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Magnetic Flux Density

Physics: AQA A Level, topic 7.5.9

Magnetic Force on a Wire Experiment

We can investigate how the force on a wire varies with flux density, current and length of wire in the magnetic field, by using a top pan balance.

Apparatus

Apparatus

  • Set up the apparatus as shown in the diagram.
  • For this investigation, aim to make the angle of the wire with the field as close as possible to 90°.
  • Caution: the current should not exceed 4A (as the variable resistor is likely to smoke).
Variables to investigate

Variables to investigate

  • Using the arrangement on the previous slide, we can investigate the magnitude of the force as a function of:
  • Flux density of the magnetic field
  • Current in the wire
  • Length of wire in the field
  • Angle between wire and field lines
Formula

Formula

  • The formula for the magnetic force is:
  • F=BILsin⁡θF = BIL\sin \theta
  • Where F is the force, B is the magnetic flux density, L is the length of wire in the field and θ is the angle between the field lines and the wire.
Example analysis

Example analysis

  • An example of the analysis possible:
  • Plot a graph of force against current.
  • The line of best fit should be a straight line that passes through the origin.
  • By comparison with “y = mx + c”, the gradient of the line should be equal to BLsinθ.

Test yourself on Magnetic Flux Density

  • What is the correct formula for the magnetic force on a wire?

  • Magnetic Force on a Wire Experiment

  • Why should current not exceed 4A?

Practise this topic for free