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Magnetic Flux Density

Physics: AQA A Level, topic 7.5.1

Magnetic Force

Magnetic force is the attractive or repulsive force felt by a charge when it is moving in a magnetic field.

Magnetic force

Magnetic force

  • Magnetic force can be calculated using the following equation:
  • Force = charge x velocity x field strength x sin(θ)sin({\theta})
  • F=qvBsin(θ)F=qvBsin({\theta})
    • Where θ{\theta} is the angle between the velocity and the field lines.
Magnetic field strength

Magnetic field strength

  • The magnetic field strength, B, is a measure of how dense the field lines are.
  • The units of magnetic field strength are Tesla (T).

Fleming's Left-Hand Rule

Fleming's left-hand rule is a useful way of telling which direction the field, the movement of the charge and the current flow in a magnetic field.

Perpendicular directions

Perpendicular directions

  • The field, the movement of the charge and the current have to be in perpendicular directions, so they must all be at 90o to each other.
  • If you hold your left hand as shown in the image:
  • Your thumb represents the direction of movement of the charge.
  • Your first finger represents the field.
  • Your middle finger represents the current.
Method

Method

  • The question will give you two out of three of these quantities.
  • You will need to line up those fingers then see which direction the last one is pointing in.
  • Remember field lines run from the north pole to the south.

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 for magnetic force

Formula for magnetic force

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

Analysis example for current

  • 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 BLsin⁡θBL\sin\theta.

Test yourself on Magnetic Flux Density

  • What is the magnetic field strength a measure of?

  • What is the equation for the force felt by a charge moving in a magnetic field?

  • Which statements about magnetic force are correct?

  • Who is the left-hand rule named after?

  • What does the gradient of a graph of magnetic force against current show?

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