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Circuit symbols

Electricity and magnetism · level 2, lesson 4 of 6

A question to start

Drawing a real battery, bulb and wires takes ages. How do engineers draw a circuit quickly, so anyone in the world can read it?

The idea

They use circuit symbols: simple shapes that everyone agrees on.

  • Cell (a single battery): a long line and a short line. The long line is the + end.
  • Battery: two or more cells joined together.
  • Bulb: a circle with a cross in it.
  • Switch: a line with a gap, and a little lever that can close it.
  • Wire: a straight line. Wires are drawn with straight edges and square corners.
+ long linecellswitchbulb

Worked examples

1. Reading a diagram. The diagram above shows one cell, one switch and one bulb in a single loop. With the switch closed, the bulb lights.

2. Drawing a torch. Two cells, a switch and a bulb in one loop: draw two cell symbols side by side, then the switch and the bulb.

3. Spot the gap. A diagram with the switch drawn open shows a circuit that is off.

Try it

  1. Which line of a cell symbol is the + end?
  2. Draw the symbol for a bulb.
  3. What is the difference between a cell and a battery?
  4. Why do engineers use symbols instead of pictures?
Answers
  1. The long line.
  2. A circle with a cross inside it.
  3. A cell is one; a battery is two or more cells joined together.
  4. They are quick to draw and everyone reads them the same way.

Common mistake

Drawing wires that do not join. In a circuit diagram, every wire must meet the next part, with no gaps, unless you mean an open switch.

Where it leads