AP Physics Physics: Thermodynamics & Modern Physics 2 — Questions and Answers
Question 1: A Carnot engine operates between a hot reservoir at 600 K and a cold reservoir at 300 K. What is its maximum theoretical efficiency?
- 25%
- 50% (Correct answer)
- 75%
- 100%
Correct answer: 50%
Carnot efficiency = 1 - T_cold/T_hot = 1 - 300/600 = 0.50, or 50%.
Question 2: Which thermodynamic process occurs at constant pressure?
- Isochoric
- Isobaric (Correct answer)
- Isothermal
- Adiabatic
Correct answer: Isobaric
An isobaric process occurs at constant pressure, allowing volume and temperature to change.
Question 3: In the photoelectric effect, increasing the intensity of light while keeping frequency constant will:
- Increase the maximum kinetic energy of ejected electrons
- Increase the number of ejected electrons per second (Correct answer)
- Increase the threshold frequency
- Decrease the work function of the metal
Correct answer: Increase the number of ejected electrons per second
Higher intensity means more photons per second, so more electrons are ejected, but each photon still delivers the same energy.
Question 4: What does the area under a P-V diagram curve represent?
- Change in internal energy
- Work done by or on the gas (Correct answer)
- Heat transferred
- Change in entropy
Correct answer: Work done by or on the gas
The area under a P-V curve equals the work done by the gas during the process W = ∫P dV.
Question 5: An electron transitions from the n=3 to n=1 energy level in a hydrogen atom. This transition produces:
- An absorbed photon in the infrared range
- An emitted photon in the ultraviolet range (Correct answer)
- An emitted photon in the visible range
- An absorbed photon in the visible range
Correct answer: An emitted photon in the ultraviolet range
The n=3 to n=1 transition in hydrogen is part of the Lyman series, which produces ultraviolet photons.
Question 6: A radioactive isotope has a half-life of 10 years. What fraction of the original sample remains after 30 years?
- 1/2
- 1/4
- 1/8 (Correct answer)
- 1/16
Correct answer: 1/8
After 3 half-lives (30 years), the fraction remaining is (1/2)^3 = 1/8.
Question 7: Which statement best describes the Second Law of Thermodynamics?
- Energy is always conserved in any process
- The entropy of an isolated system tends to increase over time (Correct answer)
- Heat always flows from cold to hot objects
- The internal energy of an ideal gas depends only on pressure
Correct answer: The entropy of an isolated system tends to increase over time
The Second Law states that the total entropy of an isolated system cannot decrease over time; natural processes increase entropy.
AP Physics Physics: Thermodynamics & Modern Physics 2
A Carnot engine operates between a hot reservoir at 600 K and a cold reservoir at 300 K.
What is its maximum theoretical efficiency?