Electromagnetic induction is the generation of a voltage in a conductor when the magnetic field through it changes. Moving a magnet into or out of a coil of wire, or rotating a coil in a magnetic field, induces an EMF that drives a current if the circuit is complete.
What is electromagnetic induction?
When a conductor experiences a changing magnetic field — either because the conductor moves through the field or because the field itself changes — a voltage is induced across the conductor. If the circuit is complete, this induced voltage drives an induced current.
Michael Faraday discovered this effect in 1831. Before working through the details, predict: if pushing a magnet into a coil generates a current, what do you expect to happen when you pull it out? The current reverses direction — because the change in the magnetic field is now opposite.
The key condition for induction is change: a stationary magnet inside a stationary coil produces no EMF. Motion or change is essential.
What factors affect the size of the induced EMF?
The magnitude of the induced EMF depends on:
| Factor | Change | Effect on induced EMF |
|---|---|---|
| Speed of magnet movement | Faster | Larger EMF — the magnetic flux changes more rapidly |
| Strength of the magnet | Stronger | Larger EMF — greater flux change per unit of movement |
| Number of turns in the coil | More turns | Larger EMF — each turn contributes, so total EMF multiplies |
| Area of the coil | Larger area | Larger EMF — more flux cuts through the coil for a given field strength |
| Presence of a soft iron core | Iron core inserted | Larger EMF — iron concentrates and guides the magnetic field through the coil |
To reverse the direction of the induced EMF (and hence the induced current), either reverse the direction of movement, or reverse the polarity of the magnet (swap N and S poles).
What does Faraday's law state?
Faraday's law of electromagnetic induction states that the induced EMF in a conductor is proportional to the rate of change of magnetic flux through the circuit.
In everyday language: the faster the magnetic field changes, or the more coil turns are present, the larger the induced voltage. Doubling the speed of movement doubles the induced EMF; doubling the number of coil turns also doubles the induced EMF.
Lenz's law (which follows from energy conservation) states that the induced current always flows in a direction that opposes the change causing it. If you push a magnet into a coil, the induced current creates a magnetic field that repels the approaching magnet — you have to do work against this opposition, which is where the electrical energy comes from.
How does an AC generator work?
An alternating current (AC) generator (also called an alternator or dynamo) converts kinetic energy into electrical energy using electromagnetic induction:
- A rectangular coil is mounted between the poles of a magnet on an axle.
- When the coil rotates, different parts of the coil cut through the magnetic field lines at different angles.
- When the coil is perpendicular to the field (sides moving parallel to the field lines), the rate of cutting field lines is zero — EMF is zero.
- When the coil is parallel to the field (sides cutting through field lines at 90°), the rate is maximum — EMF is at its peak.
- As the coil continues to rotate, the direction of cutting reverses — the EMF reverses, producing alternating current.
The coil connects to the external circuit via slip rings and brushes: the slip rings rotate with the coil, and the brushes are stationary contacts that maintain a connection, allowing the coil to spin continuously while staying connected to the circuit.
The output of an AC generator is a sinusoidal (sine wave) voltage — it rises to a maximum, falls back through zero, reaches a negative maximum, and returns to zero with each complete rotation.
How does a bicycle dynamo work?
A bicycle dynamo is a small AC generator. A permanent magnet (usually a cylindrical magnet) is spun by the friction of the bicycle tyre against a knurled wheel. As the magnet spins near a coil of wire, the changing magnetic field induces an alternating EMF in the coil, which lights the bicycle lamp. The faster the wheel turns (the faster you cycle), the higher the frequency and the larger the peak EMF — the lamp glows brighter.
How is electromagnetic induction different from the motor effect?
| Electromagnetic induction | Motor effect | |
|---|---|---|
| Input | Motion (kinetic energy) | Electrical energy (current in a field) |
| Output | Electrical energy (EMF and current) | Motion (kinetic energy) |
| Device | Generator, dynamo, transformer | Electric motor |
| Key requirement | Changing magnetic flux in a conductor | Current-carrying conductor in a magnetic field |
The two effects are reverses of each other — a motor converts electrical energy to kinetic energy; a generator converts kinetic energy to electrical energy. Both rely on the interaction between current and magnetic field, but in opposite cause-and-effect directions.
Frequently asked questions
What is electromagnetic induction in GCSE physics?
Electromagnetic induction is the generation of a voltage (EMF) in a conductor when the magnetic flux through it changes. This happens when a conductor moves through a magnetic field, when a magnet moves near a conductor, or when the current in a nearby conductor changes. The induced EMF is proportional to the rate of change of magnetic flux and to the number of turns in any coil. If the circuit is complete, the induced EMF drives an induced current.
How can you increase the induced EMF in a coil?
The induced EMF can be increased by moving the magnet faster (greater rate of flux change), using a stronger magnet, increasing the number of turns in the coil (each turn adds its contribution to the total EMF), increasing the cross-sectional area of the coil, or inserting a soft iron core to concentrate the magnetic field through the coil. Any of these changes increases the rate at which magnetic flux cuts through the coil, which is directly proportional to the induced EMF.
What is the difference between an AC generator and a DC generator?
Both use a rotating coil in a magnetic field. An AC generator uses slip rings — complete rings that allow the coil to spin continuously while maintaining contact — so the alternating voltage produced by the rotating coil reaches the external circuit as AC. A DC generator uses a split-ring commutator (a ring split into two halves), which reverses the connection at the moment the EMF would naturally reverse, so the output voltage always has the same sign — a pulsating DC. Most power station generators produce AC; battery chargers and some motors use DC.
Why does the induced current oppose the motion of the magnet?
Lenz's law states that the induced current flows in a direction that opposes the change producing it, which is a consequence of the conservation of energy. If the induced current helped the motion of the magnet rather than opposing it, the magnet would accelerate, producing more current, which would accelerate it further — energy would be created from nothing, violating conservation of energy. By opposing the motion, the system requires the operator to do work (push the magnet against the opposing force), and this work input is what provides the electrical energy output.
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