Modern Physics and Electronics - photons, nuclei and logic
Modern Physics and Electronics
The modern-physics questions cover the photoelectric effect, wave–particle duality, atomic energy levels and nuclear changes; the electronics questions cover diodes and rectification, the transistor as a switch, and logic gates. Most marks come from applying one equation cleanly and from reading truth tables without error.
Core rules
- Photon energy E = hf = hc/λ. Einstein's equation hf = W + KE_max, where W = hf₀ is the work function; the stopping potential gives eV_s = KE_max.
- Emission happens only when f > f₀, that is λ < hc/W. Intensity changes the number of photoelectrons (the current), not their maximum kinetic energy.
- de Broglie wavelength λ = h/p = h/(mv).
- Bohr model: hf = E₂ − E₁ for a spectral line; ionisation energy removes the electron from its level.
- Nuclear notation ᴬ_Z X: alpha decay reduces A by 4 and Z by 2; beta-minus raises Z by 1 and leaves A unchanged; gamma changes neither.
- Radioactive decay N = N₀(½)^(t/T); mass defect gives energy by E = Δm c².
- Electronics: a diode conducts in forward bias and blocks in reverse; it gives half-wave rectification, a bridge gives full-wave. Logic gates obey fixed truth tables - AND gives 1 only for 1,1; OR for any 1; NOT inverts; NAND and NOR are the negations of AND and OR.
Worked example
Light of wavelength 400 nm falls on a metal of work function 2.0 eV. Take hc = 1240 eV·nm.
Photon energy E = hc/λ = 1240/400 = 3.1 eV. KE_max = 3.1 − 2.0 = 1.1 eV = 1.1 × 1.6 × 10⁻¹⁹ = 1.8 × 10⁻¹⁹ J. The threshold wavelength is hc/W = 1240/2.0 = 620 nm, so light longer than this emits nothing.
Traps
Feeding nanometres straight into hc/λ without care; thinking brighter light raises KE_max; confusing threshold frequency with threshold wavelength; forgetting to convert eV to joules; changing Z instead of A in alpha decay; treating half-life as depending on sample size; mixing up NAND with AND in a truth table; assuming a diode follows Ohm's law.