OpenRevise GCSE

Viewing as

Student Teacher

Teacher view: planning notes, the answers to every question, and the teacher copies of the files.

Physics · Magnetism and electromagnetism

Transformers

Explain how a transformer works, and use \(\dfrac{V_p}{V_s} = \dfrac{n_p}{n_s}\) and \(V_s I_s = V_p I_p\) (Physics only; Higher tier only).

  • Higher
  • 6 key terms
  • All boards
Download the teacher pack · 5 files

Teacher resources

The teacher copies: slides with the questions built in, the answers, and anything else attached to this lesson for whoever is teaching it.

Student handouts

The same files the students see, to print or hand out.

Warm-up

Answer each one, then check.

  1. 1

    What is the National Grid?

    Show answerHide answer

    The network that transfers electrical power

  2. 2

    Why is high voltage used in the grid?

    Show answerHide answer

    To reduce energy lost as heat

  3. 3

    What is power?

    Show answerHide answer

    The rate of transfer of energy

  4. 4

    What is a.c.?

    Show answerHide answer

    Alternating current

  5. 5

    What is a coil?

    Show answerHide answer

    Loops of wire

Learning Objectives

  1. 1Describe the structure and function of a transformer.
  2. 2Explain how a step-up and a step-down transformer work.
  3. 3Use \(V_p / V_s = n_p / n_s\).
  4. 4Use \(V_s I_s = V_p I_p\) for a 100% efficient transformer.
  5. 5Explain why the National Grid uses transformers.

THE TRANSFORMER EQUATIONS

\(\dfrac{V_p}{V_s} = \dfrac{n_p}{n_s}\) and \(V_s \times I_s = V_p \times I_p\)

\(V_p\), \(I_p\), \(n_p\) are for the primary coil; \(V_s\), \(I_s\), \(n_s\) for the secondary. The second equation assumes 100% efficiency (power in = power out).

How a Transformer Works

Follow the steps.

  1. 1 a.c. in the primary

    An alternating current flows in the primary coil.
  2. 2 Changing field

    It produces a changing magnetic field in the iron core.
  3. 3 Field through the secondary

    The changing field passes through the secondary coil.
  4. 4 Generator effect

    An alternating p.d. is induced in the secondary coil.
  5. 5 Turns ratio

    The ratio of turns sets the ratio of potential differences.

Step-up or Step-down?

Step-up transformer

  • More turns on the secondary coil.
  • Increases the potential difference.
  • Used at power stations before the grid.

Step-down transformer

  • Fewer turns on the secondary coil.
  • Decreases the potential difference.
  • Used near homes to bring the p.d. down to 230 V.

Using the Turns Ratio

A transformer has 200 turns on its primary coil and 1000 on its secondary. The input is 12 V. Calculate the output potential difference.

Show the solutionHide the solution
  1. 1 Write the equation \(V_p / V_s = n_p / n_s\)
  2. 2 Rearrange \(V_s = V_p \times n_s \div n_p\)
  3. 3 Substitute \(V_s = 12 \times 1000 \div 200\)
  4. 4 Answer \(V_s = 60\) V

Answer60 V

Power In Equals Power Out

A 100% efficient transformer has a primary p.d. of 230 V and current of 2.0 A. The secondary p.d. is 11.5 V. Calculate the secondary current.

Show the solutionHide the solution
  1. 1 Write the equation \(V_s I_s = V_p I_p\)
  2. 2 Rearrange \(I_s = V_p I_p \div V_s\)
  3. 3 Substitute \(I_s = 230 \times 2.0 \div 11.5\)
  4. 4 Answer \(I_s = 40\) A

Answer40 A

Why the National Grid Uses Them

High voltage, low current.

  • Step-up at the power station

    Raises the p.d. (up to 400 000 V) and lowers the current.

  • Lower current

    Less energy is wasted as heat in the cables.

  • Step-down near homes

    Reduces the p.d. to 230 V for safe use.

  • Efficiency

    The grid is efficient because transformers waste little energy.

Transformer Sums

A transformer has \(n_p = 50\) and \(n_s = 500\). Input 10 V a.c. Find \(V_s\). If the input current is 2.0 A, find \(I_s\) assuming 100% efficiency.

1. Step-up: p.d. up, current down.

A good answer shows: Vs = 10 × 500 ÷ 50 = 100 V. Power in = 20 W, so Is = 20 ÷ 100 = 0.20 A.

Can I...?

  1. 1Name the parts.
  2. 2Explain the working.
  3. 3State step-up and step-down.
  4. 4Use the turns ratio.
  5. 5Use power in = power out.
  6. 6Explain why a.c. is needed.
  7. 7Explain the National Grid.
  8. 8Give typical voltages.

Summary & Exam Focus

  • \(V_p / V_s = n_p / n_s\).
  • \(V_s I_s = V_p I_p\) for 100% efficiency.
  • Step-up increases the p.d. and decreases the current.
  • a.c. is needed for a changing field.

Exam focus

A transformer has 200 primary turns and 1000 secondary turns. The input is 12 V. Calculate the output. (2 marks) (2 marks)

Vs = Vp × ns ÷ np.

Key terms

The vocabulary this lesson expects you to use. Each one is linked from the first place it appears above.

Transformer
A device that changes the potential difference of an alternating supply.
Primary coil
The input coil of a transformer.
Secondary coil
The output coil of a transformer.
Step-up transformer
A transformer that increases the potential difference.
Step-down transformer
A transformer that decreases the potential difference.
Soft iron core
A core that easily becomes magnetised and demagnetised, linking the coils.

Questions and answers

10 questions set on this lesson, with the mark schemes and model answers open.

1. Exam question Calculate 2 marks Easier

A transformer has 200 turns on its primary coil and 1000 turns on its secondary coil. The input potential difference is 12 V. Calculate the output potential difference. Use the equation: Vp ÷ Vs = np ÷ ns

Mark scheme — 2 marks available

  • Correct rearrangement and substitution — 1 mark
  • 60 V — 1 mark

Model answer

\(V_s = 12 \times 1000 \div 200 = 60\) V

2. Exam question Explain 4 marks Easier

Explain how a transformer changes the potential difference of an alternating supply.

Mark scheme — 4 marks available

  • a.c. in the primary makes a changing magnetic field — 1 mark
  • Field carried by the iron core to the secondary — 1 mark
  • Induced p.d. in the secondary (generator effect) — 1 mark
  • Turns ratio sets the p.d. ratio — 1 mark

Model answer

An alternating current in the primary coil produces a changing magnetic field in the iron core. This changing field passes through the secondary coil and induces an alternating potential difference in it. The ratio of the turns determines the ratio of the potential differences.

3. Exam question Calculate 3 marks Easier

A transformer is 100% efficient. The primary p.d. is 230 V and the primary current is 2.0 A. The secondary p.d. is 11.5 V. Calculate the secondary current.

Mark scheme — 3 marks available

  • Vs Is = Vp Ip — 1 mark
  • Correct substitution — 1 mark
  • 40 A — 1 mark

Model answer

\(I_s = 230 \times 2.0 \div 11.5 = 40\) A

4. Exam question Explain 3 marks Easier

Explain why a transformer does not work with a direct current.

Mark scheme — 3 marks available

  • d.c. produces a constant field — 1 mark
  • No change in field in the secondary — 1 mark
  • No p.d. induced — 1 mark

Model answer

A direct current gives a steady magnetic field in the core. No change in field passes through the secondary coil, so no potential difference is induced.

5. Exam question Explain 4 marks Easier

Explain why electricity is transmitted in the National Grid at a very high potential difference.

Mark scheme — 4 marks available

  • Step-up transformer raises p.d. — 1 mark
  • Current is lower for the same power — 1 mark
  • Less energy wasted as heat — 1 mark
  • Step-down near homes for safety — 1 mark

Model answer

A step-up transformer increases the potential difference and so, for the same power, the current is lower. A lower current means less energy is wasted as heat in the cables, so the transmission is more efficient. Step-down transformers then reduce the p.d. for use in homes.

6. Multiple choice 1 mark Easier

A step-up transformer has...

  1. A more turns on the secondary Correct
  2. B more turns on the primary
  3. C equal turns
  4. D no core

Why: It increases p.d.

7. Multiple choice 1 mark Core

A transformer needs...

  1. A direct current
  2. B alternating current Correct
  3. C no current
  4. D static charge

Why: A changing field.

8. Multiple choice 1 mark Core

Vp = 10 V, np = 100, ns = 500. Vs =

  1. A 2 V
  2. B 5 V
  3. C 50 V Correct
  4. D 500 V

Why: 10 × 500 ÷ 100.

9. Multiple choice 1 mark Core

In a step-up transformer the current...

  1. A increases
  2. B stays the same
  3. C becomes d.c.
  4. D decreases Correct

Why: Power is conserved.

10. Multiple choice 1 mark Stretch

The core of a transformer is...

  1. A soft iron Correct
  2. B copper
  3. C plastic
  4. D steel only

Why: Easily magnetised.