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Home · IGCSE 0625 · Electricity and magnetism · Transformers · Practice
Core · Practice questions · Transformers

Turns, voltage, ratio.

Six original Cambridge-style questions on transformers: naming the parts, the turns calculation both ways, step-up versus step-down, why a.c. is needed, and the use in power transmission.

Original questions All questions on this page are original work, written in the Cambridge IGCSE style. They are not from past papers. They test the same concepts and skills the syllabus rewards.
What the examiner wants

The ratio is everything.

01
Recall
[3 marks]

Name the three main parts of a simple transformer, and state which material the core is made from.

  • A primary coil and a secondary coil. ✓
  • Wound on a core. ✓
  • The core is made of soft iron. ✓
02
Calculation
[3 marks]

A transformer has 100 turns on the primary and 500 turns on the secondary. The input voltage is 12 V. Calculate the output voltage and state the type of transformer.

Vₛ = Vₚ × Nₛ ÷ Nₚ = 12 × 500 ÷ 100

Vₛ = 60 V

output larger than input, so step-up

03
Calculation
[3 marks]

A charger steps the 230 V mains down to 11.5 V. The primary coil has 2000 turns. Calculate the number of turns needed on the secondary coil.

Nₛ = Nₚ × Vₛ ÷ Vₚ = 2000 × 11.5 ÷ 230

Nₛ = 100 turns

04
Analysis
[2 marks]

A student connects a battery (d.c.) to the primary of a transformer and measures the output voltage. State what they find, and explain why.

  • The output voltage is zero (apart from a brief moment when switching on or off). ✓
  • A steady d.c. gives a steady field, and a transformer needs a changing field to induce a voltage. ✓
05
Application
[2 marks]

Explain why electricity is sent across the country at very high voltage, using a step-up transformer at the power station.

  • A higher voltage means a smaller current for the same power. ✓
  • A smaller current wastes less energy as heat in the cables, so transmission is more efficient. ✓
06
Analysis
[2 marks]

Explain briefly how an alternating voltage in the primary coil produces a voltage in the secondary coil, even though the two coils are not connected by a wire.

  • The alternating current in the primary makes a changing magnetic field in the soft-iron core. ✓
  • This changing field passes through the secondary coil and induces an alternating voltage in it. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.