The unit in one line
Physics after 1897: the electron, the photon, the atom that only allows certain orbits, the nucleus that pays for a reaction out of its own mass, and the devices and cosmology that followed. 12 of Physics's 50 questions over seven chapters — the largest unit in the subject, and the one most often left until it is too late.
The chapters
| Chapter | What it covers | Read |
|---|---|---|
| Nuclear physics | Charge, size, mass and density of the nucleus, mass defect and binding energy per nucleon, and fission and fusion | 18 min |
| Electron | An electron moving in electric and magnetic fields, Thomson's measurement of e/m, and Millikan's oil-drop experiment | 18 min |
| Photon and the photoelectric effect | The quantum of light, the laws of photoemission, threshold frequency, work function and stopping potential | 16 min |
| Wave–particle duality and X-rays | Bohr's hydrogen atom and its spectral series, de Broglie waves and the uncertainty principle, and X-ray production, properties and Bragg's law | 29 min |
| Radioactivity | Alpha, beta and gamma rays, the decay law, half-life and mean life, carbon dating, and medical uses and hazards | 22 min |
| Solids and semiconductor devices | Energy bands, intrinsic and extrinsic semiconductors, the p–n junction and biasing, the diode as a rectifier, and the five logic gates | 12 min |
| Particle physics and recent trends | Particles and antiparticles, leptons and quarks, the Higgs boson, nanotechnology, and the Big Bang with Hubble's law | 12 min |
A little over two hours in all.
Read in this order
Four of these chapters form one argument and should be read as one: electron, then photon and the photoelectric effect, then wave–particle duality and X-rays. Thomson and Millikan establish the particle whose charge and mass everything else is quoted in; the photoelectric effect forces light to be a particle too; and duality then turns that symmetry round and gives the electron a wavelength, which is exactly what Bohr's quantised orbits need. Read out of order, the duality chapter — the longest note in Physics — reads as a list of unrelated results.
Nuclear physics and radioactivity are the other pair, in that order: binding energy per nucleon explains why a heavy nucleus decays at all, so the decay law lands on prepared ground.
Solids and semiconductor devices and particle physics and recent trends stand alone and can be read first if you want two short, factual chapters to open the unit with. Semiconductors is also the chapter Alternating currents points at for how a diode rectifies.
How it is asked
MEC sets the paper roughly half recall, a third understanding and a fifth application. Recall dominates more here than anywhere else in Physics: constants, discoverers, particle charges, decay rules, truth tables. Understanding is why brightness does not help photoemission, why iron sits at the top of the binding-energy curve, or why a NAND gate is universal. Application is a half-life count, a photon energy from a wavelength, a de Broglie wavelength, or a Bragg angle.
Then practise
Each chapter has a fixed-length chapter test under Practice › Topics, drawn from questions written to that chapter's scope. Twelve marks makes this the unit worth re-testing most.