Thermal Physics: Heat Transfer and Changes of State
Thermal physics: energy transfer and change of state
Temperature measures the average kinetic energy of particles; thermal energy is the total internal energy transferred due to a temperature difference. Two objects in contact reach thermal equilibrium when they have the same temperature and no net heat flows. Heat transfers by conduction (particle collisions and free electrons in metals), convection (density changes in fluids) and radiation (infrared electromagnetic waves, needing no medium). Dark, matt surfaces absorb and emit radiation better than shiny, light surfaces; larger surface areas increase the rate.
Specific heat capacity c is the energy needed to raise 1 kg of a substance by 1 °C: Q = mcΔT. Specific latent heat L is the energy needed to change state of 1 kg without a temperature change: Q = mL. During melting or boiling, supplied energy breaks bonds between particles, so temperature stays constant. Thermal expansion occurs because particles vibrate more and occupy slightly more space; bimetallic strips bend because metals expand by different amounts.
Traps: using the final temperature instead of the temperature change ΔT; mixing joules and kilojoules; forgetting that temperature is constant during a state change; assuming all supplied energy is absorbed rather than lost to surroundings; and confusing specific heat capacity with latent heat.
Worked example: Energy to heat 2.0 kg of water from 20 °C to 50 °C (c = 4200 J/kg°C): Q = mcΔT = 2.0 × 4200 × 30 = 252 000 J = 252 kJ. If a 500 W heater supplies this, time t = E/P = 252 000/500 = 504 s, ignoring losses.