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Physics · Magnetism and electromagnetism

Induced potential

Describe the generator effect: how a potential difference and current are induced by a changing magnetic field, and what affects their size and direction (Physics only).

  • 6 key terms
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Teacher resources

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Student handouts

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

Warm-up

Answer each one, then check.

  1. 1

    What does a voltmeter measure?

    Show answerHide answer

    Potential difference

  2. 2

    What is a magnetic field?

    Show answerHide answer

    Region where a force acts on a magnet

  3. 3

    What is a current?

    Show answerHide answer

    A flow of charge

  4. 4

    What is a coil?

    Show answerHide answer

    Loops of wire

  5. 5

    What is a conductor?

    Show answerHide answer

    Allows current to flow

Learning Objectives

  1. 1Describe the generator effect.
  2. 2Describe how a potential difference is induced in a wire or coil by moving a magnet.
  3. 3Explain the direction of the induced current and its effect (Higher).
  4. 4Describe the factors that increase the size of the induced potential difference.

THE GENERATOR EFFECT

The generator effect is when a potential difference is induced across a conductor that moves relative to a magnetic field, or when the magnetic field through a conductor changes. If the conductor is part of a complete circuit, a current is induced.

Nothing is induced if there is no change: a stationary magnet in a stationary coil produces no potential difference.

What Changes the Induced p.d.

Learn the factors.

  • Faster movement of the magnet

    Effect on the induced potential difference: Larger

  • Stronger magnet

    Effect on the induced potential difference: Larger

  • More turns on the coil

    Effect on the induced potential difference: Larger

  • Reverse the movement

    Effect on the induced potential difference: Direction of p.d. reverses

  • Reverse the magnet

    Effect on the induced potential difference: Direction of p.d. reverses

  • Magnet stationary

    Effect on the induced potential difference: No induced p.d.

Why Does a Current Flow?

Explain why a current is produced when a magnet is moved into a coil connected in a circuit.

Show the solutionHide the solution
  1. 1 Magnetic field changes As the magnet moves in, the field through the coil changes
  2. 2 p.d. induced A potential difference is induced across the coil
  3. 3 Complete circuit The induced p.d. makes a current flow

AnswerA changing field induces a p.d., and a current in a complete circuit.

Direction of the Induced Current

Higher tier.

  • Its own field

    The induced current produces a magnetic field of its own.

  • Opposes the change

    This field opposes the change that produced it.

  • Energy

    You have to do work to push the magnet in.

  • Conservation

    That work is what gets transferred to electrical energy.

Magnet in a Coil

Push a bar magnet into a coil connected to a galvanometer. Record the deflection: slowly, quickly, out, stationary. What do you notice?

1. Keep the coil the same.

A good answer shows: Faster gives a bigger deflection; out reverses it; stationary gives zero.

Can I...?

  1. 1State the generator effect.
  2. 2Say what induces a p.d.
  3. 3Say when a current flows.
  4. 4List ways to increase it.
  5. 5Reverse the direction.
  6. 6Explain no movement, no p.d.
  7. 7Explain the opposing field.
  8. 8Link to energy.

Summary & Exam Focus

  • A changing magnetic field induces a potential difference.
  • Bigger with faster movement, stronger magnet, more turns.
  • No movement gives no p.d.
  • Reversing the movement reverses the p.d.

Exam focus

State two ways of increasing the potential difference induced in a coil. (2 marks) (2 marks)

Faster movement, stronger magnet, more turns.

Key terms

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

Generator effect
The induction of a potential difference across a conductor when the magnetic field changes or the conductor moves in a magnetic field.
Induced potential difference
A p.d. produced by the generator effect.
Induced current
A current produced in a complete circuit by an induced p.d.
Coil
Loops of wire.
Flux
The magnetic field passing through a coil.
Galvanometer
A sensitive meter that shows small currents.

Questions and answers

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

1. Exam question State 2 marks Easier

A magnet is pushed into a coil of wire connected to a voltmeter. State two ways of increasing the reading on the voltmeter.

Mark scheme — 2 marks available

  • One way — 1 mark
  • Another way — 1 mark

Model answer

Move the magnet faster. Use a stronger magnet. Use more turns on the coil. (any two)

2. Exam question Explain 3 marks Easier

A student holds a bar magnet still inside a coil of wire connected to a voltmeter. The voltmeter reads zero. Explain why and what they should do to get a reading.

Mark scheme — 3 marks available

  • No change in field — 1 mark
  • No p.d. induced — 1 mark
  • Move the magnet — 1 mark

Model answer

There is no change in the magnetic field through the coil so no potential difference is induced. They should move the magnet in or out of the coil.

3. Exam question Describe 3 marks Easier

Describe what happens to the reading on the voltmeter when the magnet is pulled out of the coil at the same speed as it was pushed in.

Mark scheme — 3 marks available

  • A reading is shown — 1 mark
  • Same size — 1 mark
  • Opposite direction — 1 mark

Model answer

The voltmeter shows a reading of the same size but in the opposite direction (reversed polarity).

4. Exam question Explain 3 marks Easier

Explain why a current flows when a magnet is moved into a coil that is part of a complete circuit.

Mark scheme — 3 marks available

  • Changing field through the coil — 1 mark
  • p.d. induced — 1 mark
  • Complete circuit so current flows — 1 mark

Model answer

The movement of the magnet changes the field through the coil, which induces a potential difference. Because the circuit is complete, the p.d. makes a current flow.

5. Exam question Explain 3 marks Easier

Explain why it is harder to push a magnet into a coil connected in a complete circuit than into a coil that is not connected.

Mark scheme — 3 marks available

  • Induced current produces its own field — 1 mark
  • Opposes the motion — 1 mark
  • Work done is transferred to electrical energy — 1 mark

Model answer

The induced current produces a magnetic field that opposes the change (the magnet's motion), so work must be done to push the magnet in. That work is transferred to electrical energy. With no complete circuit there is no current and so no opposing force.

6. Multiple choice 1 mark Easier

A p.d. can be induced in a coil by...

  1. A a steady magnetic field
  2. B a hot coil
  3. C a changing magnetic field Correct
  4. D a cold coil

Why: Generator effect.

7. Multiple choice 1 mark Core

A stationary magnet in a coil induces...

  1. A a large p.d.
  2. B a current always
  3. C sound
  4. D no potential difference Correct

Why: No change.

8. Multiple choice 1 mark Core

Moving the magnet faster makes the p.d...

  1. A larger Correct
  2. B smaller
  3. C zero
  4. D constant

Why: Faster change.

9. Multiple choice 1 mark Core

Reversing the movement makes the p.d...

  1. A increase
  2. B reverse direction Correct
  3. C zero
  4. D double

Why: Direction flips.

10. Multiple choice 1 mark Stretch

The induced current's field...

  1. A helps the change
  2. B has no direction
  3. C opposes the change Correct
  4. D is zero

Why: Conservation of energy.