Thermal Physics and Energy Transfer
Thermal physics and energy transfer covers temperature measurement, heat quantities, the gas laws and the first law of thermodynamics. Questions often require careful sign handling and unit conversion.
Heat and temperature
Temperature measures average kinetic energy; heat is energy transferred because of a temperature difference. Q = mcΔθ for heating and Q = mL for a change of state, where no temperature change occurs.
Gas laws
Boyle's law: pV = constant at constant T. Charles's law: V/T = constant at constant p. Pressure law: p/T = constant at constant V. Combine these as pV/T = constant, or pV = nRT with T in kelvin. Convert °C to K by adding 273.
Energy transfer
Conduction needs a temperature gradient and mobile free electrons or lattice vibrations; convection needs a fluid and a density change; radiation needs no medium and follows Stefan's law (P ∝ T⁴ for a black body). The first law is ΔU = Q − W, where W is work done by the gas.
Worked example
An electric heater carries 2.0 A at 12 V for 5.0 minutes in 1.0 kg of water. Energy = VIt = 12 × 2.0 × 300 = 7200 J. Δθ = Q/mc = 7200/(1.0 × 4200) = 1.71 K. If the measured rise is smaller, heat losses to the surroundings explain the difference.
Traps
Using °C in gas-law calculations, forgetting that latent heat changes state without changing temperature, and getting the sign of work in the first law wrong.