Electromagnetic Induction

This lesson covers electromagnetic induction — the generation of a voltage (and current) by a changing magnetic field — as required by the Edexcel GCSE Physics specification (1PH0), Topic 8: Magnetism and Electromagnetism. You need to understand how a voltage is induced, the factors that affect its size, and Lenz's law (Higher tier).

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What Is Electromagnetic Induction?

Electromagnetic induction is the process by which a voltage (also called an electromotive force, or e.m.f.) is induced across a conductor when it experiences a changing magnetic field.

This can happen in two ways:

1. A conductor moves through a magnetic field.
2. A magnetic field changes around a stationary conductor (e.g. a magnet moves near a coil).

In both cases, the key requirement is that there is relative motion between the conductor and the magnetic field — or more precisely, the magnetic flux through the conductor must be changing.

Exam Tip: Electromagnetic induction requires a change in the magnetic field experienced by the conductor. If the conductor is stationary in a constant magnetic field, no voltage is induced. The conductor must move, or the magnetic field must change.

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How to Demonstrate Electromagnetic Induction

Moving a Magnet into a Coil

1. Connect a coil of wire to a sensitive ammeter (or voltmeter).
2. Push a bar magnet into the coil — the ammeter shows a deflection (current flows).
3. Hold the magnet still inside the coil — the ammeter reads zero (no change in field = no induction).
4. Pull the magnet out of the coil — the ammeter deflects in the opposite direction (current reverses).

Key Observations

Action	Ammeter Reading	Explanation
Magnet pushed into coil	Deflects one way	Changing field induces a voltage → current flows
Magnet held still in coil	Zero	No change in magnetic field → no induction
Magnet pulled out of coil	Deflects opposite way	Field changes in opposite direction → current reverses
Magnet pushed in faster	Larger deflection	Faster change → larger induced voltage

graph LR
    A["Magnet MOVES<br/>relative to coil"] --> B["Magnetic flux<br/>through coil CHANGES"]
    B --> C["Voltage (e.m.f.)<br/>INDUCED across coil"]
    C --> D["If circuit is closed:<br/>induced CURRENT flows"]
    D --> E["Lenz’s law:<br/>induced current OPPOSES<br/>the change causing it"]

    F["Magnet STATIONARY<br/>inside coil"] --> G["No change in flux"]
    G --> H["NO induced voltage<br/>(galvanometer reads 0)"]

    style A fill:#2980b9,color:#fff
    style B fill:#e67e22,color:#fff
    style C fill:#27ae60,color:#fff
    style D fill:#8e44ad,color:#fff
    style E fill:#c0392b,color:#fff
    style F fill:#95a5a6,color:#fff
    style G fill:#7f8c8d,color:#fff
    style H fill:#34495e,color:#fff

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Faraday's Law

Faraday's law states that the size of the induced voltage (e.m.f.) depends on:

1. The rate of change of the magnetic field — faster movement produces a larger voltage.
2. The strength of the magnetic field — a stronger magnet produces a larger voltage.
3. The number of turns on the coil — more turns produces a larger voltage.
4. The area of the coil — a larger area means more magnetic flux is cut.

How to Increase the Induced Voltage

Method	Effect
Move the magnet faster	The magnetic field changes more quickly
Use a stronger magnet	Greater change in magnetic flux
Use more turns on the coil	Each turn contributes to the total induced voltage
Use a coil with a larger area	More magnetic flux passes through the coil

Exam Tip: For the Edexcel exam, you need to know that the induced voltage increases if the magnet moves faster, the magnet is stronger, or the coil has more turns. A very common exam question asks you to describe how to increase the voltage — always mention at least two of these factors.

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Lenz's Law (Higher Tier)

Lenz's law states that the direction of the induced current always acts to oppose the change that is producing it.

What Does This Mean?

• When a magnet is pushed into a coil, the induced current flows in a direction that creates a magnetic field to repel the incoming magnet (opposing the motion).
• When a magnet is pulled out of a coil, the induced current creates a field that attracts the departing magnet (again, opposing the change).

Why Lenz's Law Exists

Lenz's law is a consequence of the conservation of energy:

• If the induced current aided the change (instead of opposing it), the magnet would accelerate, inducing more current, producing more force, accelerating more — creating energy from nothing.
• This would violate the law of conservation of energy, so the induced current must oppose the change.

Exam Tip (Higher): Lenz's law is often tested in 6-mark questions. Remember: the induced current always opposes the change causing it. If a magnet moves into a coil, the coil acts like a magnet that repels the incoming magnet. If the magnet is pulled out, the coil attracts it. This is explained by the conservation of energy.

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Electromagnetic Induction vs the Motor Effect

It is important to distinguish between these two related but opposite phenomena:

Feature	Motor Effect	Electromagnetic Induction
Input	Current + magnetic field	Movement + magnetic field
Output	Movement (force)	Voltage (and current if circuit is complete)
Rule	Fleming's LEFT-hand rule	Lenz's law (and right-hand rule for generators)
Converts	Electrical energy → kinetic energy	Kinetic energy → electrical energy
Example	Electric motor	Generator, dynamo

Exam Tip: A very common mistake is to confuse the motor effect with electromagnetic induction. Remember: motor effect = current in a field produces a force (electricity → movement). Electromagnetic induction = movement in a field produces a voltage (movement → electricity). They are essentially the reverse of each other.

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Applications of Electromagnetic Induction

Generators

• A coil rotating in a magnetic field (or a magnet rotating inside a coil) induces a voltage.
• This is the principle behind all electrical generators (covered in detail in the next lesson).

Microphones

• Sound waves cause a diaphragm to vibrate, which moves a coil back and forth in a magnetic field.
• This induces a varying voltage that matches the pattern of the sound wave.
• The varying voltage is the electrical signal that represents the sound.

Dynamos