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Physics · Magnetism and electromagnetism

The motor effect

Explain the motor effect, use Fleming's left-hand rule and calculate the force on a conductor using \(F = BIl\) (Higher tier only; Physics only).

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  • 6 key terms
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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 a force?

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    A push or a pull

  2. 2

    What is the unit of force?

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    Newton (N)

  3. 3

    What is a magnetic field?

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    A region where a force acts on a magnet

  4. 4

    What is current?

    Show answerHide answer

    A flow of charge

  5. 5

    What is a conductor?

    Show answerHide answer

    A material that lets current flow

Learning Objectives

  1. 1Explain why a current-carrying wire in a magnetic field experiences a force.
  2. 2Use Fleming's left-hand rule to find the direction of the force.
  3. 3Recall and use \(F = BIl\).
  4. 4State when the force is greatest and when it is zero.
  5. 5Explain how the force can be reversed.

THE MOTOR EFFECT

force \(=\) magnetic flux density \(\times\) current \(\times\) length \(F = BIl\)

\(F\) in newtons (N), \(B\) in tesla (T), \(I\) in amperes (A) and \(l\) in metres (m). The conductor must be at right angles to the field. \(F = BIl\) is on the equation sheet.

Calculating the Force

A wire of length 0.20 m carries a current of 3.0 A at right angles to a magnetic field of flux density 0.40 T. Calculate the force.

Show the solutionHide the solution
  1. 1 Write the equation \(F = BIl\)
  2. 2 Substitute \(F = 0.40 \times 3.0 \times 0.20\)
  3. 3 Answer \(F = 0.24\) N

Answer0.24 N

Finding the Current

A force of 0.60 N acts on a 0.30 m wire in a field of 0.50 T. Calculate the current.

Show the solutionHide the solution
  1. 1 Rearrange \(I = F \div (Bl)\)
  2. 2 Substitute \(I = 0.60 \div (0.50 \times 0.30)\)
  3. 3 Answer \(I = 4.0\) A

Answer4.0 A

Reversing and Changing the Force

Learn what happens.

  • Reverse the current

    Effect on the force: Force reverses

  • Reverse the magnetic field

    Effect on the force: Force reverses

  • Increase the current

    Effect on the force: Force increases

  • Stronger magnets (larger B)

    Effect on the force: Force increases

  • Wire parallel to the field

    Effect on the force: No force

Why Does It Happen?

Two magnetic fields.

  • Two fields

    The magnet's field and the field round the wire interact.

  • Result

    The two fields push on each other and the wire feels a force.

  • Right angles

    The force is greatest when the wire is at 90° to the field.

  • Parallel

    No force when the wire is parallel to the field.

Hand Rule

A current flows left to right through a wire between a north pole (above) and a south pole (below). Use Fleming's left-hand rule. Which way is the force? Then calculate F for B = 0.30 T, I = 5.0 A, l = 0.10 m.

1. Thumb = force.

2. First finger = field.

3. Second finger = current.

A good answer shows: The field points downwards (north to south), the current points to the right, so the force is into the page. F = 0.30 × 5.0 × 0.10 = 0.15 N.

Can I...?

  1. 1State the motor effect.
  2. 2Use the left-hand rule.
  3. 3Write \(F = BIl\).
  4. 4Give units.
  5. 5Use right angles.
  6. 6Reverse the force.
  7. 7Change the force.
  8. 8Calculate F, I, B or l.

Summary & Exam Focus

  • \(F = BIl\) (tesla, amperes, metres).
  • Left-hand rule: thumb force, first finger field, second finger current.
  • Reversing current or field reverses the force.
  • No force if parallel to the field.

Exam focus

A 0.20 m wire carries 3.0 A at right angles to a 0.40 T field. Calculate the force. (2 marks) (2 marks)

\(F = BIl\).

Key terms

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

Motor effect
The force on a current-carrying conductor in a magnetic field.
Magnetic flux density
A measure of the strength of a magnetic field, in tesla (T).
Tesla
The unit of magnetic flux density.
Fleming's left-hand rule
A rule for finding the direction of the force on a current-carrying wire in a magnetic field.
Conductor
A material that allows a current to flow.
Thumb
Gives the direction of the force in Fleming's left-hand rule.

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 wire of length 0.20 m carries a current of 3.0 A at right angles to a magnetic field of flux density 0.40 T. Calculate the force on the wire. Use the equation: force = magnetic flux density × current × length

Mark scheme — 2 marks available

  • Correct substitution — 1 mark
  • 0.24 N — 1 mark

Model answer

\(F = 0.40 \times 3.0 \times 0.20 = 0.24\) N

2. Exam question State 2 marks Easier

State two ways of reversing the direction of the force on a current-carrying wire in a magnetic field.

Mark scheme — 2 marks available

  • Reverse the current — 1 mark
  • Reverse the field — 1 mark

Model answer

Reverse the direction of the current. Reverse the direction of the magnetic field (swap the magnets).

3. Exam question Calculate 3 marks Easier

A force of 0.60 N acts on a wire of length 0.30 m in a magnetic field of 0.50 T. The wire is at right angles to the field. Calculate the current in the wire.

Mark scheme — 3 marks available

  • Rearranges to I = F ÷ (B l) — 1 mark
  • Substitution — 1 mark
  • 4.0 A — 1 mark

Model answer

\(I = F \div (Bl) = 0.60 \div (0.50 \times 0.30) = 4.0\) A

4. Exam question Explain 3 marks Easier

Explain why a wire carrying a current experiences a force when it is placed between the poles of a magnet.

Mark scheme — 3 marks available

  • Current produces a magnetic field — 1 mark
  • Interacts with the magnet's field — 1 mark
  • Resulting force on the wire — 1 mark

Model answer

The current produces its own magnetic field around the wire. This interacts with the field of the magnet, and the two fields exert a force on each other so the wire moves.

5. Exam question Explain 3 marks Easier

Explain what happens to the force on the wire in a magnetic field when the wire is turned so that it is parallel to the field lines. Give a reason.

Mark scheme — 3 marks available

  • Force decreases to zero — 1 mark
  • Force is greatest at right angles — 1 mark
  • Fields no longer interact / no component at right angles — 1 mark

Model answer

The force decreases to zero because the wire's field no longer interacts with the magnet's field; the force is largest when the wire is at right angles to the field.

6. Multiple choice 1 mark Easier

The force on a current-carrying wire in a magnetic field is the...

  1. A generator effect
  2. B greenhouse effect
  3. C Doppler effect
  4. D motor effect Correct

Why: Motor effect.

7. Multiple choice 1 mark Core

In Fleming's left-hand rule the thumb shows the...

  1. A force Correct
  2. B current
  3. C field
  4. D charge

Why: Thrust.

8. Multiple choice 1 mark Core

The unit of magnetic flux density is the...

  1. A henry
  2. B tesla Correct
  3. C ohm
  4. D weber only

Why: T.

9. Multiple choice 1 mark Core

F = 0.5 T × 2 A × 0.4 m =

  1. A 0.25 N
  2. B 2.5 N
  3. C 0.4 N Correct
  4. D 4 N

Why: 0.5 × 2 × 0.4.

10. Multiple choice 1 mark Stretch

When a wire is parallel to the field the force is...

  1. A maximum
  2. B half
  3. C reversed
  4. D zero Correct

Why: No interaction.