§ 14.1 Thermal equilibrium
Key ideas
- Net thermal energy flows from a region of higher temperature to one of lower temperature.
- Two objects are in thermal equilibrium when they are at the same temperature, with no net energy flow between them.
- Temperature, not the amount of energy stored, decides the direction of flow: a small hot spark is hotter than a warm bath, though the bath holds far more energy.
Watch out: equal temperature means equilibrium, not equal internal energy. A bath and a cup at 40 °C are in equilibrium even though the bath stores vastly more thermal energy.
§ 14.2 Temperature scales
Key ideas
- A physical property that varies with temperature (a length, a resistance, a pressure) can be used to measure it.
- The thermodynamic (kelvin) scale does not depend on any particular substance; its zero, absolute zero, is the point of minimum particle energy.
- A change of 1 K equals a change of 1 °C; only the zero of the scale differs.
Equations
T / K = θ / °C + 273.15kelvin from Celsius (273 in most calculations)K
Watch out: every gas-law and kinetic-theory formula needs kelvin. Substituting Celsius for T (especially small or negative values) is the most common A2 thermal error.
§ 14.3 Specific heat capacity and specific latent heat
Key ideas
- Specific heat capacity c is the energy needed to raise the temperature of 1 kg of a substance by 1 K.
- Specific latent heat L is the energy to change the state of 1 kg at constant temperature: fusion for melting, vaporisation for boiling.
- Latent heat of vaporisation is much larger than that of fusion: molecules must be fully separated, not merely freed from the lattice.
Equations
E = m c Δθenergy to change temperatureJ
E = m Lenergy to change state at constant temperatureJ
Fig. 1 · A heating curve: along the slopes the temperature climbs (E = mcΔθ); on the flat plateaus the energy goes entirely into changing state (E = mL).
Watch out: on the flat plateaus the energy is supplied but the temperature does not rise: use E = mL there, never mcΔθ with Δθ = 0.