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Fields and Their Consequences

Magnetic flux density

  • 4 things to know
  • Papers 2 and 3

Overview

The force on a current-carrying wire, Fleming's left hand rule, and the current balance experiment.

What you need to know

Answer each one before you reveal it.

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Numbers

How is the force on a current-carrying wire calculated, and when does F = BIl apply directly?

Numbers

How is magnetic flux density defined, and what does the first finger show in Fleming's left hand rule?

Numbers

How is a balance reading converted into a force when measuring the force on a current-carrying wire?

Numbers

What apparatus is used to measure the magnetic force on a wire in the required practical investigating F = BIl?

Questions from this subtopic

Each one with the model answer underneath.

  1. 1

    A current-carrying wire sits at right angles to a magnetic field. State the formula for the force on it.

    Model answer

    F = BIl

  2. 2

    In the equation F = BIl, what does each symbol represent?

    Model answer

    F is the force on the wire, B is the magnetic flux density, I is the current, and l is the length of wire in the field.

  3. 3

    Calculate the force when magnetic flux density = 0.41 T, current = 5 A, length of wire = 0.21 m.

    Model answer

    Force = 0.41 × 5 × 0.21 = 0.43 N.

  4. 4

    Under what condition does the formula F = BIl apply directly without modification?

    Model answer

    When the magnetic field is perpendicular to the direction of the current.

  5. 5

    If the magnetic field is not perpendicular to the current direction, how must the formula F = BIl be modified?

    Model answer

    The perpendicular component of the magnetic field must be used instead of the full field strength.

  6. 6

    What rule relates the directions of force, magnetic field and current for a current-carrying conductor in a magnetic field?

    Model answer

    Fleming's left hand rule.

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