Electric circuits · Contrast
Series and parallel
Two bulbs, one battery, two ways to join them up. One arrangement dims both bulbs and fails completely if either one goes. The other keeps them bright and survives.
Start here
One blown bulb. The whole room stays lit.
A bulb goes in the kitchen. Nothing else in the house notices — the hall stays lit, the fridge keeps running. Yet a cheap string of decorations can lose one bulb and go dark from end to end.
What is different about the way those two sets of lamps are joined to their supply?
It is the number of paths. Every light in a house has its own branch off the supply, so a broken filament breaks one branch and nothing else. A cheap decoration string threads every bulb onto a single loop, so one broken filament is a gap in the only path there is, and all hundred go out at once.
There are exactly two ways to add a second component to a circuit. You can put it in the same loop, so the charge has to go through one and then the other — that is series. Or you can give it a loop of its own off the same battery, so the charge can go through one or the other — that is parallel.
Counting paths is how you tell them apart, and it is more reliable than looking at the shape of the drawing. Follow a route from one end of the battery to the other. If there is only one route, everything on it is in series. If there is a point where the route divides and later joins back up, the branches are in parallel.
The consequences are large. In series there is one current, and every extra component makes it harder for the charge to get round, so everything dims. In parallel each branch gets the battery's full push, so two lamps are as bright as one — and each branch is independent, so a break in one leaves the others working.
At the bench · two identical bulbs, one 3.0 V battery
Rewire it. Then take one out.
Change a control to begin
The same two bulbs and the same battery every time. Only the wiring changes. An ammeter beside the battery reads the total current leaving it.
Commit first. One bulb on this battery draws 0.30 A. You wire the two bulbs in parallel. What total current leaves the battery?
How they are wired
Unscrew a bulb
Paths from + to −
—
—
Total current from the battery
—
Bulb 1
—
—
Bulb 2
—
—
The figure
The same two bulbs, judged five ways
| Series | Parallel | |
|---|---|---|
| Paths the charge can take | One, through both bulbs | Two, one through each bulb |
| Current from the battery | 0.15 A — halved by the second bulb | 0.60 A — doubled by the second branch |
| Brightness of each bulb | Both dim | Both at full brightness |
| If one bulb fails | Both go out — the only path is broken | The other stays lit — its path is untouched |
| Where you meet it | A torch, a set of fairy lights, a dimmer chain, a fuse | Every socket and light in a house, every lamp in a car |
Series is not a worse circuit — it is a different tool. Anything that has to switch or protect a whole loop belongs in series with it, which is exactly where a switch and a fuse go. Anything that has to work on its own belongs in parallel.
Key fact
In series there is one path: one current through everything, and a break anywhere stops the lot. In parallel there is a path for each branch: every branch gets the battery's full push, the battery supplies the branch currents added together, and a break in one branch leaves the others working.
Think again
“In parallel the current has to split between the two bulbs, so each one is dimmer.”
The current does split — but it is not a fixed amount being shared out. Each branch decides for itself how much it draws, because each branch has the battery's full push across it, and one bulb on 3.0 V draws 0.30 A whether or not there is another bulb next to it. The battery does not ration the current; it supplies whatever the branches ask for, which here is 0.60 A. That is why adding lamps in parallel makes the battery flatten sooner rather than making the lamps dimmer.
“In series the first bulb gets the current first, so it is brighter than the second one.”
There is no first. The current is the same at every point of a single loop, at the same instant, so two identical bulbs in series are equally dim and swapping them over changes nothing. The charge does not queue up and arrive somewhere sooner: it is already spread all the way round, and it all starts moving together.
Mastery ladder
Not started yet.
Rungs 3 and 4 you mark yourself.
Rung 1 · Read the circuit
Three identical lamps are wired in parallel across a 6 V battery. Each one on its own draws 0.20 A. What total current leaves the battery, and what happens if the middle lamp is unscrewed?
Rung 2 · The one that catches people
A student adds a second identical bulb in series with the first and predicts each bulb will be as bright as before, because the battery has not changed. What is wrong?
Rung 3 · Explain
Two identical bulbs are wired in series and then rewired in parallel across the same battery. Explain what happens to the brightness of each bulb and to the current leaving the battery, and why.
Rung 4 · Take it somewhere new
A car has two headlamps, two brake lamps and one ignition switch that must kill everything. Say how you would wire it, and justify each choice by what would happen if you did it the other way.
Key note
Components in series sit in one loop, so one current passes through all of them and a break anywhere stops everything. Components in parallel sit on separate branches of the same battery: each branch has the battery's full push across it, the battery supplies the branch currents added together, and a break in one branch leaves the others working. To tell which you have, count the paths from one end of the battery to the other.
Going further
Cheap decorative light strings are wired in series, and the reason is money: in series the supply voltage is shared out between all the lamps, so each one can be a tiny low-voltage bulb instead of a mains-rated one. The price is the failure you know about. Better strings hide a trick — each bulb carries a small piece of coated wire called a shunt, which does nothing while the filament works and conducts once the filament breaks, keeping the loop closed. The string stays lit, and the surviving bulbs each get a slightly bigger share of the voltage, which is why the failures then start to come faster.
A house is parallel from the meter outwards, and every branch runs at the same 230 V. That is what makes appliances interchangeable: a kettle designed for 230 V works in any socket, in any room, whatever else is switched on. It also means the currents add up at the fuse box, which is the whole reason a house has fuses and circuit breakers at all.
Before this lesson
Connects to
At GCSE this becomes
- Adding resistances in series and in parallel, the rules for current and potential difference in each, and why a parallel combination resists less than either branch alone.
Where to next
Ask Mr Badmus AI
Got a circuit diagram and not sure whether it is series or parallel?
The bench is a teaching model. Both bulbs are treated as identical and as having a fixed resistance of 10 ohms, so one on 3.0 V draws 0.30 A exactly; a real filament's resistance rises as it heats, so two real bulbs in series draw somewhat more than a quarter of the power of one and the dimming is less extreme than the arithmetic suggests. The battery is treated as having no resistance of its own, which is why the parallel reading doubles cleanly; a real cell sags a little under the larger current. An unscrewed bulb is drawn as a clean gap.
Lesson content © MrBadmusAI.