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Energy and Voltage in Circuits » Series and Parallel Circuits

What you'll learn this session

Study time: 30 minutes

Edexcel spec: 2.7

What a series circuit and a parallel circuit are, and how to describe each one
How lamps behave in each kind of circuit
Why the lights and sockets in a house are wired in parallel
When a series connection is the right choice

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Two ways to connect a circuit

One battery and two lamps can be joined up in two different ways, and the lamps behave very differently in each. In the exam you must say which way suits a job, and explain why.

Key terms:

Series circuit: a circuit with only one loop, so there is only one path for the current. Every component is on that one path.
Parallel circuit: a circuit with more than one path for the current. The paths split apart at one junction and join again at another.
Branch: one of the separate paths in a parallel circuit.
Domestic lighting: the lighting in a home.

Picturing the two circuits

Sketch each circuit as you read.

🔁 Two lamps in series

Draw a rectangle of wire. Put a battery on the top side. Put a switch and two lamps on the other sides, one after another.

Trace round from the battery with your finger. You pass through the switch and both lamps, then arrive back at the battery. There is no other route.

🔀 Two lamps in parallel

Draw a rectangle of wire with a battery on the top side and one lamp on the bottom side. Now draw a second wire across the middle of the rectangle, from the left side to the right side, and put the second lamp on it.

Trace round from the battery. At the first junction you can go through one lamp or the other. Each lamp is on its own branch.

A quick test for any circuit diagram: count the routes from one side of the supply to the other. One route means series. Two or more routes mean parallel.

How lamps behave in series

Everything depends on the single loop. Learn these three results.

One switch controls everything. Open the switch and the only path is broken, so every lamp goes off together. You cannot switch one lamp off and leave the other on.
If one lamp fails, they all go out. A lamp that has 'blown' has a break inside it. That break is a gap in the only loop, so the current stops everywhere.
The lamps share the supply voltage. The voltage of the battery is split between the lamps, so each lamp gets only part of it. Each lamp is dimmer than it would be if it were connected to the battery by itself.

How lamps behave in parallel

Each branch is connected straight across the supply. This turns all three results round.

Each lamp gets the full supply voltage. The lamps do not have to share. Each one is as bright as it would be if it were connected to the battery by itself, and adding another lamp on a new branch does not make the others dimmer.
Each lamp can be switched separately. Put a switch on a branch and it controls only the lamp on that branch.
If one lamp fails, the others keep working. The break is in one branch only. The other branches are still complete paths, so the current still flows through them.

	Lamps in series	Lamps in parallel
Paths for the current	One loop	A separate branch for each lamp
Voltage for each lamp	A share of the supply voltage	The full supply voltage
Brightness	Dimmer	Full brightness
Switching	All on or all off together	Each lamp can have its own switch
One lamp fails	All the lamps go out	The other lamps stay on

Why house lights and sockets are wired in parallel

The spec names domestic lighting, so expect a question on it. The lights in a house are wired in parallel with each other.

💡 Full brightness

Mains lamps are made to work on the full mains voltage. In parallel every lamp gets it. In series the lamps would share the mains voltage and every one would be dim.

🏠 Separate switches

Each lamp is on its own branch with its own switch. You can turn the kitchen light on and leave the bedroom light off. In series it would be every light in the house or none.

✅ One fails, the rest work

When the landing lamp blows, the other rooms stay lit. In series the whole house would go dark.

The sockets in a house are wired in parallel for the same reasons. Every appliance you plug in gets the full mains voltage it was made for. You can switch the television off without stopping the fridge. If one appliance breaks, the others carry on working.

When series is the right choice

Sometimes you want one component to affect another. Then series is exactly right.

A switch goes in series with the thing it controls. When the switch is opened, it must break the path to that lamp, so it must be on the same path.
A fuse goes in series with the appliance it protects. When the fuse melts, it has to stop the current in the appliance. It can only do that if the same current passes through both.
A variable resistor goes in series with a lamp to make a dimmer, or with a motor to control its speed. It only changes the current in the lamp or motor if it is on the same path.
Decorative lights are sometimes wired as a string of many small lamps in series. Each small lamp needs only a low voltage. In series they share the supply voltage, so each lamp gets the small share it was made for. The cost is that one failed lamp can put the whole string out.

So a house uses both ideas. The lamps are in parallel with each other, but each lamp has its own switch in series with it, on its own branch.

Worked example

A bathroom has a ceiling lamp and an extractor fan. One switch must turn the lamp and the fan on and off together. Explain how to connect the switch.

Step 1: decide what the switch must control. Here it is both the lamp and the fan.

Step 2: put the switch in series with everything it controls. Connect the lamp and the fan on two separate branches, in parallel, so that each gets the full supply voltage. Put the switch on the single wire before the junction where the branches split.

Step 3: check it. Opening the switch breaks the path to both branches, so both go off together. If the lamp blows, the fan's branch is still complete, so the fan still works.

Common mistakes

Writing only 'parallel is better'. That gets no marks. Give the reason: full voltage, separate switching, or the others still work if one fails.
Saying that lamps in series are dimmer 'because the current gets used up'. Current is not used up. They are dimmer because they share the supply voltage.
Saying that a blown lamp in series 'takes all the current'. A blown lamp is a break in the circuit, so there is no current anywhere.
Deciding by how the diagram looks on the page. Count the paths for the current instead.
Putting a switch or fuse in parallel with the appliance. It must be in series with the thing it controls or protects.

Exam-style question

A camper van has two identical lamps that run from a 12 V battery. Each lamp is made to work at its normal brightness on 12 V.

(a) Describe how the two lamps and the battery are connected in a series circuit. [1 mark]

(b) The owner connects the two lamps in parallel instead. Explain two advantages of connecting the lamps in parallel. [4 marks]

(c) The owner wants to be able to make one of the lamps dimmer without changing the other lamp. State the component that is needed and explain where it must be connected. [3 marks]

Model answer

(a) The lamps and the battery are connected one after another in a single loop, so there is only one path for the current (1).

(b) Each lamp gets the full 12 V of the battery (1), so each lamp is at its normal brightness, not dim (1). Each lamp is on its own branch (1), so if one lamp fails the other one stays on, or so each lamp can be switched on and off separately (1).

(c) A variable resistor (1), connected in series with that lamp (1), on that lamp's own branch so that it does not change the current in the other lamp (1).

Exam tip

Part (b) says 'explain', and it has 4 marks for two advantages. That is one mark for each fact about the circuit (full voltage, own branch) and one for what it means for the lamps (normal brightness, the other stays on). A list of two advantages with no reasons would get only half the marks.

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