What does AQA A-Level Physics module 3.5 Electricity cover?

It covers current as the rate of flow of charge and the equation $I = nAvq$, potential difference and resistance, the current-voltage characteristics of ohmic conductors, filament lamps and diodes, resistivity and superconductivity, combining resistors in series and parallel with Kirchhoff's two laws, electrical power, the potential divider, and EMF with internal resistance. It is an AS-level module reinforced in the full A-Level.

Which equations recur most across module 3.5?

Current $I = \frac{Q}{t}$ and $I = nAvq$, resistance $R = \frac{V}{I}$, resistivity $R = \frac{\rho L}{A}$, the series and parallel resistor rules, power $P = VI = I^2 R = \frac{V^2}{R}$, the potential divider equation $V_{out} = V_{in}\frac{R_2}{R_1 + R_2}$, and the EMF equation $\varepsilon = I(R + r)$ with terminal pd $V = \varepsilon - Ir$.

Why is the terminal potential difference less than the EMF?

A real cell has internal resistance, so when it drives a current some energy per coulomb is dissipated inside the cell. These lost volts equal $Ir$, so the terminal potential difference $V = \varepsilon - Ir$ is always less than the EMF whenever current flows, and the drop grows with the current.

What is the single most common mistake in this module?

Reading the resistance of a component as the gradient of a curved I-V graph. Resistance is always $\frac{V}{I}$ at the chosen point; only a straight line through the origin has resistance equal to the inverse gradient. A close second is adding parallel resistances directly instead of using the reciprocal sum.

How should I revise the circuit calculations?

Learn the series and parallel rules cold, then practise applying Kirchhoff's two laws to multi-loop circuits. For power, match the equation to whether current or voltage is fixed: use $I^2 R$ in series and $\frac{V^2}{R}$ in parallel. Then drill potential divider and internal-resistance problems, which are AQA favourites.

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AQA A-Level Physics 3.5 Electricity: a complete overview of current, resistance, circuits, potential dividers and EMF

A deep-dive AQA A-Level Physics guide to module 3.5 Electricity. Covers current and charge, current-voltage characteristics, Ohm's law, resistivity and superconductivity, series and parallel circuits, Kirchhoff's laws, power, the potential divider, and EMF with internal resistance, including the calculations and exam patterns AQA repeats.

Generated by Claude Opus 4.820 min read3.5Updated 2026-06-02

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

Jump to a section

What module 3.5 actually demands
Current, charge and characteristics
Resistivity and superconductivity
Circuits, Kirchhoff's laws and power
Potential dividers and EMF
How module 3.5 is examined
Check your knowledge

What module 3.5 actually demands

Electricity is the circuit-analysis core of AQA A-Level Physics. Module 3.5 starts from what current and charge are at the level of moving carriers, builds the idea of resistance and how it depends on the material and shape, then combines components into series and parallel circuits analysed with Kirchhoff's laws, and finishes with two practical tools: the potential divider and the real cell with internal resistance. The examiners test clean definitions, the reading of characteristic graphs, and multi-step circuit calculations.

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

Current, charge and characteristics

Current and charge defines current as the rate of flow of charge, $I = \frac{Q}{t}$ , and links it to the carriers through $I = nAvq$ , where the drift velocity is surprisingly small because the number density of electrons is huge. Kirchhoff's first law is conservation of charge at a junction.

Current-voltage characteristics defines potential difference ( $V = \frac{W}{Q}$ ) and resistance ( $R = \frac{V}{I}$ ), states Ohm's law, and explains the I-V graphs of an ohmic conductor (straight line), a filament lamp (curves over as it heats) and a diode (conducts only in forward bias). A thermistor's resistance falls with temperature and an LDR's falls with light.

Resistivity and superconductivity

Resistivity introduces $R = \frac{\rho L}{A}$ , so resistance rises with length and falls with cross-sectional area, while resistivity is a property of the material. A metal's resistivity increases with temperature as the ions vibrate more. A superconductor has zero resistivity below its critical temperature, enabling lossless currents in MRI magnets and accelerators.

Circuits, Kirchhoff's laws and power

Series and parallel circuits give the resistor rules (series add; parallel combine by reciprocal sum), the application of Kirchhoff's two laws (charge conservation at junctions, energy conservation around loops), and the three power equations $P = VI = I^2 R = \frac{V^2}{R}$ . In series the current is shared as voltage; in parallel the voltage is common and current is shared.

Potential dividers and EMF

The potential divider supplies a chosen fraction of the supply voltage, $V_{out} = V_{in}\frac{R_2}{R_1 + R_2}$ , and becomes a sensor circuit when a thermistor or LDR replaces one resistor. EMF and internal resistance closes the module: the EMF is energy per coulomb supplied, internal resistance causes lost volts $Ir$ , and $\varepsilon = I(R + r)$ with $V = \varepsilon - Ir$ . Plotting terminal pd against current gives the EMF as the intercept and the internal resistance as the magnitude of the gradient.

How module 3.5 is examined

A typical AQA profile for Electricity:

Multiple choice and short answer. Defining current, resistance and EMF, classifying components as ohmic or non-ohmic, and recalling the effect of temperature on a thermistor or metal.
Calculations. Drift velocity from $I = nAvq$ , resistivity from wire data, series and parallel combinations, power, potential divider outputs, and current and terminal pd with internal resistance.
Graph and data questions. Reading I-V characteristics, explaining the filament-lamp curve, and finding EMF and internal resistance from a terminal-pd against current graph.
Extended answers. Explaining why a filament lamp is non-ohmic, designing a thermistor or LDR sensor circuit, and why the terminal pd falls as current increases.

Check your knowledge

A mix of recall and calculation questions covering module 3.5. Attempt them under timed conditions, then check against the solutions.

Define electric current. (1 mark)
A current of 3.0 A flows for 20 s. Calculate the charge that passes. (1 mark)
Explain why the I-V characteristic of a filament lamp curves. (3 marks)
A wire of length 2.0 m and area $5.0 \times 10^{-7} \text{ m}^2$ has resistivity $1.7 \times 10^{-8} \text{ ohm m}$ . Find its resistance. (2 marks)
Two resistors, 6.0 ohm and 12 ohm, are connected in parallel. Find the combined resistance. (2 marks)
A 9.0 V supply drives 0.30 A through a heater. Find the power dissipated. (1 mark)
A potential divider has a 2.0 k-ohm and a 3.0 k-ohm resistor across 10 V. Find the output across the 3.0 k-ohm resistor. (2 marks)
A cell of EMF 1.5 V and internal resistance 0.40 ohm drives a current of 0.50 A. Find the terminal potential difference. (2 marks)

Solutions

Q1: The rate of flow of electric charge.
Q2: $Q = It = 3.0 \times 20 = 60 \text{ C}$ .
Q3: As the current rises the filament heats up; its ions vibrate more, impeding the electrons, so the resistance increases. The gradient ( $1/R$ ) therefore decreases as voltage rises, curving the line, so the lamp is non-ohmic.
Q4: $R = \dfrac{\rho L}{A} = \dfrac{(1.7 \times 10^{-8})(2.0)}{5.0 \times 10^{-7}} = 0.068 \text{ ohm}$ .
Q5: $\dfrac{1}{R} = \dfrac{1}{6.0} + \dfrac{1}{12} = \dfrac{2}{12} + \dfrac{1}{12} = \dfrac{3}{12}$ , so $R = 4.0 \text{ ohm}$ .
Q6: $P = VI = 9.0 \times 0.30 = 2.7 \text{ W}$ .
Q7: $V_{out} = 10 \times \dfrac{3.0}{2.0 + 3.0} = 10 \times \dfrac{3.0}{5.0} = 6.0 \text{ V}$ .
Q8: $V = \varepsilon - Ir = 1.5 - 0.50 \times 0.40 = 1.5 - 0.20 = 1.3 \text{ V}$ .

Sources & how we know this

AQA A-level Physics (7408) specification — AQA (2017)