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15.7 Transformers

GCSE Physics · AQA · Chapter 15: Electromagnetism

A free GCSE Physics lesson for AQA (8463) from OBA School: higher tier, physics only. What transformers do, how they are built, how they work and why they need alternating current. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

  • (Higher tier) state that transformers change the size of an alternating p.d., and describe step-up and step-down transformers
  • (Higher tier) describe the structure of a transformer and explain why the core is iron and the wires are insulated
  • (Higher tier) explain how an alternating current in the primary coil induces an alternating p.d. in the secondary coil
  • (Higher tier) explain why a transformer does not work with direct current
  • (Higher tier) relate step-up and step-down to the numbers of turns on the two coils

Key words

  • Transformer: A device that changes the size of an alternating potential difference.
  • Primary coil: The input coil, connected to the a.c. supply.
  • Secondary coil: The output coil, in which a p.d. is induced.
  • Step-up transformer: Makes the output (secondary) p.d. bigger than the input (primary) p.d.
  • Step-down transformer: Makes the output p.d. smaller than the input p.d.
  • Iron core: The loop of iron that both coils are wound on. It carries the changing magnetic field from one coil to the other.

What a transformer does

A transformer changes the size of an alternating p.d. A step-up transformer increases it; a step-down transformer decreases it. Uses include:

  • chargers and adaptors: step the 230 V mains down to a few volts for phones, laptops and toys;
  • the National Grid: step-up transformers at power stations raise the p.d. to hundreds of thousands of volts for transmission, and step-down transformers lower it again for homes and businesses (more in 15.8);
  • appliances such as microwave ovens step the mains up to a few thousand volts.

How a transformer is built

  • Two coils of insulated wire: the primary (input) and the secondary (output).
  • Both coils are wound on the same iron core.
  • Iron is used because it is easily magnetised and demagnetised, so its magnetism can follow the alternating current, and it guides the field lines through the secondary coil.
  • The wire is insulated so that the current goes round every turn instead of taking a short cut through touching turns or the iron core.

How a transformer works

1. An alternating current in the primary coil produces a magnetic field in the iron core that keeps changing size and direction.

2. The iron core carries this changing magnetic field through the secondary coil.

3. A changing field through a coil induces a p.d. (the generator effect), so an alternating p.d. is induced across the secondary coil, at the same frequency as the supply.

4. If the secondary is part of a complete circuit (for example with a lamp), an alternating current flows in it. Energy has been transferred from the primary circuit to the secondary circuit by the magnetic field.

Exam tip: In an exam answer use the chain: alternating current → changing magnetic field in the core → field changes through the secondary → alternating p.d. induced.

Why only a.c.?

A p.d. is only induced when the magnetic field through the secondary coil is changing. A steady direct current makes a steady field, so no p.d. is induced in the secondary (except for a brief flick at the moment the d.c. is switched on or off).

Step-up, step-down and the number of turns

The changing field induces the same p.d. in every turn. So the coil with more turns has the bigger p.d. across it. If the secondary has more turns than the primary, the transformer is step-up; if it has fewer, it is step-down. The p.d.s are in the same ratio as the numbers of turns (you will use this equation in 15.8).

Worked example: step-up or step-down?

A transformer has 600 turns on its primary and 30 turns on its secondary.
The secondary has fewer turns, and 600 ÷ 30 = 20.

Answer: step-down: the output p.d. is 20 times smaller than the input

Practical: Model transformer investigation. Wind a primary coil on a C-core and connect it to a low-voltage a.c. supply (about 2 V). Wind a secondary coil and connect it to an a.c. voltmeter. Change the number of secondary turns and record the secondary p.d. Keep the primary turns, supply p.d. and core the same. Switch off between readings: the coils can get hot. Never connect a homemade coil to the mains.

Common mistakes

  • Saying the current flows through the core from one coil to the other. The coils are linked only by the changing magnetic field.
  • Saying a transformer works with a battery. It needs a.c. so that the field keeps changing.
  • Choosing steel for the core. Iron magnetises and demagnetises easily, so it follows the alternating field.
  • Getting step-up and step-down the wrong way round. Step-up has more turns on the secondary.

Revision questions

Try these, then check your answers below each one.

Question 1. (Higher tier) What does a transformer do? (Choose from: Changes the size of an alternating p.d., Changes a.c. into d.c., Stores electrical energy, Increases the power supplied.)

Answer: Changes the size of an alternating p.d.

Working: A transformer steps an alternating p.d. up or down. It cannot increase the power.

Question 2. (Higher tier) Which material is used for the core of a transformer? (Choose from: Iron, Steel, Copper, Plastic.)

Answer: Iron

Working: Iron is easily magnetised and demagnetised, so it carries the changing field through the secondary coil.

Question 3. (Higher tier) True or false? A transformer will step down the p.d. of a 12 V battery to 6 V.

Answer: False

Working: False. A battery gives d.c., which makes a steady field. A p.d. is only induced in the secondary when the field changes, so a transformer needs a.c.

Related lessons

Continue in the full course

This lesson is section 15.7 (AGCPH1507) of our free GCSE Physics course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Physics course.

 

 

 

 

 

 

 

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