Thermal Physics (Years 10–11) — Science

Specific heat capacity and latent heat with real calculations, plus how heat travels by conduction, convection and radiation.

Why does a metal spoon in hot soup burn your fingers while the wooden handle stays cool? Why does the sea warm up slowly in summer but the sand scorches your feet by lunchtime? Thermal physics answers both. It is the study of heat — how much energy a substance needs to warm up, how much it needs to melt or boil, and the three ways heat moves from hot to cold.

Why this matters Every heating and cooling system — kettles, radiators, fridges, insulation, even a flask keeping tea warm — is engineered using these ideas. In the exam these are calculation topics, so getting the equations and units right earns method marks reliably.

Specific heat capacity

Different substances need different amounts of energy to heat up. The specific heat capacity (c) is the energy needed to raise the temperature of 1 kg of a substance by 1 °C. The equation is:

E = m c ΔT — energy (J) = mass (kg) × specific heat capacity (J/kg °C) × temperature change (°C)
Water has a very high c of 4200 J/kg °C, which is why it heats and cools slowly — and why it is used in central heating and car radiators.

Substance	c (J/kg °C)
Water	4200
Aluminium	900
Copper	385

The three ways heat is transferred: conduction, convection and radiation.

Latent heat — melting and boiling

When a solid melts or a liquid boils, its temperature stays constant even though heat is still flowing in. That energy is breaking the bonds between particles rather than raising the temperature. The specific latent heat (L) is the energy needed to change the state of 1 kg without a temperature change:

Q = m L — energy (J) = mass (kg) × specific latent heat (J/kg)
Latent heat of fusion is for melting/freezing; latent heat of vaporisation is for boiling/condensing. For water these are 334 000 J/kg and 2 260 000 J/kg.

The three ways heat travels are worth learning precisely. Conduction passes energy through a solid as vibrating particles jostle their neighbours — metals do this well because free electrons carry energy quickly. Convection happens only in fluids (liquids and gases): heated fluid expands, becomes less dense and rises, while cooler fluid sinks to replace it, setting up a circulating convection current. Radiation is infrared electromagnetic waves; dark, matt surfaces are the best emitters and absorbers, while shiny, silver surfaces reflect radiation — which is why a vacuum flask has silvered walls.

During a change of state the temperature stays flat while latent heat flows in.

Worked example (3 marks). How much energy is needed to heat 0.5 kg of water from 20 °C to 80 °C? (c = 4200 J/kg °C.) ΔT = 80 − 20 = 60 °C [1]. E = m c ΔT = 0.5 × 4200 × 60 [1] = 126 000 J = 126 kJ [1].

Common exam mistake: using the final temperature instead of the temperature change (ΔT), or forgetting that temperature stays constant during melting and boiling. Always compute ΔT = final − start.

Memory hook: "heat to warm uses E = mcΔT; heat to change state uses Q = mL." Conduction needs contact (solids), convection needs a flowing fluid, radiation needs no medium at all — it even crosses empty space from the Sun.