⚡ Primary side
An alternating current flows in the primary coil. It keeps changing size and direction, so it makes a changing magnetic field.
Electromagnetic Induction » Transformers
What you'll learn this session
Study time: 30 minutes
Edexcel spec: 6.17P
- The parts of a transformer: two coils on a soft iron core
- How a changing current in one coil makes a voltage in the other
- Why a transformer needs alternating current and does not work with d.c.
- The difference between a step-up and a step-down transformer
This whole lesson is Paper 2 only content. It is not examined in Paper 1.
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Unlock This CourseWhat is a transformer?
Many things in your home need a different voltage from the mains. A phone charger needs a small voltage, but the mains socket in the UK gives about 230 V. A transformer is the device that changes the size of an alternating voltage. It can make the voltage bigger or smaller.
A transformer has no moving parts. It works using what you met in the lesson on Electromagnetic Induction: a changing magnetic field induces a voltage in a coil.
Key terms:
- Transformer: a device that changes the size of an alternating voltage, using two coils on an iron core.
- Primary coil: the coil on the input side. The alternating voltage you start with is connected to it.
- Secondary coil: the coil on the output side. The new voltage appears across it.
- Step-up transformer: a transformer that increases the voltage.
- Step-down transformer: a transformer that decreases the voltage.
The structure of a transformer
To draw a transformer in the exam, follow these steps:
- Draw a rectangular ring. This is the core. It is made of soft iron, which is easy to magnetise and demagnetise.
- Draw a coil of insulated wire wound round one side of the ring. This is the primary coil. Connect it to an alternating voltage supply.
- Draw a second coil of insulated wire wound round the opposite side of the ring. This is the secondary coil. Connect it to the output, such as a lamp.
- Label the input side and the output side. Show a different number of turns on each coil.
| Part | What it is | Job |
|---|---|---|
| Primary coil | Insulated wire, input side | Carries the input alternating current and makes the magnetic field |
| Secondary coil | Insulated wire, output side | Has the output voltage induced in it |
| Soft iron core | A ring that links the two coils | Carries the changing magnetic field from the primary to the secondary |
The wire is insulated so the current has to go round the turns of each coil. It cannot cross from one turn to the next, or into the iron. Notice that the two coils are not connected by wire. They are linked only by the magnetic field in the core.
How a transformer works
🧲 Secondary side
The soft iron core carries this changing field through the secondary coil. The changing field induces an alternating voltage in the secondary coil.
In order, the steps are:
- An alternating current flows in the primary coil.
- This makes a changing magnetic field in the iron core.
- The changing field passes through the secondary coil.
- An alternating voltage is induced in the secondary coil.
The induced voltage is alternating too, because the field is changing all the time. If a lamp or other device is connected to the secondary coil, the circuit is complete and an alternating current flows in it.
Why a transformer does not work with d.c.
A battery gives direct current (d.c.). This flows in one direction and stays at a steady size. A steady current in the primary coil makes a steady magnetic field. A steady field is not changing, so no voltage is induced in the secondary coil.
Learn this
A voltage is only induced when the magnetic field changes. Alternating current makes a changing field. Direct current makes a steady field. So a transformer works with a.c. but not with d.c.
So with d.c. there is no continuous output from the secondary coil.
Step-up and step-down
The size of the output voltage depends on the number of turns on each coil. A transformer changes the voltage by having a different number of turns on the input and output sides.
| Type | Turns on the secondary compared with the primary | Effect on the voltage |
|---|---|---|
| Step-up | More turns on the secondary | Output voltage is bigger than input voltage |
| Step-down | Fewer turns on the secondary | Output voltage is smaller than input voltage |
A phone charger contains a step-down transformer. It takes the mains voltage of about 230 V and gives a much smaller voltage. Its secondary coil has fewer turns than its primary coil. The calculations that link turns and voltages are in the lesson on Transformer Calculations.
Worked example
A transformer has 200 turns on its primary coil and 800 turns on its secondary coil. Is it step-up or step-down? Explain your answer.
The secondary coil has more turns (800) than the primary coil (200).
More turns on the secondary means the output voltage is bigger than the input voltage.
So it is a step-up transformer.
Common mistakes
Common mistakes
- Saying the current flows from the primary coil into the secondary coil. The coils are not connected. The link is the magnetic field in the core.
- Writing that the core is a magnet or is made of steel. It is soft iron.
- Saying a transformer works with a battery. A battery gives d.c., which makes a steady field, so no voltage is induced.
- Mixing up the two types. Step-up means more turns on the secondary. Step-down means fewer.
- Leaving out the word changing when explaining how the voltage is induced.
Exam-style question
A transformer has a primary coil and a secondary coil wound on a soft iron core. The primary coil is connected to an a.c. supply.
(a) Describe how the transformer makes a voltage across the secondary coil. [3 marks]
(b) Explain why the transformer does not work if the primary coil is connected to a battery. [2 marks]
(c) The secondary coil has fewer turns than the primary coil. State the type of transformer. [1 mark]
Model answer
(a) The alternating current in the primary coil produces a changing magnetic field (1). The iron core carries this changing field through the secondary coil (1). The changing field induces an alternating voltage in the secondary coil (1).
(b) A battery gives direct current, which makes a steady magnetic field (1). A steady field is not changing, so no voltage is induced in the secondary coil (1).
(c) Step-down (1).
Exam tip
In "how does it work" questions, put the steps in order: a.c. in the primary, changing field in the core, voltage induced in the secondary. Each step is usually a mark.