What does OCR Module 4 Electrons, waves and photons cover?

It covers charge and current with the elementary charge and mean drift velocity, energy, power and resistance including resistivity and the I-V characteristics of components, electrical circuits with Kirchhoff's laws, internal resistance and potential dividers, wave properties with the wave equation, intensity and polarisation, superposition and interference including stationary waves, the double slit and the diffraction grating, refraction with Snell's law and total internal reflection in optical fibres, and quantum physics with the photon model, the photoelectric effect and wave-particle duality.

What is the most common mistake in this module?

In electricity, treating internal resistance as negligible so that the terminal voltage is wrongly taken as the emf, and confusing series and parallel resistor rules. In waves, taking the node-to-node distance as a full wavelength rather than half, and using the grating line density directly as the slit spacing. In quantum physics, assuming brighter light always ejects photoelectrons even below the threshold frequency, and forgetting to convert the work function between electronvolts and joules.

How is quantum physics examined in OCR Physics A?

Expect the photon energy equation $E = hf$, the photoelectric effect explained as evidence for quantisation (instantaneous emission, a threshold frequency, and maximum kinetic energy depending on frequency not intensity), calculations with Einstein's equation $hf = \phi + KE_{\max}$ and the threshold frequency, the electronvolt as an energy unit, and the de Broglie wavelength with electron diffraction as evidence for the wave nature of matter.

How should I revise Electrons, waves and photons?

Drill the circuit routines first: current and charge, resistance and resistivity, power, Kirchhoff's laws, internal resistance and potential dividers, with I-V characteristics for the diode, filament lamp and thermistor. Then master the wave toolkit: the wave equation, stationary-wave harmonics, the double slit and grating, and refraction with total internal reflection. Finally secure the quantum ideas and their evidence. This module is assessed in Paper 2 alongside Module 6, so practise its calculations until automatic.

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OCR A-Level Physics A Electrons, waves and photons: electricity, circuits, waves and quantum physics

Q: Which equations recur most across Electrons, waves and photons?

Current as charge per second $I = \frac{\Delta Q}{\Delta t}$ and the drift equation $I = nAve$, resistance $R = \frac{V}{I}$, resistivity $R = \frac{\rho L}{A}$, power $P = IV = I^2R = \frac{V^2}{R}$, Kirchhoff's laws and the emf relation $\varepsilon = I(R + r)$, the wave equation $v = f\lambda$, the double-slit fringe equation $w = \frac{\lambda D}{a}$ and the grating equation $d\sin\theta = n\lambda$, Snell's law $n_1\sin\theta_1 = n_2\sin\theta_2$ with the critical angle $\sin\theta_c = \frac{n_2}{n_1}$, the photon energy $E = hf = \frac{hc}{\lambda}$, Einstein's photoelectric equation $hf = \phi + KE_{\max}$, and the de Broglie wavelength $\lambda = \frac{h}{mv}$.

A deep-dive OCR A-Level Physics A guide to Module 4, Electrons, waves and photons. Covers charge and current, energy, power and resistance, electrical circuits with Kirchhoff's laws and potential dividers, wave properties, superposition and interference, refraction and the electromagnetic spectrum, and quantum physics with the photoelectric effect and wave-particle duality.

Generated by Claude Opus 4.818 min readH556 Module 4Updated 2026-06-02

Reviewed by: AI editorial process; not yet individually human-reviewed

Jump to a section

What this module actually demands
Charge, current and circuits
Waves, interference and refraction
Quantum physics
How this module is examined
Check your knowledge

What this module actually demands

Electrons, waves and photons is the broadest content module in OCR Physics A. It begins with electric current as a flow of charge, builds the laws of circuits, develops the full behaviour of waves from the wave equation to interference and refraction, and ends with the quantum physics that bridges the wave and particle pictures of light and matter. The examiners reward fluent circuit analysis, careful unit work, and precise statements of the evidence behind the photon model.

This guide walks through the topics in order and sets out the exam patterns OCR repeats. Each topic has a matching dot-point page with practice; this overview ties them together.

Charge, current and circuits

Charge and current defines the elementary charge, treats current as the rate of flow of charge $I = \frac{\Delta Q}{\Delta t}$ , applies conservation of charge at junctions, and links current to carrier number density through the drift equation $I = nAve$ . Energy, power and resistance defines potential difference and emf, uses $R = \frac{V}{I}$ and resistivity $R = \frac{\rho L}{A}$ , interprets the I-V characteristics of a resistor, filament lamp, diode and thermistor, and applies the power relations $P = IV = I^2R = \frac{V^2}{R}$ .

Electrical circuits applies Kirchhoff's current and voltage laws, combines resistors in series and parallel, treats the internal resistance of a source with $\varepsilon = I(R + r)$ , and analyses potential dividers including those with thermistors and light-dependent resistors.

Waves, interference and refraction

Wave properties distinguishes transverse from longitudinal waves, defines the wave quantities, applies the wave equation $v = f\lambda$ , relates intensity to amplitude squared, and explains polarisation. Superposition and interference applies the principle of superposition, describes stationary waves and their harmonics, and uses the double-slit fringe equation $w = \frac{\lambda D}{a}$ and the grating equation $d\sin\theta = n\lambda$ . Refraction and the electromagnetic spectrum uses refractive index and Snell's law, finds the critical angle for total internal reflection, explains optical fibres, and orders the electromagnetic spectrum.

Quantum physics

Quantum physics introduces the photon model with $E = hf = \frac{hc}{\lambda}$ , explains the photoelectric effect as evidence for quantisation, applies Einstein's equation $hf = \phi + KE_{\max}$ with the work function and threshold frequency, and uses the de Broglie wavelength $\lambda = \frac{h}{mv}$ with electron diffraction to show that matter has wave properties.

How this module is examined

A typical OCR profile for Electrons, waves and photons:

Calculations. Current, charge and drift velocity, resistance and resistivity, power, circuit analysis with Kirchhoff's laws and internal resistance, potential dividers, fringe and grating problems, refraction and critical angle, photon energy, photoelectric and de Broglie calculations.
Graph questions. I-V characteristics, resistance against temperature, and stationary-wave and interference patterns.
Explanation and definition. Conservation of charge, emf versus terminal voltage, coherence, total internal reflection, and the evidence for the photon model.
Extended answers. Designing and analysing potential-divider sensors, explaining why the photoelectric effect needs the photon model, and comparing the wave and particle behaviour of light.

Check your knowledge

A mix of recall and calculation questions covering the module. Attempt them under timed conditions, then check against the solutions.

State the equation linking current, charge and time, and define each term. (2 marks)
A wire of resistivity $1.7 \times 10^{-8}\ \Omega\ \text{m}$ , length $2.0\ \text{m}$ and cross-sectional area $5.0 \times 10^{-7}\ \text{m}^2$ has what resistance? (2 marks)
State Kirchhoff's first law and the physical principle behind it. (2 marks)
A double-slit pattern has slits $0.30\ \text{mm}$ apart, a screen $2.0\ \text{m}$ away, and fringes $4.0\ \text{mm}$ apart. Find the wavelength. (2 marks)
Calculate the energy of a photon of wavelength $6.0 \times 10^{-7}\ \text{m}$ . (2 marks)
State one piece of evidence from the photoelectric effect that supports the photon model. (1 mark)

Solutions

Q1: $I = \frac{\Delta Q}{\Delta t}$ : current is the charge $\Delta Q$ passing a point per unit time $\Delta t$ .
Q2: $R = \frac{\rho L}{A} = \frac{(1.7 \times 10^{-8})(2.0)}{5.0 \times 10^{-7}} = 0.068\ \Omega$ .
Q3: The total current into a junction equals the total current out of it; it expresses conservation of charge.
Q4: $\lambda = \frac{wa}{D} = \frac{(4.0 \times 10^{-3})(0.30 \times 10^{-3})}{2.0} = 6.0 \times 10^{-7}\ \text{m}$ .
Q5: $E = \frac{hc}{\lambda} = \frac{(6.63 \times 10^{-34})(3.0 \times 10^{8})}{6.0 \times 10^{-7}} = 3.3 \times 10^{-19}\ \text{J}$ .
Q6: Any one of: emission is instantaneous, there is a threshold frequency, or the maximum kinetic energy depends on frequency not intensity.

Sources & how we know this

OCR AS and A Level Physics A (H156, H556) specification — OCR (2015)