Electric charge

Have you ever heard a crackle when taking off your jumper? This is caused by tiny electric charges on your clothes. It's called static electricity.

Your jumper and shirt get electrically charged as they rub together, and then negative electric charges jump from one to the other. This makes sparks that crackle - you can even see them in a dark room.

Electric current

When electric charges move in a wire, we say that an electric current flows in the wire. It's like the way a current of water flows in a river.

For an electric current to flow, we need two things:

• something to make the electricity flow, such as a battery or power pack

• a complete path for the current to flow in. This is called an electric circuit.

Electric circuits

An electric current will not flow if we do not have a power source (a cell, battery or power pack). It also won't flow if the circuit is not complete. One end of the power source must be joined to the other end by the wires and components of the circuit.

The simplest complete circuit is a piece of wire from one end of a battery to the other. An electric current can flow in the wire from one end of the battery to the other, but nothing useful happens. The wire just gets hot and the battery goes flat.

To do something useful with the electric current, we need to put an electrical component into the circuit, such as a lamp or motor, that can use the current to make something happen.

The bulb will only light if there is a battery and a complete circuit

We usually add in a switch to the circuit, so that we can break the circuit and stop the electric current when we want to.

Circuit diagrams

The idea of a circuit diagram is to use circuit symbols instead of drawing each component in the circuit. Always try to make the wires straight lines, and don't be tempted to make them wiggly.

The whole point is to make it easier to see what is connected to what. Here you can see how the symbols for a cell (not a battery!) and a lamp look in a circuit diagram.

If you have to draw a circuit diagram from scratch, it is usually easier to draw the circuit symbols first, and then add all the wires. If you have to draw wires to join circuit symbols that are already shown, use a ruler and don't let the wires cross each other

Series & parallel circuits

There are two types of circuit we can make, called series and parallel.

The components in a circuit are joined by wires.

• if there are no branches then it's a series circuit

• if there are branches it's a parallel circuit

Series circuits

In a television series, you get several episodes, one after the other. A series circuit is similar. You get several components one after the other.

If you follow the circuit diagram from one side of the cell to the other, you should pass through all the different components, one after the other, without any branches.

If you put more lamps into a series circuit, the lamps will be dimmer than before.

In a series circuit, if a lamp breaks or a component is disconnected, the circuit is broken and all the components stop working.

Series circuits are useful if you want a warning that one of the components in the circuit has failed. They also use less wiring than parallel circuits.

Parallel circuits

In parallel circuits different components are connected on different branches of the wire. If you follow the circuit diagram from one side of the cell to the other, you can only pass through all the different components if you follow all the branches.

In a parallel circuit, if a lamp breaks or a component is disconnected from one parallel wire, the components on different branches keep working. And, unlike a series circuit, the lamps stay bright if you add more lamps in parallel.

Parallel circuits are useful if you want everything to work, even if one component has failed. This is why our homes are wired up with parallel circuits.

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