20.2 Force on a current-carrying conductor

A current-carrying conductor experiences magnetic force when its current has a component perpendicular to an external magnetic field.

Reverse current or field direction to reverse force. Parallel current and field produce no force in the ideal straight-wire model.

A wire between magnet poles deflects when current flows, forming the basis of a simple motor.

The force is not caused by current alone; it requires interaction with an external magnetic field.

For wire length L carrying current I in field B at angle θ, force magnitude is F=BIL sinθ.

Use θ between current direction and field, and apply Fleming’s left-hand rule or a vector cross product for direction.

A 0.20 m wire carrying 3 A perpendicular to 0.50 T field experiences 0.30 N.

Using BIL without checking angle overestimates force when the conductor is not perpendicular.

For a wire perpendicular to a field, B=F/(IL), measured in tesla, so 1 T=1 N A⁻¹ m⁻¹.

The definition assumes the conductor is perpendicular; otherwise divide by IL sinθ or resolve the perpendicular component.

A 0.40 N force on a 0.20 m wire carrying 2.0 A gives B=1.0 T when perpendicular.

Tesla is not force per charge; that relates to electric field, while B describes magnetic force on current or moving charge.