Science · Electric circuits

Parallel circuits

When one of the lamps in your home blows, why do the others keep working? Find out how each bulb in a parallel circuit has its own branch, why the brightness does not drop, and what price you pay for it.

18 min
1 simulation
Mini quiz

Updated: 22 October 2026

Preview

The question

When the lamp in your room blows, the kitchen lamp does not go out, and the fridge keeps running. Yet in a series circuit, when one bulb was removed, they all went out. So the wiring in your home must be set up in a different way. What could that be, and why is it chosen?

Try it yourself

No account needed: change the values and check your answer. Some simulations ask for a prediction before they start; it doesn't matter whether it's right or wrong.

Electric circuit

One switch in parallel branches

Tap the switches to open or close them. If the bulb lights with only Branch 2 closed, submit your answer.

Predict first

In the series circuit all the switches had to be closed; how many switches do you think must be closed in the parallel circuit to light the bulb?

It doesn’t matter whether you’re right; the simulation opens once you choose.

Lab

Series and parallel circuits

Open full page in the Lab →

Fig. 1Series and parallel circuits

Tap a switch or a bulb

Series connection

Ammeter

0.00 A

Bulb 1off
Bulb 2off
Equivalent resistanceopen circuit

The switch is open: the circuit is not complete.

Parallel connection

Ammeter

0.00 A

Bulb 1off
Bulb 2off
Equivalent resistanceopen circuit

The switch is open: the circuit is not complete.

Values

Both boards use the same parts. Type your own value, then press Enter or leave the box.

Source voltage

Bulb 1 resistance

Bulb 2 resistance

Learn

Progress is kept on this page only; no points are given.

1Predict
2Try
3Explain

On the series board, with both bulbs lit, what happens to bulb 2 if you remove bulb 1?

Purple track: copper carrying current (dots move in the conventional current direction)Gold handle: knife switchThe numbers come from the same Ohm’s law model as the worked example in this article: a 12 V source and two identical 12 Ω bulbs (resistance taken as constant). In series each bulb gets 6 V and 3 W; in parallel each gets 12 V and 12 W, so the parallel bulbs glow brighter and the source supplies four times the current. Remove a bulb: in series the path breaks, in parallel the other bulb stays lit. Change the values and the same model works it out again: in series the higher-resistance bulb glows brighter, in parallel the lower-resistance one does.

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Frequently asked

4 questions

Why doesn't adding bulbs in parallel make them dimmer?

Each bulb is connected directly to both ends of the battery on its own branch and gets the same voltage, so each one shines as brightly as a single bulb. The battery pays the price: the total current drawn from it increases, so it runs out faster.

How can I tell whether a circuit is series or parallel?

In a battery circuit, remove one of the bulbs: if the others go out too, it is series; if they stay lit, it is parallel. On a circuit diagram, trace the path of the current with your finger; if it follows a single path, it is series, and if it splits into branches and joins again, it is parallel.

Why is it dangerous to plug many devices into one extension lead?

Devices plugged into an extension lead are connected in parallel and each draws its own current; the current through the lead is the sum of them all. If powerful devices such as heaters or irons are used together, the lead can overheat and cause a fire.

What does a fuse or circuit breaker do in a home's wiring?

As more devices are connected in parallel, the total current in the line increases. A fuse or circuit breaker cuts the circuit when the current goes above a safe limit, stopping the wires from overheating. If a breaker trips, do not switch it back on without finding the cause, and report the problem to an electrician.