Making electricity.

Making electricity

You can make an electric current in a wire if a wire is at right angles to a magnetic field and the magnetic field is changing. We say that the electric current has been induced.

You can change the magnetism by moving the wire past the magnet, or moving the magnet past the wire. It will show up better on a meter if you use a coil of wire.

A current is generated only when the magnet is moving.

The current stops if the magnet stops moving - even if it is inside the coil.

The current goes the other way if the magnet moves in the opposite direction

To make the current greater:

use more coils
move the magnet faster
use a stronger magnet

The magnetic field goes around each loop of wire in the coil, so if you increase the number of coils there are more places where the magnetism changes.

The faster the magnet moves, the faster the magnetism changes.

If you keep the magnet still and move the wire, you induce a current in exactly the same way.

Generators

Generators induce a current by moving a magnet inside a coil of wire, or by moving a coil of wire ...

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To make the current greater:

use more coils
move the magnet faster
use a stronger magnet

The magnetic field goes around each loop of wire in the coil, so if you increase the number of coils there are more places where the magnetism changes.

The faster the magnet moves, the faster the magnetism changes.

If you keep the magnet still and move the wire, you induce a current in exactly the same way.

Generators

Generators induce a current by moving a magnet inside a coil of wire, or by moving a coil of wire inside a magnetic field. One example of a generator is a bicycle dynamo.

Bicycles sometimes have dynamos which rub against the back tyre. As the bike moves, a wheel on the top of the dynamo turns a magnet inside a coil.

Generators from motors

Simple electric motors generate electric currents when you spin them. Instead of using electric current to turn a coil, turning the coil produces an electric current.

Transformers

Making the magnet stronger and weaker will also produce a changing magnetic field. You can do this by changing the current in an electromagnet (the primary coil). If another coil of wire (the secondary coil) is within the magnetic field of the primary coil, the changing magnetic field will go past the secondary coil and induce a current.

Voltage and transformers

The main use of transformers is to change the voltage of an a.c. supply. Remember that the magnetic field must always be changing, so transformers will only work on a.c.

The equation linking voltages is:

Voltage across coil 1/voltage across coil 2 = number of turns in coil 1/number of turns in coil 2

Transformers in real life

Transformers are not 100% efficient. As the current flows through the coils some heat will be produced. The output of a real transformer will be less than predicted by this calculation, so sometimes you will be asked for the percentage efficiency of a transformer.

The National Grid

Electricity is carried around the country by the National Grid. There is less energy loss at high voltages, so the electricity is sent through the wires at 275kV and 400kV. Power stations usually work at 25kV, so they use transformers to increase the voltage. Transformers are also used at the other end to bring the voltage down to 240V.