WPH12 Waves and Electricity

Lessons in WPH12

1. The wave equation and resistivity
   v = fλ, T = 1/f. R = ρL/A. V = IR.
2. Describing waves: transverse, longitudinal and polarisation
   Transverse: oscillation perpendicular to energy transfer; longitudinal: parallel. Only transverse waves can be polarised.
3. Core practical 4: speed of sound in air
   Each return to in phase = one wavelength moved. Measure over many wavelengths; v = fλ or the gradient of λ against 1/f.
4. Superposition, coherence and interference
   Path difference nλ: constructive; (n + 1/2)λ: destructive (for in-phase sources). Phase difference = 2π x path difference / λ.
5. Standing waves and waves on strings
   Nodes λ/2 apart; fixed string fundamental λ = 2L; v = sqrt(T/μ).
6. Core practical 5: frequency of a vibrating string
   f = (1/2L) sqrt(T/μ): f ∝ 1/L, f ∝ sqrt(T), f ∝ 1/sqrt(μ). Change one variable at a time.
7. Refraction and refractive index
   n = c/v; n1 sin θ1 = n2 sin θ2 with angles from the normal; frequency stays the same.
8. Critical angle and total internal reflection
   sin C = 1/n (into air). TIR needs travel towards a lower n and an angle of incidence greater than C.
9. Intensity, reflection and pulse-echo techniques
   I = P/A. At a boundary a wave is partly reflected, partly transmitted. Pulse-echo: d = vt/2; detail limited by wavelength and pulse length.
10. Diffraction and the diffraction grating (with core practical 6)
    Diffraction is greatest when the gap is about λ. Grating: nλ = d sin θ, d = 1/(lines per metre), tan θ = x/D on a screen.
11. Electron diffraction and the de Broglie wavelength
    Electron diffraction shows electrons act as waves; λ = h/p = h/mv.
12. Photons and the electronvolt
    E = hf = hc/λ. 1 eV = 1.60 x 10^-19 J. Wave model: diffraction and interference; photon model: photoelectric effect.
13. The photoelectric effect
    hf = φ + (1/2)mv²_max; f0 = φ/h. One photon frees one electron: frequency sets the energy, intensity sets the number.
14. Energy levels and line spectra
    Photon energy = difference between levels: hf = E1 - E2. Discrete levels give line spectra.
15. Current, potential difference and resistance
    I = ΔQ/Δt; V = W/Q; R = V/I. Ohm's law (I ∝ V) holds only for some conductors at constant temperature.
16. Series and parallel circuits
    Series: R = R1 + R2 + ...; parallel: 1/R = 1/R1 + 1/R2 + .... Current conserved at junctions; p.d.s around a loop add to the e.m.f.
17. Electrical power and energy
    P = VI = I² R = V²/R; W = VIt.
18. I-V graphs and why resistance changes
    R = V/I at each point. Hot metal: more lattice vibrations, lower drift velocity, higher R. NTC thermistor and LDR: more charge carriers (n rises), lower R.
19. Core practical 7: measuring resistivity
    Plot R against l: gradient = ρ/A, so ρ = gradient x A. Small currents; micrometer for diameter; intercepts do not affect the gradient.
20. Charge carriers and drift velocity
    I = nqvA. Metals have huge n, so low resistivity; semiconductors and insulators have far fewer charge carriers.
21. Potential along a wire and potential dividers
    V_out = V_in x R2/(R1 + R2). The bigger resistance takes the bigger share. Potential falls linearly along a uniform wire.
22. E.m.f. and internal resistance (with core practical 8)
    ε = I(R + r); V = ε - Ir. Plot V against I: intercept = ε, gradient = -r.

Next exams for WPH12

• January 2027: Wed 13 Jan 2027, afternoon, 1h 30m
• Summer 2027: Wed 12 May 2027, afternoon, 1h 30m

From Pearson's final timetables. All exam dates. Always confirm your entries with your exams officer.