Physics GCSE: Magnetism and Electromagnetism

Generating electricity

Creating currents

You can see that any current in a magnetic field will produce a force, which may make something move.

The opposite affect is that if a wire is moved in a magnetic field, a voltage is produced, and if there is a complete loop, a current will flow. This is how electricity is generated.

There are two main ways to generate electricity:

1. Moving a wire in a magnetic field:

• If the wire is moved in the opposite direction, the current will flow the other way.
• If the wire is moved faster, a larger current will flow.
• If the wire is stationary, no current will flow.

2. Moving a magnet in a coil of wire:

• If the magnet changes direction, the current will flow the other way.
• If the magnetic poles are swapped around, the current will flow the other way.
• If the magnet moves faster, a larger current will flow.
• If the magnet is stationary, no current will flow. 

Generating electricity 

In industry, electricity is generated by spinning a coil of wire in a magnetic field.

To increase the voltage or current generated:

1. Spin the coil faster.
2. Put more loops on the coil.
3. Use a stronger magnetic field.
4. Use a coil with a larger area.

When a current is generated we say that it has been induced in a conductor.

Note: A voltage is always induced when a conductor moves in a magnetic field, but a current is only induced if there is a complete circuit for it to flow around.

In the diagram, you can see that there are carbon brushes and slip rings.

The carbon brushes act as contacts with the slip rings, so that the current can always flow from the coil to an external circuit.

Why is alternating current produced?

The current produced is an alternating current as during each rotation, each side of the coil moves up and down in the magnetic field. This change in direction produces a change in direction of the current. 

Transformers

Transformers are able to change the voltage of an alternating current. This is used on the national grid. The larger the voltage, the lower the amount of wasted heat energy in the cables. However, these large voltages are too dangerous to use in the home, so transformers are used to reduce the voltage to a safe level.

An alternating current has a changing magnetic field around it. This changing magnetic field can induce a current in another nearby conductor. This is how a transformer works.

The alternating current in the primary coil has a changing magnetic field around it. This changing field induces an alternating current and voltage in the secondary coil. The size of this voltage depends on the difference in the number of loops on the coils.

Calculating the size of the output voltage

You can work out the size of the voltage using the following equation:

This means that if there are twice as many loops on the secondary coil, then twice the voltage will be across the secondary coil, and so on.

Have a go filling in the following table: