Electromagnetic InductionEdexcel IGCSE Physics: Revision notes
Section 1
What is electromagnetic induction?
Electromagnetic induction is the production of a voltage (and current, in a complete circuit) in a conductor when it experiences a changing magnetic field. This happens when:
- a conductor (wire or coil) moves through a magnetic field, or
- the magnetic field through a stationary conductor changes (e.g. a magnet is pushed in and out of a coil).
The size of the induced voltage is increased by: moving the conductor/magnet faster, using a stronger magnet, and using a coil with more turns.
When asked for factors affecting induced voltage, always give at least two: speed of movement/change, strength of magnetic field, and number of turns on the coil.
Section 2
Generating electricity
Electricity generators use electromagnetic induction by either:
- rotating a magnet inside a coil of wire, or
- rotating a coil of wire within a magnetic field.
Either method continuously changes the magnetic field experienced by the coil, inducing a voltage. Because the coil (or magnet) rotates, the direction of the induced voltage repeatedly reverses — this is why generators produce alternating current (a.c.).
The induced voltage can be increased by spinning faster, using a stronger magnet, or increasing the number of turns on the coil.
A bicycle dynamo induces a voltage by rotating a small magnet next to a coil as the wheel turns — pedal faster and the lamp shines brighter, because the field changes more rapidly.
Section 3
Transformers
A transformer changes the size of an alternating voltage. It consists of a primary coil and a secondary coil, both wound around a common iron core, but not electrically connected to each other.
An alternating current in the primary coil creates a constantly changing magnetic field in the iron core. This changing field induces an alternating voltage in the secondary coil — transformers only work with a.c., because a constant (d.c.) current produces no changing field.
- A step-up transformer has more turns on the secondary coil than the primary, and increases voltage.
- A step-down transformer has fewer turns on the secondary coil than the primary, and decreases voltage.
A common error is thinking transformers work with direct current (d.c.) — they only work with alternating current, since a changing field is essential.
Section 4
Transformer equations and power transmission
The turns ratio relates the primary and secondary voltages:
Vp / Vs = Np / Ns
For an ideal (100% efficient) transformer, input power equals output power:
Vp × Ip = Vs × Is
In the National Grid, step-up transformers raise the voltage (and lower the current) for long-distance transmission, and step-down transformers reduce it again for safe domestic use. Transmitting at high voltage and low current reduces energy losses to heating in the cables, since heating losses depend on the current, not the voltage.
A transformer has 100 turns on the primary and 500 on the secondary, with 230 V input. Vs = Vp × (Ns/Np) = 230 × (500/100) = 1150 V.
Must Know
- a voltage is induced when a conductor moves through a magnetic field, or when the field through it changes
- induced voltage increases with faster movement, a stronger magnet, and more coil turns
- generators rotate a magnet in a coil (or a coil in a field) to produce a.c.
- transformers only work with a.c., using a changing magnetic field in a shared iron core
- Vp/Vs = Np/Ns; step-up transformers have Ns > Np, step-down have Ns < Np
- for 100% efficiency: Vp × Ip = Vs × Is
- the National Grid uses step-up then step-down transformers to transmit at high voltage/low current, minimising energy loss
That's the notes covered.
Carry on to the next subtopic.