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AP Physics 2 Unit 1: Thermodynamics

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Unit 1: Thermodynamics

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Question 1

A $P V$ diagram shows a gas undergoing a rectangular cycle (two isobaric and two isochoric processes). In which segment does the gas do the most positive work?

A.The isochoric (constant volume) expansion at high pressure.B.The isobaric compression at low pressure.C.The isobaric expansion at high pressure.D.The isochoric compression at low volume.

Question 2

A heat engine absorbs $Q_{H} = 800 J$ from a hot reservoir and does $W = 200 J$ of work per cycle. What is the change in entropy of the cold reservoir per cycle if it is at $T_{C} = 400 K$ ?

Δ S_{C} = 0.5 J/K

Δ S_{C} = 1.5 J/K

Δ S_{C} = 2.0 J/K

Δ S_{C} = - 1.5 J/K

Question 3

Which of the following processes is impossible according to the second law of thermodynamics?

A.A heat engine converts 40% of absorbed heat into work and rejects the rest.B.Heat flows from a hot object to a cold object when they are in thermal contact.C.A refrigerator removes heat from a cold interior and expels heat to a warm room, using electrical work.D.A cyclic heat engine converts all absorbed heat into work with no heat rejected to a cold reservoir.

Question 4

An ideal gas expands adiabatically and does $W = 500 J$ of work on its surroundings. What is the change in internal energy of the gas?

Δ U = + 500 J

Δ U = 0

Δ U = - 500 J

D.Cannot be determined without knowing the pressure.

Question 5

An ideal gas undergoes the following four-step cycle: (1) isobaric expansion at $P_{H}$ , (2) isochoric pressure drop to $P_{L}$ , (3) isobaric compression at $P_{L}$ , (4) isochoric pressure rise back to $P_{H}$ . If the volume changes from $V_{1}$ to $V_{2} > V_{1}$ , what is the net work done by the gas per cycle?

W_{net} = P_{H} V_{2} - P_{L} V_{1}

W_{net} = (P_{H} - P_{L}) (V_{1} + V_{2})

W_{net} = (P_{H} - P_{L}) (V_{2} - V_{1})

W_{net} = P_{H} (V_{2} - V_{1}) - P_{L} (V_{2} - V_{1}) /2

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Question 6

Which of the following best describes the zeroth law of thermodynamics?

A.Energy cannot be created or destroyed in an isolated system.B.If two systems are each in thermal equilibrium with a third system, they are in thermal equilibrium with each other.C.Heat flows spontaneously from cold objects to hot objects.D.The entropy of a perfect crystal at absolute zero is zero.

Question 7

A gas undergoes an isothermal process. Which of the following is true about the change in internal energy $Δ U$ of an ideal gas during this process?

Δ U > 0

because work is done on the gas.B.

Δ U < 0

because the gas must release heat to maintain constant temperature.C.

Δ U = 0

because internal energy of an ideal gas depends only on temperature.D.

Δ U = Q

because no work is done during an isothermal process.

Question 8

Two identical samples of an ideal gas start at the same state $(P_{0}, V_{0}, T_{0})$ . Sample A undergoes an adiabatic expansion to volume $2 V_{0}$ . Sample B undergoes an isothermal expansion to volume $2 V_{0}$ . Which sample ends at a higher pressure, and why?

A.Sample A, because adiabatic processes conserve energy better.B.Sample B, because isothermal expansion keeps temperature constant while adiabatic expansion causes cooling.C.Both samples end at the same pressure, because they expand to the same volume.D.Sample A, because adiabatic expansion curves are steeper, reaching higher pressures.

Question 9

One mole of an ideal monatomic gas undergoes an isobaric expansion from volume $V_{1}$ to $2 V_{1}$ at pressure $P$ . Which expression correctly gives the heat $Q$ added to the gas? (Use $R$ for the ideal gas constant.)

Q = P V_{1}

Q = \frac{3}{2} P V_{1}

Q = \frac{5}{2} P V_{1}

Q = 2 P V_{1}

Question 10

A heat engine operates between a hot reservoir at $T_{H} = 600 K$ and a cold reservoir at $T_{C} = 300 K$ . What is the maximum (Carnot) efficiency of this engine?

25%

50%

75%

100%

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Question 11

An ideal gas is held at constant volume while heat $Q$ is added to it. Which expression correctly gives the change in internal energy $Δ U$ ?

Δ U = Q - W

, where

W = P Δ V > 0

Δ U = Q

Δ U = - Q

Δ U = 0

, because temperature does not change at constant volume.

Question 12

The average kinetic energy of molecules in an ideal monatomic gas at temperature $T$ (in kelvin) is given by:

\overset{ˉ}{K} = k T

\overset{ˉ}{K} = \frac{1}{2} k T

\overset{ˉ}{K} = \frac{3}{2} k T

\overset{ˉ}{K} = \frac{3}{2} n R T

AP Physics 2 Unit 1: Thermodynamics — Free with Answer Explanations | Test Practice Hub