Thermodynamics and heat engines · Grade 12
Thermodynamics is the physics of heat and work. The first law is conservation of energy for gases: heat supplied either raises internal energy or is used to do work. The second law limits every heat engine: some heat must always be passed to a colder place, so no engine can be 100% efficient.
Key facts
- First law: ΔU = Q − W (Q = heat supplied to the gas, W = work done BY the gas).
- Work done by a gas at constant pressure: W = pΔV (area under a p–V graph).
- Isothermal: ΔU = 0. Adiabatic: Q = 0. Constant volume: W = 0.
- Heat engine: Q_h = W + Q_c; efficiency = W ÷ Q_h.
- Maximum (Carnot) efficiency = 1 − T_c ÷ T_h (kelvin).
- Refrigerator COP = Q_c ÷ W; heat pump COP = Q_h ÷ W. Entropy change ΔS = Q ÷ T at constant temperature.