Heat, Temperature and Thermal Physics
What the exam tests
Heat, Temperature and Thermal Physics examines the difference between heat and temperature, thermometric properties, specific heat capacity, latent heat, the three modes of heat transfer and the gas laws. Numerical questions on mixing hot and cold water, melting ice and the ideal gas equation are common.
Core rules
- Temperature measures average kinetic energy; heat is energy transferred because of a temperature difference.
- Q = mcΔT applies within one phase; during a change of state use Q = mL and the temperature stays constant.
- Kelvin = Celsius + 273; gas-law calculations must use kelvin.
- Boyle: pV = constant at constant T; Charles: V/T = constant at constant p; Gay-Lussac: p/T = constant at constant V.
- Combined gas law: p₁V₁/T₁ = p₂V₂/T₂.
- Conduction needs particles and a temperature gradient; convection needs a fluid that can expand; radiation needs no medium and travels at c.
Worked example
How much energy is needed to turn 0.20 kg of ice at 0 °C into water at 20 °C? Take L = 3.34 × 10⁵ J/kg and c = 4.18 × 10³ J/(kg·K). Melting: Q₁ = mL = 0.20 × 3.34 × 10⁵ = 6.68 × 10⁴ J. Heating: Q₂ = mcΔT = 0.20 × 4.18 × 10³ × 20 = 1.67 × 10⁴ J. Total = 8.35 × 10⁴ J. Melting dominates, which is a common exam point.
Traps
- Using Q = mcΔT while a substance is melting or boiling.
- Forgetting to add the two stages in a change-of-state problem.
- Entering °C into a gas-law ratio.
- Assuming a good conductor is a good radiator; shiny surfaces are poor emitters.
- Ignoring heat lost to the container in calorimetry.