Current can be calculated using the equation: current = p.d. ÷ resistance (I = V ÷ R).
The rules of current and p.d. for a parallel 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 p.d. 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 I = V ÷ R is shown for a component, circuit rules need to be applied to the rest of the circuit.

Keywords

Current - the rate of flow of charge in a circuit
Branch - a separate series loop in a parallel circuit
Resistance - a property of materials that makes it harder for current to flow
Potential difference (p.d.) - a measure of how 'hard' an electric field can push charge
Parallel - a type of circuit in which there are two or more separate branches

Common misconception

To solve circuit problems 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.

Use online simulations to explore and discuss the behaviour of circuits. Use past exam questions to give your pupils lots of practice at answering circuit questions, once you are confident they understand how to answer them.

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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Starter quiz

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6 Questions

Q1.

How many parallel branches does the circuit shown have?

none

one

two

Correct answer: three

four

Q2.

A series circuit contains a battery and one lamp. A second, identical lamp is added, in parallel with the first lamp. Which of the following statements is correct?

The current through the first lamp increases.

The current through the first lamp decreases.

Correct answer: The current through the first lamp stays the same.

Q3.

A series circuit contains a battery and one lamp. A second, identical lamp is added, in parallel with the first lamp. Which of the following statements is correct?

Correct answer: The current through the battery increases.

The current through the battery decreases.

The current through the battery stays the same.

Q4.

A series circuit contains a battery and one lamp. A second, identical lamp is added, in parallel with the first lamp. Which of the following statements is correct?

The total resistance of the circuit increases.

Correct answer: The total resistance of the circuit decreases.

The total resistance of the circuit stays the same.

Q5.

What potential difference is needed to make a current of 2.0 A flow through a 10 Ω resistor?

0.20 V

5.0 V

12 V

Correct answer: 20 V

Q6.

A lamp in a circuit has a potential difference 1.5 V across it and a current 3.0 A flowing through it. What is the resistance of the lamp?

Correct answer: 0.50 Ω

1.5 Ω

2.0 Ω

4.5 Ω

Exit quiz

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6 Questions

Q1.

The diagram shows a circuit with parallel branches. Which of the following quantities must be the same for every branch?

current

Correct answer: potential difference (p.d.)

resistance

Q2.

What is the missing current in the parallel circuit shown?

1.2 A

Correct answer: 2.2 A

3.7 A

4.6 A

6.1 A

Q3.

In the circuit shown, the two resistors are identical and the p.d. across the battery is 6 V. The potential difference across the resistor on the left is V.

Correct Answer: 3, 3.0, 3 V, 3 volts, three

Q4.

In the circuit shown, the potential difference across the resistor is V.

Correct Answer: 8.8, 8.80, 8.8 V, 8.8 volts, 8.80 V

Q5.

What is the resistance of the resistor in the circuit shown?

0.33 Ω

0.44 Ω

Correct answer: 3.0 Ω

3.5 Ω

4.0 Ω

Q6.

In the circuit shown, the potential difference across the lamp is V.

Correct Answer: 6, 6.0, 6 V, 6 volts, six