Heat and Thermal Physics
Heat and Thermal Physics
Heat and Thermal Physics is Section B. It tests the difference between heat and temperature, how energy is transferred, thermal expansion, and the behaviour of gases through the kinetic model.
Core rules and concepts
- Heat and temperature. Heat is energy transferred because of a temperature difference; temperature measures the average kinetic energy of particles. Thermal energy flows from hot to cold until thermal equilibrium.
- Specific heat capacity. Q = mcΔT, where c is the energy needed to raise 1 kg by 1 K. For water, c is about 4200 J kg⁻¹ K⁻¹.
- Latent heat. During melting, boiling or freezing, temperature stays constant while energy changes the arrangement of particles. Q = ml, with l the specific latent heat.
- Thermal expansion. Solids, liquids and gases generally expand when heated because particles vibrate or move more; water behaves anomalously between 0 °C and 4 °C, becoming denser as it warms.
- Gas laws. At constant temperature, pV = constant (Boyle). At constant pressure, V/T = constant (Charles). At constant volume, p/T = constant (pressure law). Temperatures must be in kelvin: T/K = θ/°C + 273.
- Heat transfer. Conduction needs particle collisions and free electrons; convection needs a fluid that expands and becomes less dense; radiation is infrared electromagnetic transfer and needs no medium.
Worked example
How much energy is needed to heat 2.0 kg of water from 20 °C to 70 °C? Take c = 4200 J kg⁻¹ K⁻¹. Reasoning: temperature change ΔT = 70 − 20 = 50 K. Use Q = mcΔT = (2.0)(4200)(50) = 420 000 J = 4.2 × 10⁵ J. A 50 K rise is the same size as a 50 °C rise, so no conversion of the interval is needed.
Traps that cost marks
- Using °C instead of kelvin in gas-law calculations; the temperature must be absolute.
- Forgetting latent heat when a substance changes state, or assuming temperature rises during boiling.
- Mixing up Q = mcΔT and Q = ml.
- Using one mass in a mixture problem: each substance has its own m, c and ΔT.
- Claiming convection occurs in solids, or that radiation needs a medium.