Electromagnetic Induction: Experiments & Laws Revision Notes | A Level AQA Physics

1. The Phenomenon: Why Does a Moving Magnet Produce a Current?

Picture a coil of wire connected to a sensitive galvanometer. The coil is sitting still, the needle reads zero. Now push a bar magnet into the coil — the needle deflects. Pull the magnet out — it deflects the other way. Hold the magnet stationary inside the coil — the needle returns to zero. That sequence of observations is electromagnetic induction, and every rule in this topic flows from it.

The key variable is not the presence of a magnetic field, but the change in the magnetic flux passing through the coil. Flux, given the symbol $\Phi$, measures how much of the magnetic field threads through a given area. When that threading changes — because the magnet moves, the coil moves, or the field strength changes — an EMF is induced.

When a coil has $N$ turns, every turn contributes its own flux. The total is called the flux linkage: $N\Phi$. Doubling the number of turns doubles the flux linkage for the same field — and, as you will see, doubles the induced EMF for the same rate of change. This is why real transformers and generators use many-turn coils rather than a single loop.

2. Mapping the Experimental Phenomena

Before formalising the laws, it is worth cataloguing exactly what the experiments show, because AQA questions often ask you to describe or explain observations rather than quote a formula. The table below summarises the classic magnet-and-coil results.

Every row in that table is a direct experimental test of Faraday's law. The magnitude of the deflection tracks the rate of flux linkage change; the direction of the deflection tracks the sign of that change. These two strands become the two laws you need to master.

3. Faraday's Law: Quantifying the Induced EMF

Faraday's law takes the experimental pattern — bigger rate of change gives bigger EMF — and turns it into a precise, calculable relationship. The induced EMF equals the rate of change of flux linkage.

Notice that the formula contains $N\Phi$ as a single quantity — the flux linkage — not $N$ and $\Phi$ separately. This matters when the number of turns is fixed (as it usually is) and only $\Phi$ changes: you can write $\varepsilon = -N\frac{\Delta\Phi}{\Delta t}$. Both forms are equivalent; use whichever the question gives you data for.

The gradient of a flux linkage–time graph gives the induced EMF directly. A steep gradient means a large EMF; a flat section (constant flux linkage) means zero EMF. This graph-reading skill appears regularly in AQA data questions.

Worked example: Calculating Induced EMF Using Faraday's Law

A coil of 200 turns has a cross-sectional area of $4.0 \times 10^{-4}$ m². A uniform magnetic field perpendicular to the coil increases from 0.10 T to 0.55 T in 0.030 s. Calculate the magnitude of the induced EMF. [3 marks]

1. $|\varepsilon| = N\frac{\Delta\Phi}{\Delta t}$ — [M1] Selecting the correct formula. Since $N$ is constant and the field is perpendicular to the coil, $\Delta\Phi = \Delta B \times A$.
2. $\Delta\Phi = \Delta B \times A = (0.55 - 0.10) \times 4.0 \times 10^{-4}$ $\Delta\Phi = 0.45 \times 4.0 \times 10^{-4} = 1.8 \times 10^{-4}$ Wb — The field is perpendicular to the coil so $\cos\theta = 1$ and $\Phi = BA$. The change in $B$ is $0.55 - 0.10 = 0.45$ T.
3. $|\varepsilon| = 200 \times \frac{1.8 \times 10^{-4}}{0.030}$ — [M1] Correct substitution with $N = 200$, $\Delta\Phi = 1.8 \times 10^{-4}$ Wb, $\Delta t = 0.030$ s.
4. $\frac{1.8 \times 10^{-4}}{0.030} = 6.0 \times 10^{-3}$ V per turn $|\varepsilon| = 200 \times 6.0 \times 10^{-3} = 1.2$ V — [A1] Division: $1.8 \times 10^{-4} \div 3.0 \times 10^{-2} = 6.0 \times 10^{-3}$. Then $200 \times 6.0 \times 10^{-3} = 1.2$ V. Units check: Wb s$^{-1}$ = V. Correct.

Answer: $|\varepsilon| = 1.2$ V

4. Lenz's Law: The Direction of the Induced Current

Faraday's law tells you how much EMF is induced. Lenz's law tells you which way the current flows. The two laws are inseparable in practice, and the negative sign in Faraday's equation is the mathematical expression of Lenz's law.

Here is how to apply Lenz's law step by step. First, identify whether the flux through the coil is increasing or decreasing. Second, determine the direction the induced current must flow to create a field that opposes that change. If flux is increasing into the page, the induced current creates a field out of the page inside the coil — use the right-hand grip rule to find the current direction around the coil.

The energy conservation argument is worth holding onto. When you push a magnet into a coil, you feel a resistive force — the induced current creates a magnetic field that repels the incoming magnet. You do work against that force, and that work is the source of the electrical energy delivered to the circuit. If the induced current instead attracted the magnet, it would accelerate indefinitely and generate unlimited energy, violating conservation of energy.

5. Connecting the Two Laws: The Negative Sign and What It Means

The full statement of Faraday's law, $\varepsilon = -\frac{\Delta(N\Phi)}{\Delta t}$, carries both laws simultaneously. The magnitude of the right-hand side gives you the size of the EMF (Faraday); the negative sign tells you the EMF acts to oppose the flux change (Lenz). In most AQA calculations you work with magnitudes and state the direction separately using Lenz's law reasoning — but recognising that the negative sign is not a mathematical accident is important for higher-mark explanation questions.

A useful way to think about it: the induced EMF drives a current that tries to maintain the status quo. Flux increasing? The induced current fights the increase. Flux decreasing? The induced current fights the decrease. The coil is, in effect, magnetically conservative — it resists change, much like inertia resists changes in motion.

6. Required Practical: Investigating Electromagnetic Induction

The standard AQA investigation uses a magnet, a coil, and a data logger or galvanometer to test Faraday's law quantitatively. The practical links directly to both spec requirements: you observe the phenomena and then use the data to verify the proportionality between induced EMF and rate of flux change.

One reliability point worth noting: the data logger captures the full EMF–time trace as the magnet passes through. You will see a positive peak as the magnet enters and a negative peak as it exits. The area under each peak is proportional to the total change in flux linkage — a neat graphical check of Faraday's law that examiners sometimes ask about.

7. Exam Technique and Common Mistakes

The most reliable way to lose marks on induction questions is to confuse flux with flux linkage, or to state Lenz's law without explaining the energy conservation reasoning behind it. The guidance below targets the patterns that appear most often in AQA mark schemes.

For 6-mark extended response questions on induction, the mark scheme typically rewards: a clear statement of Faraday's law with correct terminology; a statement of Lenz's law with the energy conservation justification; application to the specific scenario described; and a conclusion that ties the physics to the observable outcome. Practise structuring your answer in that order.