Electromagnetic Induction and Transformers
Electromagnetic induction for Cambridge IGCSE Physics 0625. Induced voltage, the generator, transformers and why the national grid uses high voltage.
What you need to know
- A voltage is induced when a conductor cuts magnetic field lines, or when the magnetic field through a coil changes.
- No relative movement and no changing field means no induced voltage. A magnet resting inside a coil induces nothing.
- The size of the induced voltage increases with a stronger magnet, more turns on the coil, and faster movement.
- A transformer only works with alternating current, because it needs a continuously changing magnetic field in the core.
- The grid transmits at high voltage so the current is low, and low current means far less energy wasted as heat in the cables, because power loss is I squared R.
Equations
- Vp / Vs = Np / Ns
- for an ideal transformer Vp x Ip = Vs x Is
- power loss in a cable = I^2 R
Electromagnetic Induction and Transformers questions students ask
How do you induce a voltage in a conductor?
Move the conductor so it cuts magnetic field lines, or change the magnetic field passing through a coil. Either way there must be a change.
Does a magnet sitting still inside a coil induce a voltage?
No. Nothing is changing, so no voltage is induced. The magnet has to move, or the field has to change.
What increases the size of the induced voltage?
A stronger magnet, more turns on the coil, and faster relative movement between the magnet and the coil.
How does a generator work?
A coil rotates in a magnetic field, so the field through it changes continuously and an alternating voltage is induced. Slip rings connect it to the circuit.
Why does a transformer not work on direct current?
A transformer needs a continuously changing magnetic field in the core. Direct current gives a steady field, so nothing is induced in the secondary coil.
What is the transformer equation?
The ratio of primary voltage to secondary voltage equals the ratio of primary turns to secondary turns. Vp over Vs equals Np over Ns.
What is a step up transformer?
One with more turns on the secondary than the primary, so the output voltage is higher than the input voltage and the current is correspondingly lower.
Why does the grid transmit at high voltage?
For a given power, high voltage means low current. Energy wasted heating the cables is I squared R, so a smaller current wastes far less energy.
Does a transformer create energy?
No. In an ideal transformer the input power equals the output power. If the voltage goes up, the current goes down by the same factor.
Mistakes that cost marks
- Saying a transformer works on direct current.
- Explaining the grid with "high voltage means less energy lost" without the step through low current and I squared R.
- Confusing the primary and secondary coils in the turns ratio.
- Writing that the coil creates current. It induces a voltage. Current only flows if the circuit is complete.
Every Cambridge and Edexcel paper from 2016 to today, with its mark scheme, no login.
Open the paper libraryLive online classes across Egypt and the Gulf. No online payment.
Request accessOnline classes