Thermal Physics
Temperature, heat and expansion
Distinguish temperature (a measure of average kinetic energy of particles) from internal energy (total kinetic plus potential energy of particles) and heat (energy transferred due to a temperature difference). Know that solids, liquids and gases expand on heating because particles vibrate or move more, and that this can be problematic (railway gaps, bimetallic strips) as well as useful (thermostats). The Celsius and kelvin scales are related by T(K) = θ(°C) + 273.
Specific heat capacity and latent heat
Use E = mcΔθ for heating and E = mL for changes of state. A frequent trap is applying mcΔθ across a phase change; during melting or boiling the temperature stays constant while latent heat is absorbed. Read heating curves carefully: flat sections indicate latent heat. Also note that specific heat capacity is the energy to raise 1 kg by 1 °C.
Conduction, convection and radiation
Conduction transfers energy by particle collisions and free electrons, best in metals. Convection occurs in fluids because warmer, less dense fluid rises. Radiation needs no medium, travels at the speed of light, and is absorbed best by dull black surfaces and reflected by shiny surfaces. Explain insulation in terms of reducing all three, and describe experiments such as comparing black and silver cans.
Worked example
How much energy heats 0.5 kg of water from 20 °C to 80 °C, given c = 4200 J/(kg °C)? E = mcΔθ = 0.5 × 4200 × 60 = 126 000 J = 126 kJ. If a 500 W heater does this, time t = E/P = 126 000/500 = 252 s, ignoring losses. Real apparatus loses energy to surroundings, so measured times are longer and calculated efficiency below 100%.