GCSE Combined Science (AQA)

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Calculating the Force on a Wire

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Magnetism and electromagnetism The motor effect

Calculating the Force on a Wire

5:56 The Motor Effect
Spec 4.7.2.2
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  • The force on a wire carrying a current perpendicular to a magnetic field is described by the equation: F = B * I * L.
  • This equation represents the motor effect, where a wire experiences a force when it carries a current through a magnetic field.
  • Variables in the equation:
  • F: Force experienced by the wire (in Newtons).
  • B: Magnetic flux density (in Teslas), indicating the strength of the field.
  • I: Current (in amperes), with a larger current leading to a larger force.
  • L: Length of the wire in the field (in meters), with more wire in the field resulting in a larger force.
  • In a motor effect demonstration, the current should be perpendicular to the magnetic field.
  • The equation can be adapted for non-perpendicular angles, but this is not required for GCSE level.
  • In examples, the field and current will always be perpendicular.
  • Only the length of the wire within the field is measured.
  • Fleming's left-hand rule can be used to determine the direction of the force.
  • Practice question: Calculate the force on a wire of length 40 cm carrying a current of 20 A, perpendicular to a magnetic field with a flux density of 0.02 Teslas.
  • Steps to solve:
  • 1. Draw a diagram to understand the values and directions.
  • 2. Recall the equation F = I * B * L.
  • 3. Substitute the values in correct units (convert 40 cm to 0.4 m).
  • 4. Calculate the force: F = 0.02 * 20 * 0.4 = 0.16 Newtons.
  • 5. Use Fleming's left-hand rule to find the direction of the force (out of the page).
  • Final answer: The force is 0.16 Newtons, directed out of the page.

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