and reservoir form a closed, isolated system. True statements about entropy for this reversible situation include which of the following? Select two answers.

A The entropy of the gas decreases because work is done on the gas while its temperature remains constant.

B The entropy of the reservoir increases because thermal energy is transferred to it from the gas.

C The system has a net increase in entropy as a result of the process.

D The change in entropy of the system depends on how quickly the isothermal compression occurs.

▶️Answer/Explanation

Ans:A , B

The temperature is constant, so the change in entropy is proportional to the change in thermal energy. Since thermal energy is being transferred from the gas to the reservoir, the thermal energy of the gas, and hence the entropy of the gas, is decreasing.
The temperature is constant, so the change in entropy is proportional to the change in thermal energy. Since thermal energy is being transferred from the gas to the reservoir, the thermal energy of the reservoir, and hence the entropy, is increasing.

Question

Which of the following describes the microscopic difference between the change in entropy of a gas during reversible processes (which are theoretical) and irreversible processes (which actually occur)?

▶️Answer/Explanation

Ans:C

Entropy is a measure of how many different microscopic ways a system may be arranged to give the same measurable macroscopic system quantities, such as volume, pressure, and temperature. Thus, entropy is often described as a measure of the disorder of a system.

Question

A very hot gas composed of equal numbers of positive and negative ions is in a closed, thermally insulated container and is in thermal equilibrium at temperature \(T_0\). The figure above represents the initial distribution of the ions. A strong uniform electric field directed toward the bottom of the page is now created in and around the container, and the gas is allowed to reach a new thermal equilibrium at temperature \(T_1\).

Which of the following best represents a possible distribution of the ions at temperature \(T_1\) ?

▶️Answer/Explanation

Ans:C

Question

The electric field is now turned off, and the gas is allowed to reach a final equilibrium temperature \(T_2\). How does \(T_2\) compare to \(T_1\), and why?

(A) \(T_2\) is less, because the electric potential energy added by the field to separate the ions is gone once the field is turned off.

(B) \(T_2\) is less, because pairs of positive and negative ions bond to become molecules once the field is turned off, and energy is required to create the bonds.

(D) \(T_2\) is greater, because the electric potential energy of the separated ions is converted to kinetic energy as they mix and collide.</

▶️Answer/Explanation

Ans:D

Question

Two identical samples of helium gas are in identical sealed flasks at room temperature. One flask is just sitting in the room, and the other is inside a large, insulated vacuum container. Both flasks are opened and the samples are released, so the helium in one flask spreads throughout the room and the helium in the other flask spreads in the sealed container, as shown in the figures. Which of the following is true of the change in entropy that occurs in each case when the flasks are opened?

A The entropy increases in both cases because each system is now in a more disordered state.

B The entropy increases as the helium mixes with air but not as it spreads out into the sealed container.

C There is no change in entropy for either case because the temperature of the helium does not change.

D The entropy changes in both cases, but there is not sufficient information to determine whether it increases or decreases.

▶️Answer/Explanation

Ans:A

The helium that mixed with the air and the helium that spread out into the container are both more disordered than they were in their original states.

Entropy and the Second Law of Thermodynamics AP Physics 2 MCQ