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AP Physics 2 study materials
AP Physics 2 study resources

AP Physics 2: Thermodynamics

AP Physics 2 · Work through your exam, one topic at a time.

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Thermodynamics

Core concepts

AP Physics 2 thermodynamics centers on the ideal gas law ($PV=nRT$), the first law ($\Delta U = Q - W$ with $W$ the work done by the gas), and the second law (entropy, heat engines, efficiency). Know the four thermodynamic processes: isothermal ($\Delta U=0$, $Q=W=nRT\ln(V_f/V_i)$), adiabatic ($Q=0$, so $\Delta U=-W$), isobaric ($W=P\Delta V$), and isochoric ($W=0$). Internal energy of a monatomic ideal gas is $U=\tfrac{3}{2}nRT$; for a diatomic gas use $\tfrac{5}{2}nRT$. Kinetic theory links temperature to average molecular kinetic energy: $\bar{K}=\tfrac{3}{2}k_BT$.

Traps to watch

  • Sign of work: many questions define $W$ as work done on the gas, flipping the sign in the first law. Read the convention stated.
  • Assuming a process is isothermal when only the endpoints have equal temperature.
  • Forgetting that no work is done in a rigid container (isochoric), so all heat changes internal energy.
  • Confusing heat and temperature: a large object at low temperature can hold more thermal energy than a small hot one.
  • Carnot efficiency $e=1-T_C/T_H$ uses kelvin temperatures.

Worked example

One mole of monatomic ideal gas expands isothermally at $T=300,\mathrm{K}$ from $V$ to $2V$. Then $\Delta U=0$, so $Q=W=nRT\ln 2\approx (1)(8.31)(300)(0.693)\approx 1.73\times10^3,\mathrm{J}$. All heat added becomes work.

Rule of thumb: for an ideal gas, temperature alone fixes internal energy, so any two states at the same $T$ have the same $U$.