Topic P6: Global challenges lesson
Introduction to Transformers: Retained Combined-scope transformer-structure content split from the standalone OCR Physics mixed lesson.
Introduction to Transformers is part of P6.2 Powering Earth in GCSE OCR Higher Combined Science Physics.
4 min
Reading time
17
Lessons in this topic
GCSE OCR Higher
Pathway
Expert reviewed
Content review
Introduction to Transformers: Retained Combined-scope transformer-structure content split from the standalone OCR Physics mixed lesson.
Use this lesson to stay inside P6.2 Powering Earth while connecting the detail back to the wider Topic P6: Global challenges topic.
Read the core explanations from Introduction to Transformers before testing yourself from memory.
Picture the electricity arriving at your house. It started its journey at a power station at 25,000 volts, travelled across the country at 400,000 volts, then dropped to 230 volts before reaching your kettle. How does voltage change so dramatically along the way?
The answer is transformers - devices that can step voltage up or down without losing much energy. They're everywhere in our electrical grid, from massive units at power stations to small adaptors for your phone charger. Understanding how they work reveals one of the most elegant applications of electromagnetic induction.
Transformers only work with alternating current (AC), not direct current (DC). This is crucial because they rely on changing magnetic fields to function. The alternating nature of AC provides exactly the changing conditions needed for electromagnetic induction to occur.
Every transformer has the same basic design: two separate coils of wire wound around a shared iron core. The input coil is called the primary coil, and the output coil is the secondary coil. These coils are not electrically connected - they're completely separate circuits.
The iron core is crucial to the transformer's operation. Iron is easily magnetised, which means it can concentrate and direct the magnetic field from the primary coil through to the secondary coil. Without this iron core, very little magnetic field would link the two coils, making the transformer extremely inefficient.
When AC flows through the primary coil, it creates a changing magnetic field in the iron core. This changing magnetic field then passes through the secondary coil, inducing a voltage across it. The beauty is that no electrical connection is needed between the coils - energy transfers through the magnetic field.
Electromagnetic induction underpins the process. When alternating current flows through the primary coil, it creates a magnetic field that constantly changes direction and strength. This changing magnetic field passes through the iron core and links with the secondary coil.
According to Faraday's law, a changing magnetic field through a coil induces an electromotive force (EMF) in that coil. The faster the magnetic field changes, the greater the induced EMF. Since AC naturally alternates 50 times per second in the UK, it provides the perfect changing conditions for induction.
The key insight is that the voltage induced in the secondary coil depends on how many turns of wire it has compared to the primary coil. More turns in the secondary means higher induced voltage - this is how transformers can step voltage up or down.
**Trap**: Many students think the primary and secondary coils are electrically connected. They're not - energy transfers purely through the changing magnetic field in the iron core. This is what makes transformers so efficient and safe.
The relationship between input and output voltage in a transformer follows a simple pattern. The voltage ratio equals the turns ratio. If the secondary coil has twice as many turns as the primary, the output voltage will be twice the input voltage.
Transformer questions follow predictable patterns in GCSE exams. The most common mistake is mixing up which coil is primary and which is secondary. Always read the question carefully to identify which coil is connected to the input supply.
When explaining how transformers work, use the key sequence: AC in primary coil → changing magnetic field in iron core → magnetic field links with secondary coil → EMF induced in secondary coil. This logical chain shows understanding of the electromagnetic induction process.
Published by Revise Right. Use the official specification reference to check the requirements for your course. Content updated: 2026-03-23.
Move to the related lessons or back to the topic page when you need the wider sequence before exam-style practice.
Retained Combined-scope transformer-structure content split from the standalone OCR Physics mixed lesson.
By the end of this lesson, you should be able to Explain how alternating current in one coil induces current in another coil using electromagnetic induction; Describe the basic structure of a transformer with primary and secondary coils wound on an iron core.
Read each section, then close the notes and explain the idea from memory. Check your explanation against the page and finish with an exam question from Topic P6: Global challenges.