Electric circuits, conductors and insulators

Press a switch and a bulb lights up. Behind this simple act is an electric circuit: a complete, unbroken path along which electric current can flow.

Parts of a simple circuit

• A source: an electric cell or a battery (two or more cells joined together). A cell has two terminals, positive (+) and negative (−). Chemicals inside it provide the push that drives the current.
• Connecting wires: usually copper, to carry the current.
• A device: something that uses the electricity, such as a bulb, a buzzer or a motor.
• A switch: to open or close the path.

Closed and open circuits

When the switch is ON, the path is complete. Current flows from the positive terminal of the cell, through the wire, the bulb and the switch, back to the negative terminal. This is a closed circuit, and the bulb glows.

When the switch is OFF, there is a gap in the path. Current cannot jump across the gap, so it stops everywhere at once. This is an open circuit, and the bulb goes off. A loose wire or a broken filament in a bulb also breaks the circuit; we then say the bulb has "fused".

Conductors and insulators

Materials that allow electric current to pass through them easily are called conductors. Metals such as copper, aluminium, iron and silver are good conductors. So are the graphite in a pencil lead, salty or impure water and the human body.

Materials that do not allow current to pass are called insulators. Rubber, plastic, dry wood, glass and dry cloth are insulators.

We need both. Wires are made of copper so that current flows freely, and they are covered with plastic so that we can touch them safely. Switches, plug tops and the handles of an electrician's tools are made of or coated with insulating material.

Circuit symbols

To draw circuits quickly, scientists use symbols. A cell is shown as two parallel lines, a long one for positive and a short thick one for negative. A bulb is a circle with a cross or a loop, a wire is a straight line, and a switch is a break in the line with a small arm.

Safety with electricity

The cells used in torches and toys are safe to experiment with. Electricity from wall sockets is very dangerous and can kill.

• Never touch a switch or an appliance with wet hands.
• Never push fingers, pins or wires into a socket.
• Do not fly kites near electric lines or touch a fallen wire.
• If someone gets a shock, do not touch the person directly. Switch off the mains or push the person away with a dry wooden stick, and call an adult.