Thermal and Kinetic Theory
What this domain tests
Temperature and heat, specific heat capacity, latent heat, thermal expansion, the gas laws and the molecular model that explains pressure and temperature.
Core rules
- Q = mcΔθ for a temperature change; Q = mL for a change of state.
- Kelvin: T/K = θ/°C + 273.
- During melting or boiling, energy is absorbed while temperature stays constant - latent heat breaks bonds rather than raising kinetic energy.
- Boyle's law: pV constant at fixed T. Charles's law: V/T constant at fixed p. Pressure law: p/T constant at fixed V. Combined: p₁V₁/T₁ = p₂V₂/T₂.
- Ideal gas: pV = nRT. Kinetic theory assumes random motion, negligible molecular volume, no intermolecular forces and elastic collisions.
- Heat transfer: conduction (solids, free electrons), convection (fluids, density change), radiation (infrared, needs no medium).
Worked example
How long will a 2000 W kettle take to heat 0.5 kg of water from 20 °C to 100 °C? Q = mcΔθ = 0.5 × 4200 × 80 = 168 000 J. t = Q/P = 168 000/2000 = 84 s. In practice it takes longer because some energy heats the kettle and escapes to the room - an exam answer should say so rather than claim the kettle is perfectly efficient.
Traps that cost marks
- Using °C where the gas law demands kelvin.
- Applying Q = mcΔθ across a change of state.
- Assuming a 2000 W kettle transfers all 2000 W to the water.
- Confusing heat (energy transferred) with temperature.
- Forgetting heat lost to the surroundings in mixing problems.