Thermal Physics
This section covers Thermal Physics. The exam tests the molecular model of matter, internal energy, specific heat capacity, latent heat, and ideal gases.
Core rules
- Temperature measures the average kinetic energy of particles; internal energy is the total random kinetic and potential energy of the particles.
- Heating without a phase change uses Q = mcΔT. Heating during a phase change at constant temperature uses Q = mL, where L is specific latent heat.
- For an ideal gas, pV = nRT, with T in kelvin. Pressure results from particle collisions with walls; average kinetic energy is proportional to absolute temperature.
- Thermal equilibrium means no net energy transfer by heating because objects have the same temperature.
- The kinetic model assumes particles are in random motion, collisions are elastic, and intermolecular forces are negligible except during collisions.
Worked example
How much energy is needed to heat 0.50 kg of water from 20 °C to 80 °C? Use c = 4200 J kg⁻¹ K⁻¹. Temperature change ΔT = 80 − 20 = 60 K. Q = mcΔT = 0.50 × 4200 × 60 = 126000 J = 1.3 × 10⁵ J. The same temperature interval in kelvin is also 60 K, so no conversion of a temperature difference is needed.
Traps
Use kelvin in pV = nRT; Celsius is acceptable only for a temperature difference. Do not use c during a phase change; use L instead. Temperature stays constant during melting or boiling even though energy is being supplied. Do not confuse internal energy with temperature: two different masses at the same temperature can have different internal energies.