Physics

18.2 - Series Circuits

7 min

What a series circuit is

P.d. and current in series

Resistance in series

P.d. across one component

note 1 of 4

What a series circuit is

In a series circuit, the components are connected end to end in a single loop between the positive and negative ends of the power supply.
If one component is removed or breaks, the circuit is broken and all the components stop working.

A battery, switch and resistor joined in one loop labelled single loop, and after an arrow the same circuit in grey with the resistor lifted out, labelled removed and circuit is broken.

Connecting meters

An ammeter is connected in series, so the current it measures flows through it.
A voltmeter is connected in parallel across a component.

Two circuits split by a dashed line: on the left an ammeter in the loop with a battery and lamp, labelled in series; on the right a voltmeter across the lamp, labelled in parallel.

note 2 of 4

The series rules - E.m.f., current and resistance

Current is the same everywhere

The same current flows through every part of a series circuit:

$I_{1} = I_{2} = \dots$

The size of the current depends on the total e.m.f. and total resistance ( $I = \frac{V}{R}$ ).

Resistance adds up

The total resistance is the sum of the individual resistances:

$R_{total} = R_{1} + R_{2} + \dots$

The source e.m.f. is shared

The e.m.f. of the supply is shared between the components, so the potential differences round the circuit add up to the source e.m.f.:

$V = V_{1} + V_{2}$

note 3 of 4

Worked example - Current in a series circuit

A 24 V supply is connected in series with two resistors of 3 Ω and 5 Ω. Calculate the current in the circuit.

Step 1 - Find the total resistance

The resistors are in series, so add them:

$R_{total} = R_{1} + R_{2} = 3 + 5 = 8 Ω$

Step 2 - Rearrange $V = I R$ to make $I$ the subject

$I = \frac{V}{R}$

Step 3 - Substitute the values

Use the supply voltage and the total resistance:

$I = \frac{V}{R} = \frac{24}{8} = 3 A$

The current is 3 A, and because this is a series circuit, the same 3 A flows through both resistors.

note 4 of 4

Source e.m.f.s add up in series

When energy sources, such as cells, are connected in series the same way round, their e.m.f.s add together.
For example, two cells each with an e.m.f. of 1.5 V connected in series supply a total e.m.f. of 3 V.

Two loops divided by a dashed line, each with a lamp and switch: the left has one cell labelled 1.5 V, the right two cells labelled 1.5 V + 1.5 V = 3 V.

key terms

TermDefinition

Series circuitA circuit where the components are connected end to end in a single loop. Components connected like this are said to be in series.

end of notes

Key points to remember

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18.3 - Parallel CircuitsNext up