Electromagnetic Induction (Higher)

This lesson covers Higher Tier content. Questions based on electromagnetic induction may appear only on Higher Tier papers in the Edexcel GCSE Combined Science specification (1SC0).

This lesson explains how a changing magnetic field can induce a voltage (and current) in a conductor, the factors that affect the size of the induced voltage and a qualitative understanding of Lenz's law.

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What Is Electromagnetic Induction?

Electromagnetic induction is the process of generating a voltage (and, if the circuit is complete, a current) in a conductor by changing the magnetic field around it.

There are two ways to induce a voltage:

Method	Example
Move a conductor through a magnetic field	Pushing a wire between the poles of a magnet
Change the magnetic field around a stationary conductor	Moving a magnet into or out of a coil of wire

In both cases, the conductor must cut through magnetic field lines (or the number of field lines passing through the coil must change).

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Demonstrating Electromagnetic Induction

Magnet and Coil Experiment

1. Connect a coil of wire to a sensitive galvanometer (a meter that shows direction of current).
2. Push a bar magnet into the coil — the galvanometer needle deflects (a voltage and current are induced).
3. Hold the magnet still inside the coil — the needle returns to zero (no change in field → no induced voltage).
4. Pull the magnet out — the needle deflects in the opposite direction.

Key Observation

A voltage is induced only while the magnetic field through the coil is changing. No change → no induced voltage.

Exam Tip: Students often lose marks by saying "moving a magnet induces a current." Be precise: the magnet must move relative to the conductor, and there must be a change in the magnetic field through the conductor.

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Factors Affecting the Size of the Induced Voltage

Factor	Effect of Increasing It
Speed of movement (magnet or wire)	Larger induced voltage
Strength of the magnet	Larger induced voltage
Number of turns on the coil	Larger induced voltage

Doubling the speed, field strength or number of turns each produces a proportionally larger voltage.

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Lenz's Law (Qualitative)

Lenz's law states:

The direction of the induced current is always such that it opposes the change that produced it.

What Does This Mean?

• If you push a north pole into a coil, the induced current flows in a direction that makes the end of the coil nearest the magnet a north pole — this repels the incoming magnet, opposing the motion.
• If you pull the north pole out, the nearest end becomes a south pole — this attracts the magnet, again opposing the motion.

Lenz's law is a consequence of the conservation of energy — if the induced current aided the change, energy would be created from nothing.

graph LR
    subgraph "Lenz’s Law — Magnet Pushed into Coil"
    direction LR
    Mag["N pole approaching →"] -->|"Induced N pole repels"| CoilEnd["Coil end becomes N"]
    end

Exam Tip: Lenz's law is about opposition. The key word in any answer is "opposes." The induced effect always works against the change causing it.

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Alternating Current from Electromagnetic Induction

When a magnet is repeatedly pushed in and out of a coil, the induced voltage alternates — it switches direction with each change of motion. This produces an alternating current (AC) if the circuit is complete.

Motion	Voltage Direction
Magnet pushed in	One direction (e.g. positive)
Magnet stationary inside	Zero
Magnet pulled out	Opposite direction (e.g. negative)

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Worked Example

A student moves a bar magnet into a coil connected to a galvanometer. The galvanometer shows a deflection to the right. State and explain what happens if the student:

(a) Moves the magnet into the coil faster.

The galvanometer deflects further to the right — the induced voltage is larger because the rate of change of the magnetic field is greater.

(b) Holds the magnet stationary inside the coil.

The galvanometer reads zero — there is no change in the magnetic field, so no voltage is induced.

(c) Pulls the magnet out of the coil at the original speed.

The galvanometer deflects to the left — the direction of the change in magnetic field is reversed, so the induced voltage (and current) is in the opposite direction.

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Common Misconceptions

Misconception	Correction
A stationary magnet inside a coil induces a voltage	No — the field must be changing for induction to occur
A stronger magnet always induces a larger current	Only if the magnet is moving relative to the conductor
Lenz's law means the induced current stops the magnet	The induced current opposes the motion but does not completely stop it (you still have to do work against the opposition)

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Summary

• Electromagnetic induction: a voltage is induced when a conductor experiences a changing magnetic field.
• A voltage is induced only while the field is changing — not when it is static.
• The induced voltage is increased by moving faster, using a stronger magnet or using more turns on the coil.
• Lenz's law: the induced current always opposes the change producing it (conservation of energy).
• Repeatedly reversing the motion produces an alternating voltage/current.

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Deeper Dive: Magnetic Flux and Why It Matters

The modern way to describe induction uses magnetic flux (Φ), roughly "the amount of magnetic field passing through a loop." When the flux changes (by the field strength changing, by the loop changing area, or by the loop changing orientation), a voltage is induced. The induced EMF is proportional to the rate of change of flux:

$\varepsilon = -N \frac{\Delta \Phi}{\Delta t}$ε=−NΔtΔΦ​