Thermodynamics and Modern Physics
Domain 5: Thermodynamics and Modern Physics
This domain tests heat, temperature, the first law, ideal gases, entropy, and introductory modern physics: photons, the photoelectric effect, atomic spectra, radioactivity, and mass-energy. Items often hinge on energy conservation at the photon or nuclear scale.
Core rules
- Heat is energy transferred because of a temperature difference; temperature measures average molecular kinetic energy. Use kelvin in gas-law calculations.
- Specific heat: Q = mcΔT. Latent heat: Q = mL, with no temperature change during a phase change.
- First law: ΔU = Q − W, where W is work done by the system. On a PV diagram, work is area under the curve; a closed cycle has ΔU = 0.
- Ideal gas law: PV = nRT. For a fixed amount of gas, PV/T is constant.
- Heat-engine efficiency e = W/QH; Carnot maximum e = 1 − TC/TH with absolute temperatures.
- Photon energy E = hf = hc/λ. Photoelectric emission occurs when hf exceeds the work function φ; maximum kinetic energy is Kmax = hf − φ.
- Radioactive decay is random; half-life is the time for half the nuclei to decay. Mass-energy: E = mc².
Worked example
Light of wavelength 400 nm strikes a metal with work function 2.2 eV. Photon energy is E = hc/λ. Using hc = 1240 eV·nm, E = 1240/400 = 3.1 eV. The maximum photoelectron kinetic energy is 3.1 − 2.2 = 0.9 eV. Increasing intensity at the same wavelength increases the number of electrons, not their maximum energy; using shorter wavelength increases Kmax.
Traps that cost marks
- Using Celsius in PV = nRT or the Carnot formula.
- Adding latent heat while also changing temperature through the phase change.
- Confusing heat with temperature.
- Saying photon energy depends on intensity rather than frequency.
- Treating half-life as a fixed lifetime for every nucleus.
- Forgetting that mass is converted to energy in nuclear reactions.