Circuits in series and parallel
Design & Technology · Key Stage 2 · The School
One path, or more than one
A SERIES circuit has just one path for the current. Cell, bulb, bulb, back to the cell. A PARALLEL circuit splits into more than one path. Current reaches each bulb by its own separate route. Both circuits can share the very same cell. Looking at how a circuit is drawn usually shows straight away which kind it is.
What happens if a bulb fails
In a series circuit, one broken bulb breaks the whole loop. Everything on it stops. In a parallel circuit, one broken bulb only breaks its own branch. The other branches keep working. Their path back to the cell never passed through the broken one. This single fact explains almost every real choice between the two.
Choosing which to use
Old strings of fairy lights were wired in series. That is why the whole string went dark from one blown bulb. Modern house wiring is parallel instead. One broken lamp does not turn off the whole house. The choice depends on one thing: should one failure be allowed to stop everything else? That single question decides almost every real circuit design.
Twenty fairy lights are wired in series. One bulb blows. The whole string goes dark. There was only ever one path, and it is now broken. Now rewire the same twenty lights in parallel instead. Each bulb gets its own path back to the cell. One blown bulb now only removes its own light. The other nineteen carry on exactly as before.