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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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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.

Practice questions

Have a go at each one before you open its answer.

  1. Question 1 Calculate 2 marks

    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

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    Model answer

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

    Mark scheme

    • Correct substitution — 1 mark
    • 0.24 N — 1 mark
  2. Question 2 State 2 marks

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

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    Model answer

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

    Mark scheme

    • Reverse the current — 1 mark
    • Reverse the field — 1 mark
  3. Question 3 Calculate 3 marks

    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.

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    Model answer

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

    Mark scheme

    • Rearranges to I = F ÷ (B l) — 1 mark
    • Substitution — 1 mark
    • 4.0 A — 1 mark
  4. Question 4 Explain 3 marks

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

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    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.

    Mark scheme

    • Current produces a magnetic field — 1 mark
    • Interacts with the magnet's field — 1 mark
    • Resulting force on the wire — 1 mark
  5. Question 5 Explain 3 marks

    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.

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    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.

    Mark scheme

    • 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

Quick check

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

    1. Agenerator effect
    2. Bgreenhouse effect
    3. CDoppler effect
    4. Dmotor effect
    Show answerHide answer

    D: motor effect

    Motor effect.

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

    1. Aforce
    2. Bcurrent
    3. Cfield
    4. Dcharge
    Show answerHide answer

    A: force

    Thrust.

  3. The unit of magnetic flux density is the...

    1. Ahenry
    2. Btesla
    3. Cohm
    4. Dweber only
    Show answerHide answer

    B: tesla

    T.

  4. F = 0.5 T × 2 A × 0.4 m =

    1. A0.25 N
    2. B2.5 N
    3. C0.4 N
    4. D4 N
    Show answerHide answer

    C: 0.4 N

    0.5 × 2 × 0.4.

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

    1. Amaximum
    2. Bhalf
    3. Creversed
    4. Dzero
    Show answerHide answer

    D: zero

    No interaction.

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