Skip to main content
WalesCombined ScienceSyllabus dot point

How is electricity generated, and how do transformers change the voltage?

Electromagnetic induction in a generator, how a transformer changes voltage, and the difference between step-up and step-down transformers.

A focused answer to the WJEC GCSE Science Double Award Unit 3 topic on generators and transformers, covering electromagnetic induction in a generator, how a transformer changes voltage, and step-up and step-down transformers.

Generated by Claude Opus 4.89 min answer

Reviewed by: AI editorial process; not yet individually human-reviewed

Have a quick question? Jump to the Q&A page

Jump to a section
  1. What this dot point is asking
  2. Electromagnetic induction
  3. Generators
  4. Transformers
  5. Step-up and step-down
  6. Increasing the induced voltage
  7. Why transformers are useful
  8. Try this

What this dot point is asking

WJEC Double Award Unit 3 wants you to describe electromagnetic induction in a generator, explain how a transformer changes voltage, and distinguish step-up from step-down transformers.

Electromagnetic induction

A voltage is induced when a magnet is moved into or out of a coil, or when a coil is spun in a magnetic field. The faster the movement, the stronger the magnet, or the more turns on the coil, the bigger the induced voltage. Nothing is induced if the magnet is held still.

Generators

A generator uses electromagnetic induction to make electricity. A coil is spun in a magnetic field (or a magnet is spun inside a coil), so the field through the coil keeps changing, inducing a voltage. Because the coil turns, the induced voltage keeps reversing, producing alternating current (a.c.). Power stations use large generators driven by turbines.

Transformers

Step-up and step-down

The turns ratio sets how the voltage changes, given by the transformer equation:

VpVs=npns\frac{V_p}{V_s} = \frac{n_p}{n_s}

where VpV_p and VsV_s are the primary and secondary voltages and npn_p and nsn_s are the numbers of turns.

  • Step-up transformer: more turns on the secondary, so the voltage is increased.
  • Step-down transformer: fewer turns on the secondary, so the voltage is decreased.

These are central to the National Grid, which steps voltage up for transmission and down for use.

Increasing the induced voltage

Several changes increase the size of the induced voltage from a generator or moving magnet, and these are common exam points. The voltage is bigger if the magnet (or coil) moves faster, if a stronger magnet is used, and if the coil has more turns. In a generator, spinning the coil faster also makes the alternating voltage higher in peak value and higher in frequency. Linking each change to a larger or faster-changing magnetic field explains why it increases the induced voltage.

Why transformers are useful

Transformers are useful because they let the voltage of an a.c. supply be changed efficiently with almost no moving parts. This is what makes long-distance electricity transmission practical: the voltage can be stepped up to reduce energy loss and then stepped back down for safe use. Transformers are also found in chargers and adaptors, which step the mains voltage down to the lower voltage that phones and laptops need. Because they rely on a changing magnetic field, they only work with alternating current, which is one reason mains electricity is a.c.

Try this

Q1. What type of current does a simple generator produce? [1 mark]

  • Cue. Alternating current (a.c.).

Q2. A transformer increases the voltage. Which coil has more turns? [1 mark]

  • Cue. The secondary coil (it is a step-up transformer).

Exam-style practice questions

Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

WJEC style3 marksDescribe how a voltage is generated when a magnet is moved into a coil of wire.
Show worked answer →

A Unit 3 describe question. Reward: moving the magnet into the coil makes the magnetic field through the coil change (1); this induces a voltage (potential difference) across the coil (1); if the coil is part of a complete circuit, a current flows, and moving the magnet faster or using a stronger magnet induces a bigger voltage (1). Markers credit the changing field, the induced voltage and a factor that increases it. A common error is to say a voltage is induced when the magnet is held still inside the coil (it must be moving).

WJEC style4 marksA transformer has 100 turns on the primary coil and 500 turns on the secondary coil. The input voltage is 20 V. State the type of transformer and calculate the output voltage.
Show worked answer →

A Unit 3 calculation. The secondary has more turns, so it is a step-up transformer (1). Use VpVs=npns\dfrac{V_p}{V_s} = \dfrac{n_p}{n_s}, so Vs=Vp×nsnp=20×500100V_s = V_p \times \dfrac{n_s}{n_p} = 20 \times \dfrac{500}{100} (1 mark for method) =100 V= 100\,\text{V} (1 mark). State it increases the voltage (1). Markers credit identifying step-up, the transformer equation and the answer. A common error is to invert the turns ratio.

Related dot points

Sources & how we know this