Thermal physics: temperature, heat transfer, states of matter, thermal energy
What this domain tests
Thermal physics covers temperature and internal energy, the three ways heat moves, specific heat capacity, changes of state and energy balance in heating and cooling. The exam asks you to compute heat quantities in joules and kilojoules, explain everyday insulation and cooking situations, and interpret heating graphs with plateaus at melting and boiling points.
Core rules
- Temperature and internal energy. Heating raises the average kinetic energy of the particles. T(K) = t(°C) + 273.
- Heat flow goes from the hotter body to the colder one until thermal equilibrium; it never flows spontaneously the other way.
- Three mechanisms: conduction (mainly in solids, through particle collisions; metals conduct well thanks to free electrons), convection (rising warm currents in fluids) and radiation (infrared, needing no medium - that is how the Sun's energy arrives).
- Heating without a change of state: Q = mcΔT, with c in J/(kg·K). For water c = 4200 J/(kg·K).
- Change of state: energy breaks or forms bonds, so temperature stays constant during melting and boiling. Ice melts at 0 °C and water boils at 100 °C at normal pressure.
- Insulation: trapped air, wool, foam and double glazing slow conduction and convection; shiny surfaces reduce radiation.
- Conservation: in an insulated system, the heat lost by the hot body equals the heat gained by the cold one.
Worked example
A 2.0 kW kettle heats 1.5 kg of water from 20 °C to 100 °C. How long should it take if there are no losses?
Q = mcΔT = 4200 × 1.5 × 80 = 504 000 J. t = Q/P = 504 000 / 2000 = 252 s, about 4.2 minutes. A real kettle takes longer, because some energy heats the kettle body and the surrounding air: its efficiency is below 100 %.
Traps that cost marks
- Putting the final temperature into the formula instead of the change ΔT.
- Mixing grams with kilograms, or joules with kilojoules, inside one calculation.
- Claiming the temperature rises while ice is melting - it stays at 0 °C until all the ice has gone.
- Confusing heat with temperature: a bathtub of warm water holds more internal energy than a cup of boiling water.
- Forgetting that a vacuum flask blocks conduction and convection but still allows slow radiation.
- Treating cold as something that flows in; cold is simply less thermal energy.