Current can be calculated using the equation: current = p.d. ÷ resistance (I = V ÷ R).
The rules of current and p.d. for a series circuit can be applied to find some missing values of current and p.d.
The equation I = V ÷ R can be rearranged so that either V or R is the subject of the equation.
If any two values in the equation I = V ÷ R are known, the third can be calculated.
If only one value in the equation I = V ÷ R is known, rules for current or p.d. need to be applied to find a second.

Keywords

Electrical current - Electrical current is the amount of charge flowing past a given point in one second.
Series circuit - A series circuit is an electrical circuit with a single loop.
Potential difference (p.d.) - P.d is a measure of the push an electric field gives to charges.
Resistance - Resistance is a property of a material that prevents the flow of current.
Ohms (Ω) - Ohms is the unit of resistance.

Common misconception

To solve circuits, you need only to put numbers into equations.

Talk through circuits with pupils before analysing them in order to develop an intrinsic understanding of how circuits work; research shows this improves pupils’ problem solving ability for electric circuits.

A string loop or rope loop model is very useful for supporting discussion about rules in series circuits.

Teacher tip

Licence

This content is © Oak National Academy Limited (2024), licensed on Open Government Licence version 3.0 except where otherwise stated. See Oak's terms & conditions (Collection 2).

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Worksheet

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Starter quiz

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6 Questions

Q1.

Which of the following is the unit of electric current?

Correct answer: ampere

coulomb

joule

newton

watt

Q2.

Which of the following is the unit of resistance?

coulomb

joule

Correct answer: ohm

volt

watt

Q3.

There is an invisible electric around a charged object, in which other charged objects and small, uncharged objects experience forces.

Correct Answer: field, feild

Q4.

Which of the following can always be used to describe a series circuit?

Correct answer: a circuit with only one loop

a circuit with only one component

a circuit with only one power source

a circuit in which every component has the same p.d.

Q5.

A lamp is added to a series circuit, in series with the other components in the circuit. No other changes are made to the circuit. Which of the following describes what happens to the current?

It increases.

Correct answer: It decreases.

It stays the same.

It is impossible to say without more information.

Q6.

A series circuit includes one 1.5 V cell. Two more 1.5 V cells are added in series with the first cell, and no other changes are made. Which of the following occurs?

Correct answer: The current multiplies by 3.

The current doubles.

The current halves.

The current divides by 3.

The current stays the same.

Exit quiz

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6 Questions

Q1.

The circuit diagram shows a series circuit. The potential difference across each cell and component is shown, except for the resistor. The p.d. across the resistor is V.

Correct Answer: 0.9, 0.9 V, 0.9V, 0.9 volts, 0.9volts

Q2.

What potential difference is needed to make a current of 0.40 A flow through a motor with a resistance of 8.0 Ω?

Correct Answer: 3.2 V, 3.20, 3.2, 3.20 V, 3.2V

Q3.

A resistor in a circuit has a potential difference of 5.5 V across it and a current of 2.2 A flowing through it. The resistance of the resistor is Ω.

Correct Answer: 2.5, 2.50, 2.5 ohms, 2.50 ohms, 2.5 ohm

Q4.

The diagram shows three components in series with each other. The potential difference across each component, and the resistance of one component, are shown. The resistance of the resistor is Ω.

Correct Answer: 6, 6.0, 6 ohms, six, six ohms

Q5.

What is the resistance of the lamp labelled X in the circuit shown?

0.68 Ω

0.83 Ω

Correct answer: 1.2 Ω

2.8 Ω

Q6.

The diagram shows a series circuit. The power supply voltage, the p.d. across the lamp and the resistance of the resistor are shown. The reading on the ammeter is A.

Correct answer: 0.15 A

0.32 A

0.40 A

2.4 A

6.7 A