Heat and Thermal Physics
Heat, temperature and internal energy
Temperature measures the average kinetic energy of particles; heat is energy transferred because of a temperature difference. SEC Physics tests specific heat capacity, latent heat, and conduction, convection and radiation.
- Specific heat capacity: E = mcΔT, where ΔT is the temperature change, not the final temperature.
- Latent heat: E = mL at a change of state; temperature stays constant during melting or boiling.
- Conduction transfers energy by particle collisions and free electrons in metals; convection involves density changes in fluids; radiation needs no medium and depends on surface colour and texture.
- Thermal equilibrium means no net heat flow because temperatures are equal.
Worked example
0.50 kg of water is heated from 20 °C to 60 °C. Taking c = 4200 J/(kg °C), find the energy needed.
ΔT = 60 − 20 = 40 °C.
E = mcΔT = 0.50 × 4200 × 40 = 84 000 J (84 kJ).
Reasoning: the temperature change is 40 °C, not 60 °C; the mass must be in kilograms.
Traps
Using the final temperature instead of ΔT; forgetting to convert grams to kilograms; using E = mcΔT during a change of state; ignoring heat losses to surroundings; confusing temperature with heat.