Thermal Physics and Kinetic Theory
What this domain tests
Thermal Physics and Kinetic Theory tests thermal expansion, calorimetry, heat transfer, gas laws, kinetic theory, degrees of freedom and thermodynamic processes.
Core rules
- Linear expansion ΔL = LαΔT; area and volume expansions use 2α and 3α.
- Heat Q = mcΔT; latent heat Q = mL at constant temperature.
- Ideal gas PV = nRT; average translational KE per molecule = (3/2)kT.
- rms speed v_rms = √(3RT/M) = √(3kT/m).
- Degrees of freedom: monatomic 3, diatomic 5 at ordinary temperature; C_v = (f/2)R.
- Isothermal: ΔU = 0; adiabatic: Q = 0; first law ΔU = Q − W.
Worked example
For oxygen gas (M = 0.032 kg mol⁻¹) at 300 K, v_rms = √(3RT/M) = √(3 × 8.31 × 300/0.032) ≈ √(2.34×10⁵) ≈ 484 m s⁻¹. Doubling the absolute temperature multiplies v_rms by √2, not by 2.
Traps
- Treating heat and temperature as the same quantity.
- Using Celsius instead of kelvin in gas-law calculations.
- Forgetting that latent heat changes phase without changing temperature.
- Mixing up rms speed with average or most probable speed.