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Electric circuits
2a. Series and parallel: series circuits
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Lamps in series - interactive graphic
Picture 2.1. Two lamps in series are dimmer than one on its own.
What gets used up?
You can connect several lamps in series. A single switch turns them all on and off. The switch can be anywhere in the circuit.
  • When the switch is open, no current can flow.
  • When the switch is closed, the circuit is complete and the current flows.
What happens to the brightness of the lamps?
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What happens to the brightness?
Two lamps are dimmer than a single lamp on its own.

Three lamps in series are even dimmer.

The battery pushes the current through the lamps. The more lamps there are, the harder it is for the current to flow. There is more resistance in the circuit.

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What is resistance?
With one lamp, it is easy for the current to flow. So there is a big current and the lamp is bright.

With several lamps, it is more difficult; the battery can only push a small current round the circuit. So the lamps are dim. The bigger the resistance, the smaller the current.

Resistance of decorators tool
Picture 2.2. Which one of these tubes of filler will be easier to push? The orange one because it has a smaller resistance.
What is resistance like?
You can think of resistance as being like a water pipe. A long, thin pipe makes it difficult for water to flow whereas a short, fat pipe makes it easy. The short, fat pipe has a smaller resistance and, for the same pressure, lets a bigger current flow through it.

Here's another example: look at these decorator's tools in picture 2.2. The yellow one (on top) has a long thin nozzle; the orange one (underneath) has a short, wide nozzle. The decorator squeezes the handle to force out the filler (for filling cracks in the wall).

Imagine the decorator pushes with the same force on each of these, which one will have the bigger flow?
The orange one, because the big hole has a small resistance and it is easier for the gunge to flow through.

Is the current still the same everywhere?
Notice that the second lamp is not dimmer than the first. In fact, the two lamps in series are equally bright. The same current flows through each lamp, so they are equally bright.
Effect of voltage - interactive graphic
Picture 2.3. The number of cells and lamps affects the brightness:
two cells and two lamps
two cells and one lamp
four cells and two lamps.
What is voltage?
Let's have a look at changing both voltage and resistance.
  • Voltage tells us how much a battery pushes the current.
  • Resistance tells us how difficult it is for the current to flow.

When we add a lamp in series, the resistance goes up. This reduces the current and the lamps are dimmer.

However, we can make the lamps brighter again using a battery with a bigger voltage. Now the battery is pushing harder and, even though it's more difficult, the normal current flows again.

Be careful not to make the voltage too big on a single lamp. It will burn out.

Circuit Challenge 6 - Try it now!
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Question 5
Des is making some series circuits using a set of similar lamps and cells. When he connects a single lamp to two cells, it comes on with normal brightness. He then tries different numbers of lamps and cells.

These are shown in the table below. In each case decide on the brightness of the lamps and choose the right one from the drop down list.

number of lamps number of cells brightness
1 2
2 2
2 4
2 6
3 6