A straight horizontal wire of length 0.60 m carries a current of 3.0 A from west to east. The wire lies in a uniform horizontal magnetic field of magnitude 0.40 T directed from north to south. What is the magnitude and direction of the magnetic force on the wire?
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Correct answer
A. 0.72 N upward
Principle or equation
Ampere force: F = BIL sin θ, with direction given by the right-hand rule.
Why this answer is correct
The angle between current (east) and magnetic field (south) is 90°, so sin θ = 1. F = BIL = 0.40 T × 3.0 A × 0.60 m = 0.72 N. Using the right-hand rule: thumb east, fingers south, palm points upward (since the force is perpendicular to both). Thus the force is upward.
Example
If a 0.5 m wire carries 2 A east in a 0.3 T field north, F = 0.3 × 2 × 0.5 = 0.3 N upward.
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