Circuit symbols
Drawing every bulb and wire in a circuit takes time, so scientists use standard symbols.
Component | How it is drawn |
|---|---|
Electric cell | a long thin line (positive terminal) and a short thick line (negative terminal) |
Battery | two or more cells joined, long line of one to short line of the next |
Switch (key), open or OFF | a gap in the line, or a small circle not touching |
Switch, closed or ON | the line is continuous |
Bulb | a circle with a cross or a small loop inside |
Connecting wire | a straight line |
A drawing of a circuit using these symbols is a circuit diagram.
Cells and batteries
A battery is two or more cells connected with the positive terminal of one joined to the negative terminal of the next. Torches, remote controls and toys use batteries because they need more energy than a single cell gives.
Closed and open circuits
Current flows only when there is a complete (closed) path from the positive terminal, through the components, back to the negative terminal. When a switch is OFF, the circuit is open and no current flows.
The heating effect of current
When current flows through a wire, the wire gets hot. This is the heating effect of electric current.
How hot the wire gets depends on its material, length and thickness.
Electric heaters, irons, kettles, geysers and toasters contain a coil of special wire called an element, which glows red-hot.
In an incandescent bulb, a thin coiled wire called the filament, usually made of tungsten, becomes so hot it gives out light. Much of the electricity is wasted as heat. CFLs and LED bulbs give the same light using much less electricity.
Fuses and MCBs
If too much current flows, for example because of a short circuit (wires touching directly) or because too many appliances are plugged into one socket, the wires can overheat and cause a fire.
A fuse is a safety device: a short piece of special wire that melts quickly when the current is too high, breaking the circuit. A fuse must have the right rating; replacing it with an ordinary thick wire is dangerous.
Modern buildings use miniature circuit breakers (MCBs), switches that turn off automatically when the current is too large. They can simply be switched on again once the fault is fixed.
The magnetic effect of current
Place a compass near a wire and switch on the current. The needle moves. A wire carrying current behaves like a magnet. This was first noticed by Hans Christian Oersted.
Electromagnets
Wind insulated copper wire around an iron nail and connect it to a cell. The nail now attracts pins. Switch the current off and the pins fall. This is an electromagnet: a magnet that works only while current flows.
Electromagnet | Permanent magnet |
|---|---|
magnetism can be switched on and off | always magnetic |
strength can be increased with more turns or more current | fixed strength |
Electromagnets are used in cranes to lift scrap iron, in loudspeakers, and by doctors to remove small iron pieces from the eye.
The electric bell
An electric bell uses an electromagnet:
Pressing the switch completes the circuit, and the coil becomes an electromagnet.
It pulls the iron strip, called the armature, and the attached hammer strikes the gong.
As the armature moves, it breaks contact with a screw, so the circuit is broken and the coil stops being a magnet.
The armature springs back, touches the screw, completes the circuit again, and the cycle repeats rapidly. The bell rings continuously.
Summary
Circuits are drawn with standard symbols; current flows only in a closed circuit.
Current heats a wire: used in heaters and bulbs, and guarded against by fuses and MCBs.
Current makes a wire act like a magnet: used in electromagnets and electric bells.