Science · Electric circuits

Series circuits

What happens if you connect bulbs one after another along a single path? Find out why, in a series circuit, the bulbs get dimmer as you add more, and why they all go out when one is removed.

18 min
1 simulation
Mini quiz

Updated: 22 October 2026

Preview

The question

You may have heard that with some old-style fairy lights, when one bulb blows the whole string goes out. Why can one small bulb leave dozens of bulbs in the dark at once? And if you connect two bulbs to the same battery, does each one shine as brightly as a single bulb?

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

A series circuit with three switches

Tap the switches to open or close them. When the bulb lights, submit your answer.

Predict first

In the series circuit, what do you think happens to the bulb if you close only Switch 1?

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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

In a series circuit, is the current used up in the first bulb?

No. In a series circuit the current follows a single path and is the same at every point, so identical bulbs shine equally brightly. What the bulbs use up is not current but the energy supplied by the battery, which turns into light and heat.

Why do the bulbs get dimmer when you add more to a series circuit?

Each bulb resists the current. As more bulbs are connected in series, the total resistance grows, the same battery drives less current through the circuit, and each bulb shines more dimly. In return, the battery lasts longer.

What happens if I put one of two series batteries in backwards?

The reversed battery's voltage does not add to the other one; it cancels it out. If one of two identical 1.5 V batteries is reversed, the total voltage is almost zero and the bulb does not light. Place batteries so that the positive end of one meets the negative end of the next.

What is a series connection useful for in everyday life?

It is useful when several conditions must be met at the same time: in a circuit with two switches in series, both switches must be closed for the bulb to light. Batteries are lined up end to end in torches for the same reason, to add their voltages.