Main theoretical concepts fundamental to physics
This domain tests whether you can select and combine the core models of Cambridge IAL Physics - mechanics, waves, electricity, fields, quantum and nuclear physics - in unfamiliar structured questions. Examiners reward correct physics statements, equation use with consistent units, and clear reasoning; they penalise formula hunting without a physical argument.
Core rules to hold together
- Units and homogeneity: convert cm, mm, eV and kW h to SI before substitution; both sides of any physical equation must have the same base units.
- Mechanics: use Newton’s second law as a vector equation, and apply conservation of energy and momentum only when their conditions hold; for circular motion the resultant force acts towards the centre.
- Waves: use v = fλ and superpose displacements, not intensities; intensity is proportional to amplitude squared.
- Fields: E = F/q and V = W/q define field and potential; in a uniform field E = V/d; magnetic force is F = BIL sinθ or F = Bqv sinθ.
- Quantum and nuclear: photon energy E = hf = hc/λ; photoelectric equation hf = φ + K_max; nuclear equations conserve nucleon number and charge.
Worked example
A 450 nm photon strikes a metal of work function 2.20 eV. Find the maximum kinetic energy and the stopping potential.
Photon energy: E = hc/λ = (6.63 × 10⁻³⁴)(3.00 × 10⁸)/(450 × 10⁻⁹) = 4.42 × 10⁻¹⁹ J. Work function: φ = 2.20 × 1.60 × 10⁻¹⁹ = 3.52 × 10⁻¹⁹ J. K_max = 4.42 × 10⁻¹⁹ − 3.52 × 10⁻¹⁹ = 0.90 × 10⁻¹⁹ J = 0.56 eV. Stopping potential: V_s = K_max/e = 0.56 V.
The reasoning must keep energy in joules until the final eV conversion; using h = 6.63 × 10⁻³⁴ J s and c = 3.00 × 10⁸ m s⁻¹ is expected.
Traps that cost marks
- Substituting centimetres or electronvolts without converting to SI.
- Writing terminal p.d. as the e.m.f. when current flows; the correct relation is V = E − Ir.
- Adding intensities in superposition instead of adding displacements and then squaring the resultant amplitude.
- Treating speed as constant in circular motion while ignoring the changing velocity direction and centripetal force.
- Confusing the work function with the threshold frequency; φ = hf₀ links them.