Thermodynamics and Kinetic Theory
Core concepts
Understand temperature as a measure of average kinetic energy, internal energy, specific heat capacity (Q = mcΔT), latent heat (Q = mL), and the gas laws. Know the first law of thermodynamics, ΔU = Q − W (or Q = ΔU + W depending on sign convention), and that the second law forbids a perfect heat engine and gives direction to spontaneous processes. Kinetic theory links pressure to molecular collisions and gives average kinetic energy proportional to absolute temperature.
Common traps
- Temperature vs heat: temperature is intensive; heat is energy transferred. Adding heat does not always raise temperature (phase change).
- Absolute temperature must be used in gas-law and kinetic-theory calculations; 0 °C is not zero energy.
- Sign conventions in the first law - state which convention you use.
- Efficiency of a heat engine is (Qin − Qout)/Qin and can never be 100%.
- Confusing conduction, convection and radiation mechanisms.
Worked example
How much energy to heat 0.5 kg of water from 20 °C to 60 °C? Q = mcΔT = 0.5 × 4200 × 40 = 84,000 J = 84 kJ. If instead the water boiled at 100 °C, you would use Q = mL with L ≈ 2.26 × 10⁶ J/kg.
Rule of thumb
Identify the process first - heating a single phase, a phase change, or a gas-law change - then select the matching equation.