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Circuit and electromagnetism basics

A straight wire 10 cm long is placed perpendicular to a uniform magnetic field of flux density 0.5 T and carries a current of 2 A. What is the magnitude of the force on the wire?

  1. A0.1 N
  2. B10 N
  3. C0 N
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Answer: A. 0.1 N

The force on a current in a magnetic field is F = BIL sin θ; for a perpendicular wire (θ = 90°), F = 0.5 T × 2 A × 0.1 m = 0.1 N. The length must be converted to meters. The force is perpendicular to both the current and the field, with its direction given by Fleming's left-hand rule. Motors turn this force into rotation. When the current is parallel to the field, the force is zero.

  • A. Correct. 0.5 T × 2 A × 0.1 m = 0.1 N.
  • B. 10 N uses the length in centimeters (10) without converting.
  • C. The force is zero when the current is parallel to the field; it is maximum when perpendicular.
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