Electromagnetic induction
Section: Electricity & Magnetism | Syllabus: Cambridge IGCSE Physics (0625)
What is Electromagnetic Induction?
Electromagnetic induction is the production of an e.m.f. (and hence current in a closed circuit) when there is relative motion between a conductor and a magnetic field.
There are two ways to induce an e.m.f.:
- A conductor moving across a magnetic field
- A changing magnetic field linking with a conductor
Demonstrating Electromagnetic Induction
Experiment 1: Moving Wire
- Hold a wire at rest between the poles of a U-shaped magnet, connected to a sensitive centre-zero galvanometer
- Move the wire in each of six directions in turn: up, down, left, right, forwards, backwards
- Observe: the meter deflects only when the wire moves up or down — i.e. at right angles to the field, cutting the field lines
- The deflection is in opposite directions for up and down, and only lasts while the wire is moving
- Moving the wire left/right/forwards/backwards (parallel to the field, or along its own length) produces no deflection
A current is induced only when the wire moves up or down, at right angles to the field between the magnet poles
Experiment 2: Moving Magnet and Coil
- Connect a coil of wire to a galvanometer
- Move a bar magnet into the coil
- Observe: galvanometer deflects in one direction
- Move magnet out: deflection in opposite direction
- Magnet stationary inside coil: no deflection
An e.m.f. is induced only while there is relative motion between the magnet and the coil
Factors Affecting Induced E.M.F.
The magnitude of the induced e.m.f. can be increased by:
- Increasing the speed of movement (faster motion)
- Using a stronger magnet (greater field strength)
- Increasing the number of turns on the coil
- Using a larger area of coil (more field lines cut)
No e.m.f. is induced when there is no relative motion between the conductor and the magnetic field (both stationary relative to each other).
Common Mistakes
- ❌ Thinking a magnet induces an e.m.f. just by being near a coil
- ✓ an e.m.f. is only induced while there is relative motion — a magnet resting inside a stationary coil induces nothing, no matter how strong it is
Direction of Induced E.M.F.
The direction of the induced e.m.f. (and current) always opposes the change that produces it.
Lenz's Law
If you push a N pole into a coil, the induced current creates a N pole at that end of the coil, so the coil repels the incoming magnet. When the magnet is withdrawn, the top of the coil becomes a S pole instead, so the coil now attracts the magnet, hindering its removal. The induced e.m.f. and current are in opposite directions for approach and withdrawal.
The induced current always creates a magnetic pole that opposes the motion of the magnet, whether it is approaching or being withdrawn
Why the Induced E.M.F. Must Oppose the Change
This is an example of the principle of conservation of energy. If the induced current instead created a pole that attracted an approaching magnet, the magnet would accelerate inward with no effort needed — generating electrical energy from nothing. Instead, work must be done by whoever moves the magnet to overcome the opposing force, and this work is the source of the induced e.m.f.'s electrical energy.
Reversing the direction of motion or the magnetic field direction reverses the direction of the induced e.m.f.
Fleming's Right-Hand Rule (Dynamo Rule)
Use your right hand to find the relative directions of motion, magnetic field, and induced current.
- First finger = Field (N to S)
- SeCond finger = Current (induced, conventional direction)
- ThuMb = Motion (of the conductor)
Fleming's Right-Hand Rule: Field, Current, and Motion are mutually perpendicular
Left Hand vs Right Hand
| Rule | Effect | Known → Found |
|---|---|---|
| Left-hand rule | Motor effect — a cause of motion | Current + Field → Force |
| Right-hand rule | Dynamo/generator effect — an effect of motion | Motion + Field → Current |
Applications of Electromagnetic Induction
- Generators: Convert kinetic energy to electrical energy
- Transformers: Change a.c. voltages
- Microphones: Convert sound to electrical signals
- Induction hobs: Heat cooking pans using induced currents
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