Thermal Physics
Thermal Physics tests heat-transfer calculations, specific and latent heat, the ideal gas equation and the first law of thermodynamics.
Heat and phase change
Q = mcΔθ for a temperature change; Q = mL for a change of state at constant temperature. In a lagged mixture, heat lost by hot bodies equals heat gained by cold bodies. Specific heat capacity is energy per kilogram per kelvin; specific latent heat is energy per kilogram at the change of state.
Gases
pV = nRT with T in kelvin and R = 8.31 J mol⁻¹ K⁻¹. Combined law: p₁V₁/T₁ = p₂V₂/T₂. Kinetic theory assumes molecules are point masses in random motion, collisions are elastic and intermolecular forces are negligible except during collisions; absolute temperature is proportional to mean kinetic energy.
First law
Use ΔU = Q − W, where Q is heat added to the system and W is work done by the system. Adiabatic: Q = 0. Isothermal: ΔU = 0 for an ideal gas.
Worked example
Energy needed to convert 0.20 kg of ice at 0 °C to water at 20 °C: melt: Q₁ = mLf = 0.20 × 3.34 × 10⁵ = 6.68 × 10⁴ J; warm: Q₂ = mcΔθ = 0.20 × 4200 × 20 = 1.68 × 10⁴ J; total = 8.36 × 10⁴ J.
Traps
Substituting Celsius into pV = nRT; forgetting the latent-heat term when a state change occurs; using the specific heat of water for ice or steam; muddling the sign of work in the first law; ignoring heat losses to the surroundings.