WJEC A-Level Physics Unit 2 Electricity and Light: a complete overview of conduction, resistance, DC circuits, waves, refraction, photons and lasers

What Unit 2 actually demands

Unit 2 Electricity and Light pairs two strands. The electricity strand builds from charge and current through resistance and resistivity to full DC-circuit analysis with Kirchhoff's laws and internal resistance. The light strand develops the wave model (interference, diffraction and refraction), then breaks it with the photon model and the photoelectric effect, finishing with lasers. Both strands are heavily calculation-based and reward precise definitions.

This guide walks through the eight topics of the unit in specification order, then sets out the exam patterns WJEC repeats. Each topic has a matching dot-point page with practice questions; this overview ties them together.

Conduction of electricity

Current is the rate of flow of charge, $I = \frac{\Delta Q}{\Delta t}$. WJEC links current to the microscopic picture with $I = nAve$, where $n$ is the number density of charge carriers, $A$ the cross-sectional area, $v$ the drift velocity and $e$ the carrier charge. This explains why metals (high $n$) and semiconductors (lower $n$) carry current so differently.

Resistance

Resistance is $R = \frac{V}{I}$. Ohm's law holds for an ohmic conductor at constant temperature. Resistivity $\rho$ links resistance to geometry, $R = \frac{\rho L}{A}$, and is a material property. I-V characteristics for a metal, a filament lamp and a diode are standard.

DC circuits

DC analysis uses Kirchhoff's first law (current in equals current out at a junction, conserving charge) and second law (the sum of EMFs equals the sum of potential differences round a loop, conserving energy). EMF and internal resistance give $\varepsilon = I(R + r)$, and the potential divider sets an output voltage from a ratio of resistances.

The nature of waves

A wave transfers energy without transferring matter. WJEC distinguishes transverse from longitudinal waves, defines amplitude, wavelength, frequency, period and phase, uses the wave equation $v = f\lambda$, and treats polarisation as evidence that a wave is transverse.

Wave properties

Superposition produces interference: coherent sources give a stable pattern with constructive and destructive fringes set by the path difference. Diffraction spreads waves through gaps, and the diffraction grating equation $d\sin\theta = n\lambda$ lets you measure wavelength precisely.

Refraction of light

Refraction bends light at a boundary as its speed changes. Snell's law is $n_1\sin\theta_1 = n_2\sin\theta_2$, the refractive index is $n = \frac{c}{v}$, and beyond the critical angle light undergoes total internal reflection, the principle behind optical fibres.

Photons

The photon model gives each quantum of light energy $E = hf = \frac{hc}{\lambda}$. The photoelectric effect is explained by Einstein's equation $hf = \phi + E_{k(max)}$, where $\phi$ is the work function. The threshold frequency and instant emission are wave-model failures.

Lasers

A laser produces coherent light by stimulated emission with a population inversion maintained by pumping. The beam is intense, monochromatic, coherent and collimated. WJEC covers energy levels, the metastable state and the role of the optical cavity.

How Unit 2 is examined

WJEC Unit 2 is an AS written paper of about 1 hour 30 minutes carrying 80 marks. Questions come straight from the specification statements, around 40 per cent of marks are mathematical, and the specified practicals on resistivity and the diffraction grating recur. Drill circuit analysis and the photoelectric equation until both are automatic.

The eight topics, dot point by dot point

Each topic has a dot-point answer page with worked exam questions and cross-links. Browse them from this unit overview and the subject hub.

For the official specification

WJEC publishes the full specification, past papers and mark schemes at wjec.co.uk. Always revise from the current specification and WJEC's own past papers, because question style is board-specific.