Free GCSE Physics lesson: Circuit Calculations
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Free Lessons -> GCSE / Key Stage 4 -> Physics -> Circuit Calculations

Lesson 27 · GCSE / Key Stage 4 · Physics

Circuit calculations: charge, current, potential difference and resistance

Use charge, current, potential difference, resistance, energy and power equations in circuit questions.

Qualification: GCSE Subject: Physics Circuits

Electricity

Lesson overview

This lesson introduces the core physics idea, the useful equipment and the calculation or data skills used on this page.

FocusElectric circuit calculations
Time45-60 minutes
EquipmentCalculator and circuit equation list.
Practical linkNo separate practical method focus
Maths tagsM1 substitution with units, M6 ratio and percentage

What you will learn

  • Use charge flow = current x time.
  • Use potential difference = current x resistance.
  • Link energy transferred, charge and potential difference.
  • Choose between current, resistance, energy and power equations in circuits.

Circuit calculation set supplied on this page

Use the lamp, resistor and battery prompts to practise choosing the correct circuit equation from the quantities given.

Circuit calculations infographic

Infographic explaining GCSE Physics circuit calculations, including charge flow, current, time, potential difference, resistance, energy transfer, power, ammeter and voltmeter placement.
Use this visual to choose the correct circuit equation, place meters correctly and keep units clear.Download visual

Clear explanation

Circuit calculations become easier when you name the quantities. Current is rate of flow of charge. Potential difference is energy transferred per coulomb of charge.

For an ohmic resistor at constant temperature, potential difference and current are directly proportional, so V = I x R.

Power in a circuit can be found using energy transferred per second, or from potential difference and current when those quantities are given.

Key diagram

Series circuit values for resistance and current calculations A series circuit has a twelve volt supply, two resistors labelled four ohms and eight ohms, and one current path. 12 V 4 Ω 8 Ω R total = 4 Ω + 8 Ω = 12 Ω one loop, so the same current flows through both resistors
Diagram: the labelled values give pupils a concrete circuit for applying V = I R and adding series resistance.

Worked examples

Charge flow from current and time

A current of 0.8 A flows for 25 s.

charge flow = current x time

charge flow = 0.8 x 25

Answer: The charge flow is 20 C.

Quick checks

Choose an answer, then check your thinking.

1. Which equation links charge flow, current and time?

2. A 4 ohm resistor has 2 A through it. What is the potential difference?

Practice questions

Question 1

A current of 3 A flows for 10 s. Calculate charge flow.

Reveal answer and marking guidance

Answer: 30 C.

Marking: Credit Q = I x t and 3 x 10 = 30 C.

Question 2

A resistor has 12 V across it and 0.5 A through it. Calculate resistance.

Reveal answer and marking guidance

Answer: 24 ohms.

Marking: Credit R = V ÷ I and 12 ÷ 0.5 = 24 ohms.

Question 3

A charge of 40 C transfers 200 J of energy. Calculate potential difference.

Reveal answer and marking guidance

Answer: 5 V.

Marking: Credit V = E ÷ Q and 200 ÷ 40 = 5 V.

Question 4

A 230 V appliance has a current of 2 A. Calculate power.

Reveal answer and marking guidance

Answer: 460 W.

Marking: Credit P = V x I and 230 x 2 = 460 W.

Practice ladder

FluencyRecall the key definition, unit, equation or model before using the lesson questions.
ApplicationApply electric circuit calculations to an unfamiliar device, practical setup or data description.
Practical interpretationUse evidence, graph features, uncertainty, method quality or conclusion wording where the question asks you to evaluate.
Maths skillM1 substitution with units

Answers and marking guidance

The exact practice answers are hidden under each question so you can try first. For this lesson, marks come from using the correct physics model, choosing the right equation where needed, keeping units with values, and explaining changes with precise words such as transfer, resultant force, acceleration, evidence and uncertainty.

Common mistakes

  • Confusing charge in coulombs with current in amps.
  • Using V = I ÷ R instead of V = I x R.
  • Forgetting that potential difference is energy per charge.
  • Mixing up seconds and minutes in charge-flow calculations.

Extension challenge

Write one circuit question that uses Q = I x t and one that uses E = Q x V, then solve both.

Reveal answer

Example answer: A strong extension response names the physics model, uses accurate units and explains why the evidence supports the conclusion.

Exam-board guidance

Short board notes only. Learn the core physics above first.

AQA GCSE Physics

AQA GCSE Physics: often rewards clear physics explanations, correct equations, units and practical evidence for electric circuit calculations.

OCR GCSE Physics

OCR GCSE Physics: often values precise definitions, clear working, graph interpretation and links between models and evidence.

Pearson Edexcel GCSE Physics

Pearson Edexcel GCSE Physics: often combines the concept with equation use, data handling and practical interpretation.

Eduqas GCSE Physics

Eduqas GCSE Physics: learn the core explanation and practise applying it to unfamiliar contexts, data and practical questions.

WJEC Wales

WJEC Wales: often expects accurate terms, units and evidence-based explanations using the shared physics idea.

CCEA GCSE Physics

CCEA GCSE Physics: connect the idea to your current unit and use the same practical method language your class uses.

Next lesson

Next, continue with Moments, Levers and Gears.

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