Introduction to Transformers Revision Notes | GCSE AQA Higher Physics
A transformer is a device that uses electromagnetic induction to change the voltage of an alternating current supply. Free GCSE Physics notes and 13 FAQs.
Updated 20 Mar 2026 · Revise Right Team
Introduction to Transformers is part of Induced potential, transformers and the National Grid (physics only) (HT only) in GCSE AQA Higher Physics.
5 min
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What this lesson covers
A transformer is a device that uses electromagnetic induction to change the voltage of an alternating current supply. Free GCSE Physics notes and 13 FAQs.
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
Calculate voltage and current relationships in transformers using the transformer equation
Evaluate the efficiency of transformers and explain energy losses in real transformers
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Use this lesson to stay inside Induced potential, transformers and the National Grid (physics only) (HT only) while connecting the detail back to the wider Magnetism and electromagnetism topic.
Magnetism and electromagnetism
Induced potential, transformers and the National Grid (physics only) (HT only)
GCSE AQA Higher Physics
Revision notes
Read the core explanations from Introduction to Transformers before testing yourself from memory.
1. What Are Transformers and Why Do We Need Them?
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.
2. The Basic Structure - Two Coils and an Iron Core
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.
3. How Electromagnetic Induction Makes It Work
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.
4. The Transformer Equation - Turns and Voltage
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.
This equation tells us everything about how transformers change voltage. A step-up transformer has more turns on the secondary coil ($N_s > N_p$), giving higher output voltage. A step-down transformer has fewer turns on the secondary coil ($N_s < N_p$), giving lower output voltage.
For ideal transformers (100% efficient), there's also a current relationship. Since power must be conserved, when voltage steps up, current steps down proportionally. This gives us the current equation: $\frac{I_p}{I_s} = \frac{N_s}{N_p}$.
Worked example: Calculating Secondary Voltage
A transformer has 100 turns on the primary coil and 500 turns on the secondary coil. If the primary voltage is 12V, calculate the secondary voltage. [3 marks]
$\frac{V_s}{V_p} = \frac{N_s}{N_p}$ — [M1] for correct formula
$\frac{V_s}{12} = \frac{500}{100}$ — [M1] for correct substitution with units
$\frac{V_s}{12} = 5$ — 500 ÷ 100 = 5
$V_s = 12 \times 5 = 60$ V — [A1] for correct answer with units
Answer: 60 V
5. Real Transformers and Energy Losses
Real transformers aren't 100% efficient like our ideal calculations suggest. Some energy is always lost as heat, though modern transformers can achieve efficiencies above 95%. Understanding these losses helps explain why transformers get warm during operation.
The main energy losses occur in the copper coils due to their electrical resistance. When current flows through the coils, some electrical energy converts to heat energy ($P = I^2R$). This is why transformer coils are made from thick copper wire to minimise resistance.
The iron core also contributes to energy losses, though the specification doesn't require detailed knowledge of these mechanisms. What matters for GCSE is understanding that real transformers have some energy loss, making them less than 100% efficient.
Efficiency can be calculated using: Efficiency = $\frac{\text{Power Output}}{\text{Power Input}} \times 100\%$. For transformers, this is often expressed as the ratio of secondary power to primary power.
6. Exam Technique - Getting Full Marks on Transformer Questions
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.
For calculation questions, always start by writing the transformer equation clearly. Show your substitution step with units, and give your final answer with the correct unit. Remember that voltage is measured in volts (V) and current in amperes (A).
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.
Sources and content standards
Published by Revise Right. Use the official specification reference to check the requirements for your course. Content updated: 2026-03-20.
A transformer is a device that uses electromagnetic induction to change the voltage of an alternating current supply. It consists of a primary coil and a secondary coil wound around a shared iron core.
How does a transformer work?
Alternating current in the primary coil creates a changing magnetic field in the iron core. This changing magnetic field passes through the secondary coil and induces a voltage across it, which depends on its number of turns compared with the primary coil.
How do transformers work in physics?
In a transformer, AC in the primary coil creates a changing magnetic field in the iron core; this field links with the secondary coil and induces a voltage there. The output voltage depends on the ratio of turns on the two coils.
What is the transformer current equation?
For an ideal, 100% efficient transformer, Ip/Is = Ns/Np, because power must be conserved: when voltage steps up, current steps down proportionally. More turns on the secondary coil means less secondary current.
How do you calculate transformer efficiency?
Transformer efficiency = power output ÷ power input × 100%, often given as the ratio of secondary power to primary power. Modern transformers can be more than 95% efficient.
How do you explain how a transformer works?
A transformer is explained with a chain: AC in the primary coil produces a changing magnetic field in the iron core, the field links with the secondary coil, and an EMF is induced in the secondary coil.
Where are transformers found?
Transformers are everywhere in the electrical grid, from massive units at power stations to small adaptors for phone chargers. They step voltage up or down without losing much energy.
Why does a transformer have an iron core?
A transformer has an iron core because iron is easily magnetised, so it concentrates and directs the magnetic field from the primary coil through the secondary coil. Without it, very little magnetic field would link the two coils and the transformer would be extremely inefficient.
Are the primary and secondary coils of a transformer connected?
The primary and secondary coils of a transformer are not electrically connected; they are completely separate circuits. Energy transfers purely through the changing magnetic field in the iron core.
Why do transformers get warm?
Transformers get warm because some energy is always lost as heat. The main losses are in the copper coils due to their electrical resistance, which is why the coils are made from thick copper wire; the iron core also adds some losses.
What is the secondary voltage with 100 and 500 turns and 12 V in?
A transformer with 100 turns on the primary, 500 on the secondary and a 12 V primary voltage gives a secondary voltage of 60 V. The voltage ratio equals the turns ratio: 500 ÷ 100 = 5, so the secondary voltage is 12 × 5 = 60 V, making it a step-up transformer.
Why are transformer coils made of thick copper wire?
Transformer coils are made from thick copper wire to minimise their resistance. When current flows through the coils, some electrical energy converts to heat (P = I²R), which is the main energy loss.
What does Faraday's law say about transformers?
Faraday's law says a changing magnetic field through a coil induces an electromotive force (EMF) in that coil, and the faster the field changes, the greater the EMF. AC alternating 50 times per second provides these changing conditions.