AP Physics Physics: Thermodynamics & Modern Physics 5 — Questions and Answers
Question 1: The mass defect of a nucleus is directly related to its:
- Atomic number
- Neutron number
- Binding energy (Correct answer)
- Radioactive decay rate
Correct answer: Binding energy
Binding energy = mass defect × c²; the mass defect represents the energy needed to completely separate the nucleus into its constituent nucleons.
Question 2: Which of the following correctly describes the relationship between absolute temperature T and the average kinetic energy of molecules in an ideal gas?
- KE_avg = kT
- KE_avg = (3/2)kT (Correct answer)
- KE_avg = (1/2)kT
- KE_avg = 3kT
Correct answer: KE_avg = (3/2)kT
From the kinetic theory of ideal gases, the average translational kinetic energy per molecule is (3/2)k_B T.
Question 3: Alpha particles are deflected significantly by electric and magnetic fields because they:
- Have zero mass
- Are neutral particles
- Carry a charge of +2e (Correct answer)
- Travel at the speed of light
Correct answer: Carry a charge of +2e
Alpha particles consist of 2 protons and 2 neutrons, giving them a charge of +2e, which causes deflection in fields.
Question 4: A refrigerator removes 800 J of heat from a cold interior and expels 1100 J to the warm exterior. How much work does the refrigerator's compressor do?
- 200 J
- 300 J (Correct answer)
- 800 J
- 1100 J
Correct answer: 300 J
By energy conservation: W = Q_hot - Q_cold = 1100 J - 800 J = 300 J of work input required.
Question 5: What happens to the wavelength of a photon when it passes through a gravitational field (gravitational redshift)?
- It decreases as it escapes a gravitational well
- It increases as it escapes a gravitational well (Correct answer)
- It remains constant regardless of gravity
- It becomes zero at the event horizon of a black hole
Correct answer: It increases as it escapes a gravitational well
Photons lose energy climbing out of a gravitational well, decreasing frequency and increasing wavelength — this is gravitational redshift.
Question 6: In nuclear fusion reactions in the sun, four hydrogen nuclei ultimately combine to produce:
- A carbon nucleus and four electrons
- A helium-4 nucleus, two positrons, and two neutrinos (Correct answer)
- A helium-3 nucleus and a gamma ray
- Two deuterium nuclei and two photons
Correct answer: A helium-4 nucleus, two positrons, and two neutrinos
The proton-proton chain net reaction: 4¹H → ⁴He + 2e⁺ + 2ν_e + energy (as gamma rays).
Question 7: A process is thermodynamically reversible if:
- It produces no change in temperature
- It occurs infinitely slowly and the system remains in equilibrium throughout (Correct answer)
- No heat is transferred during the process
- The internal energy of the system remains constant
Correct answer: It occurs infinitely slowly and the system remains in equilibrium throughout
A reversible process is a quasi-static idealization where the system passes through a continuous series of equilibrium states with no entropy generation.
AP Physics Physics: Thermodynamics & Modern Physics 5
The mass defect of a nucleus is directly related to its: