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Magnetism and the Motor Effect

The force on a conductor, and how a motor turns

  • 3 things to know
  • Paper 2

Overview

Force = magnetic flux density × current × length, the tesla and what 1 T means, calculating force, flux density, current and length, and how a direct current motor turns, with its split-ring commutator, brushes and direction of rotation.

What you need to know

Answer each one before you reveal it.

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Key term

A current-carrying wire lies at right angles to a magnetic field. State the equation for the force acting on it.

Key term

State the unit of magnetic flux density and its symbol.

Key term

Give the unit of magnetic flux density written in terms of newtons, amperes and metres.

Questions from this subtopic

Each one with the model answer underneath.

  1. 1

    A current-carrying wire lies at right angles to a magnetic field. State the equation for the force acting on it.

    Model answer

    Force = magnetic flux density × current × length.

  2. 2

    State the unit of magnetic flux density and its symbol.

    Model answer

    The tesla, with the symbol T.

  3. 3

    Give the unit of magnetic flux density written in terms of newtons, amperes and metres.

    Model answer

    The newton per ampere metre, N/A m.

  4. 4

    Explain what a magnetic flux density of 1 T means.

    Model answer

    A wire carrying a current of 1 A at right angles to the field feels a force of 1 N on each metre of its length.

  5. 5

    Calculate the force on a 0.25 m length of wire carrying a current of 4.0 A at right angles to a magnetic field of flux density 0.30 T.

    Model answer

    Force = 0.30 × 4.0 × 0.25 = 0.30 N.

  6. 6

    A 0.12 m length of wire carrying a current of 3.0 A at right angles to a magnetic field feels a force of 0.72 N. Calculate the magnetic flux density.

    Model answer

    Magnetic flux density = 0.72 ÷ (3.0 × 0.12) = 2.0 T.

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