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

Lesson 42 · GCSE / Key Stage 4 · Physics

Transformer calculations and efficiency

Use transformer turns ratios, power relationships and efficiency ideas in National Grid questions.

Qualification: GCSE Subject: Physics Transformers

Electricity

Lesson overview

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

FocusTransformer equations, power and efficiency
Time45-60 minutes
EquipmentCalculator and transformer equation sheet if used by your course.
Practical linkNo separate practical method focus
Maths tagsM1 substitution with units, M2 rearranging/equations, M6 ratio and percentage

What you will learn

  • Use the transformer turns ratio equation.
  • Explain step-up and step-down behaviour using coil turns.
  • Use input and output power for ideal transformers.
  • Explain efficiency limits and heating losses.

Transformer calculation set supplied on this page

Use the primary coil, secondary coil and power-transfer examples to practise turns-ratio calculations.

Transformer calculations and efficiency infographic

Infographic explaining GCSE Physics transformer calculations and efficiency, including turns ratio, primary and secondary coils, power transfer, efficiency equations and the need for alternating current.
Use this visual to link transformer turns ratio, power transfer, current changes and efficiency checks.Download visual

Clear explanation

In a transformer, the ratio of potential differences equals the ratio of turns on the coils. More turns on the secondary coil means a higher output potential difference.

An ideal transformer transfers power without loss, so input power equals output power. Real transformers waste some energy by heating.

Step-up transformers increase potential difference and reduce current for the same transmitted power. This helps reduce heating losses in cables.

Key diagram

Transformer coils with different numbers of turns A transformer core has a primary coil with fewer turns and a secondary coil with more turns, labelled with input and output potential difference. primary secondary fewer turns more turns turns ratio links input p.d. to output p.d.
Diagram: the different coil counts make the voltage ratio model visible before pupils use the transformer equation.

Worked examples

Turns ratio

A transformer has 100 turns on the primary coil and 500 turns on the secondary coil.

The secondary has five times as many turns.

The secondary potential difference is five times the primary potential difference.

Answer: If the primary is 20 V, the secondary is 100 V.

Quick checks

Choose an answer, then check your thinking.

1. What kind of transformer has more secondary turns than primary turns?

2. In an ideal transformer, what happens to input and output power?

Practice questions

Question 1

A transformer has 200 primary turns and 1000 secondary turns. Is it step-up or step-down?

Reveal answer and marking guidance

Answer: Step-up.

Marking: Credit more secondary turns than primary turns.

Question 2

A transformer has 50 V on the primary and twice as many secondary turns as primary turns. What is the secondary potential difference?

Reveal answer and marking guidance

Answer: 100 V.

Marking: Credit doubling potential difference.

Question 3

An ideal transformer outputs 60 W. What is its input power?

Reveal answer and marking guidance

Answer: 60 W.

Marking: Credit ideal transformer power conservation.

Question 4

Why are real transformers less than 100% efficient?

Reveal answer and marking guidance

Answer: Some energy is dissipated, mainly by heating in coils and core.

Marking: Credit heating losses or energy transferred to thermal stores.

Practice ladder

FluencyRecall the key definition, unit, equation or model before using the lesson questions.
ApplicationApply transformer equations, power and efficiency 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

  • Reversing primary and secondary turns.
  • Assuming real transformers are perfectly efficient.
  • Saying step-up means current also increases for the same power.
  • Forgetting transformers need alternating current.

Extension challenge

Explain why stepping up voltage for transmission can reduce cable heating even when the same power is delivered.

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 transformer equations, power and efficiency.

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 Practical: Infrared Radiation.

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