Simple circuits

A circuit is a complete loop that electricity can flow around. A cell or battery pushes the electricity, wires carry it, and components such as bulbs, buzzers and motors use it. If there is a gap anywhere in the loop, nothing works – that is exactly how a switch turns things on and off.

Key facts

  • Electricity only flows round a complete loop with no gaps.
  • A switch makes (closed) or breaks (open) a gap in the loop.
  • Cells joined in a row, the same way round, add their voltages: two 1.5 V cells give 3 V.
  • More cells for each bulb makes the bulbs brighter; more bulbs in one loop makes them dimmer.
  • Circuit symbols: a cell is a long and a short line; a bulb is a circle with a cross; a switch is a gap with a lever.
  • A bulb works best at the voltage marked on it; too much voltage can blow it.

Worked examples

Example 1

Will the bulb in this circuit light?

  1. Start at the cell and trace the wire all the way round with your finger.
  2. You reach the switch: it is open, so there is a gap.
  3. No complete loop means no electricity flows, so the bulb stays off.

Answer: No – the switch is open, so the loop has a gap.

Example 2

A torch needs 4.5 V. How many 1.5 V cells must be put in a row?

  1. One cell gives 1.5 V.
  2. Two cells in a row give 1.5 + 1.5 = 3 V.
  3. Three cells give 3 + 1.5 = 4.5 V: that is three cells.

Answer: 3 cells

Example 3

Circuit A has 2 cells and 1 bulb. Circuit B has 2 cells and 2 bulbs (all in one loop). Which has brighter bulbs, and why?

  1. A: 2 cells for 1 bulb 2 ÷ 1 = 2 cells per bulb
  2. B: 2 cells shared by 2 bulbs 2 ÷ 2 = 1 cell per bulb
  3. More push per bulb means a brighter bulb, so A is brighter.

Answer: Circuit A – each bulb gets more push.