Series and parallel circuits

Jonny Nelson introduces an animated explanation of circuits

A series circuit only has one loop. The current flowing can be measured with an ammeter connected in series in the circuit.

Series circuits

The current flowing in a series circuit is the same everywhere.

The potential difference across a component in a series circuit depends on its resistance. Energy is conserved in circuits, so the total potential difference for a series circuit is:

  • the sum of the potential differences across the individual components
  • the same as the potential difference of the cell
In series circuits, the current is the same through each component. The potential difference of the power supply is shared between the components.

Parallel circuits

Parallel circuits have more than one loop.

An electron will not pass through every component on its way round the circuit. The diagram shows a circuit with two bulbs in parallel.

Circuit containing a switch, five ammeters and cell, all connected in series, and two lamps connected in parallel. Four of the ammeters are labelled as I1, I2, I3 and I4.A parallel circuit showing two bulbs, a switch and a cell. An ammeter takes readings near the cell.

If one of the bulbs is broken, the current can still flow round the circuit through the other loop, so the other bulb stays on.

The lights in a house are wired this way. If one light breaks, the others continue to work. If the lights were wired in series instead, all the lights would go out if one breaks. In a parallel circuit, measurements of the current at different points in the circuit should add up to the ammeter reading close to the cell.

In the circuit above:

  • (current at I1) = (current at I3)
  • (current at I2) + (current at I4) = (current at I1 or I3)

Since energy has to be conserved, the energy transferred around the circuit by the electrons is the same whichever path the electrons follow. This means that the potential difference across the components in a parallel circuit is always the same value.

In parallel circuits, the potential difference is the same across each loop. The total current supplied is shared between the components on different loops.