Physics

2.5 - 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

Series rules - Potential difference, current and resistance

Potential difference is shared

The total potential difference of the supply is shared between the components:

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

The bigger a component's resistance, the bigger its share of the total potential difference.

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 potential difference and total resistance ( $I = \frac{V}{R}$ ).

Resistance adds up

The total resistance is the sum of the individual resistances:

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

Adding another resistor in series increases the total resistance, reducing the total current.

A loop with a battery labelled V total, an ammeter each side labelled I1 and I2, and two resistors R1 and R2 along the bottom, each with a voltmeter across it, V1 and V2.

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

Cell potential differences add up in series

When cells are connected in series the same way round (the positive terminal of one to the negative terminal of the next), their potential differences add together.
For example, two 1.5 V cells connected in series supply a total 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

Series rules - Potential difference, current and resistance

Potential difference is shared

The total potential difference of the supply is shared between the components:

V_{total} = V_{1} + V_{2} + \dots

The bigger a component's resistance, the bigger its share of the total potential difference.

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 potential difference 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

Adding another resistor in series increases the total resistance, reducing the total current.

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.

2.5 - Series Circuits

What a series circuit is

Connecting meters

Series rules - Potential difference, current and resistance

Potential difference is shared

Current is the same everywhere

Resistance adds up

Worked example - Current in a series circuit

Step 1 - Find the total resistance

Step 2 - Rearrange V=IR to make I the subject

Step 3 - Substitute the values

Cell potential differences add up in series

Key points to remember

2.5 - Series Circuits

What a series circuit is

Connecting meters

Series rules - Potential difference, current and resistance

Potential difference is shared

Current is the same everywhere

Resistance adds up

Worked example - Current in a series circuit

Step 1 - Find the total resistance

Step 2 - Rearrange V=IR to make I the subject

Step 3 - Substitute the values

Cell potential differences add up in series

Key points to remember

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

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