Series and parallel circuits: predicting brightness
Physics · Key Stage 2 · The School
Adding a bulb in series
Add a second bulb to a SERIES circuit, one after another in the same loop, and both bulbs glow more dimly than a single bulb did alone. The same push from the cell is now shared between two bulbs instead of powering just one, so neither gets the full share it had before.
Adding a bulb in parallel
Add a second bulb in PARALLEL instead, each on its own separate branch back to the cell, and both bulbs glow at close to full brightness, almost as bright as a single bulb alone. Each branch gets its own nearly-full share of the push, because the branches do not have to divide it between them the way a series loop does.
Why the two are so different
In series, every bulb sits on the SAME path, so they share one push between them. In parallel, every bulb sits on its OWN path, so each one gets close to the full push for itself. The wiring shape, not the bulbs themselves, decides how bright each one ends up.
A circuit with one bulb and one cell glows at a certain brightness. Predict what happens with a second, identical bulb added in series: both bulbs should dim, sharing the one push. Now predict a second bulb added in parallel instead: both bulbs should stay close to full brightness, since each gets its own separate path. Building both versions and testing them proves the prediction right — same cell, same two bulbs, two very different results, decided entirely by how the wires were arranged.