Electricity is generated when a coil of wire moves in a magnetic field. This is the basis of electricity generators.

Most electricity is made in power stations by burning fuels. Transformers are used in the National Grid to reduce energy losses from the wires during transmission.

The dynamo effect

An electric current is produced when a magnet is moved into a coil of wire in a circuit. The direction of the current is reversed when the magnet is moved out of the coil. It can also be reversed if the other pole of the magnet is moved into the coil.

Notice that no current is induced when the magnet is still, even if it is inside the coil.

Increasing the induced current

To increase the induced current:

• move the magnet more quickly
• use a stronger magnet
• increase the number of turns on the coil

Batteries produce direct current, or DC, electricity. The current flows in the same direction all of the time, as shown by this typical voltage-time graph.

Generators produce alternating current, or AC, electricity. The current reverses direction regularly, as shown by this typical voltage-time graph.

Mains electricity is AC. It has a frequency of 50Hz - 50 cycles per second.

When a wire is moved in a magnetic field, the movement, magnetic field and current are all at right angles to each other. If the wire is moved in the opposite direction, the induced current also moves in the opposite direction.

One side of a coil in a generator moves up during one half-turn, and then down during the next half-turn. This means that, as a coil is rotated in a magnetic field, the induced current reverses direction every half-turn.

There are four main stages:

1. the fuel is burned to boil water to make steam
2. the steam makes a turbine spin
3. the spinning turbine turns a generator which produces electricity
4. the electricity goes to the transformers to produce the correct voltage

Power stations generate significant heat losses to the environment. Below is an energy transfer diagram for the generation of electricity from a fossil fuel such as coal.

Transformers

A transformer is an electrical device that changes the voltage of an AC supply. A transformer changes a high-voltage supply into a low-voltage one, or vice versa:

• a transformer that increases the voltage is called a step-up transformer
• a transformer that decreases the voltage is called a step-down transformer

Step-down transformers are used in mains adapters and rechargers for mobile phones and CD players.

The National Grid

When a current flows through a wire, some energy is lost as heat. The higher the current, the more heat is lost. To reduce these losses, the National Grid transmits electricity at a low current. This requires a high voltage.

Power stations produce electricity at 25,000V. Electricity is sent through the National Grid cables at 400,000V, 275,000V and 132,000V.

Step-up transformers at power stations produce the very high voltages needed to transmit electricity through the National Grid power lines. These high voltages are too dangerous to use in the home, so step-down transformers are used locally to reduce the voltage to safe levels.

The energy input and output of a power station can be summarised by this equation:

fuel energy input = waste energy output + electrical energy output

For a given input of energy from a fuel, the more waste energy output, the less electrical energy output.

efficiency = electrical energy output ÷ fuel energy input

In the example above:

efficiency = 400 ÷ 1,000 = 0.4