Thermal Physics
Thermal Physics tests the difference between heat and temperature, the Kelvin scale, specific heat capacity, latent heat, thermal expansion, and the three mechanisms of heat transfer. Expect calculations involving energy and phase change, plus explanations using the particle model.
Core rules and concepts
- Temperature measures average kinetic energy of particles; heat is energy transferred because of a temperature difference.
- T(K) = θ(°C) + 273. A temperature change in °C equals the same change in K.
- Specific heat capacity: E = mcΔθ, where c is energy needed to raise 1 kg by 1 °C.
- Latent heat: E = mL. During melting or boiling the temperature stays constant even though energy is being supplied; the energy breaks bonds between particles.
- Thermal expansion occurs because particles vibrate more and occupy slightly more space. Solids expand least, gases most for the same temperature rise.
- Conduction transfers energy by particle collisions and free electrons, mainly in solids. Convection transfers energy by fluid circulation. Radiation transfers energy by infrared waves and needs no medium.
Worked example
Find the energy needed to melt 0.20 kg of ice at 0 °C and warm the melted water to 20 °C. Latent heat of fusion of ice = 3.34 × 10^5 J/kg; specific heat capacity of water = 4200 J/(kg °C). Energy to melt: E₁ = mL = 0.20 × 3.34 × 10^5 = 66 800 J. Energy to warm water: E₂ = mcΔθ = 0.20 × 4200 × 20 = 16 800 J. Total energy = 66 800 + 16 800 = 83 600 J (about 8.4 × 10^4 J). Reasoning: treat melting and heating as separate stages; the temperature does not rise during melting.
Traps that cost marks
- Using °C instead of K in gas-law calculations.
- Forgetting latent heat during a phase change because the temperature is constant.
- Mixing up heat and temperature in explanations.
- Omitting heat losses to surroundings, which makes calculated energy a minimum.
- Using the wrong unit for c; it is J/(kg °C), not J/°C.