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Force on a current-carrying wire

A wire carrying a current in a magnetic field feels a force when the current is not parallel to the field. The force is largest when the wire is perpendicular to the field, and it is zero when they are parallel. The direction is found with the right-hand rule: the fingers from the current towards the field, and the thumb shows the force. This is the force that makes a motor turn. A stronger field, more current or a longer wire gives a larger force.

F = ILB

force when I is perpendicular to B

Symbols

$I$	current	A
$L$	length in the field	m
$B$	magnetic flux density	T
$F$	force	N

Example

A wire of 0.4 m carries 5 A perpendicular to a field of 0.2 T.

$F = 5 \cdot 0.4 \cdot 0.2 = 0.4$ N.

The force is perpendicular to both the current and the field. It does no work along the wire.

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Part of Physics 2: Electromagnetism.