Heat and Temperature
Heat and Temperature tests the distinction between heat and temperature, specific heat capacity, latent heat, gas laws, thermodynamics and heat transfer by conduction, convection and radiation.
Core rules
- Temperature is a measure of average kinetic energy; heat is energy transferred because of a temperature difference. Convert to kelvin: T(K) = T(°C) + 273.15.
- Heating without a change of state: Q = mcΔT. A change of state at constant temperature uses Q = mL, where L is specific latent heat.
- The gas laws apply to a fixed mass of gas: Boyle’s law pV = constant at constant T; Charles’s law V/T = constant at constant p; Gay-Lussac’s law p/T = constant at constant V. Use kelvin in all gas-law calculations.
- The first law of thermodynamics expresses energy conservation: heat supplied equals change in internal energy plus work done by the gas, Q = ΔU + W.
- Conduction transfers heat by molecular collisions and free electrons in metals; convection transfers heat by fluid movement; radiation transfers heat by electromagnetic waves and needs no medium.
Worked example
0.50 kg of water is heated from 20°C to 80°C. Using c = 4180 J kg⁻¹ K⁻¹, Q = mcΔT = 0.50 × 4180 × 60 = 125 400 J, or 1.25 × 10⁵ J. The temperature change is 60 K, the same numerical change as 60°C. If a 500 W heater supplies this energy with no losses, the time is t = Q/P = 125 400/500 = 251 s, about 4.2 minutes.
Traps
Use kelvin in gas-law equations, not Celsius. A temperature change in °C equals the same change in K. Latent heat calculations have no ΔT during the change of state. Do not confuse heat with temperature. In real experiments, account for heat lost to the surroundings.