A wire that carries a current in a magnetic field feels a force. For a wire at right angles to the field, the size of the force is found with F = B × I × l, where F is the force in newtons (N), B is the magnetic flux density in tesla (T), I is the current in amperes (A) and l is the length of wire in the field in metres (m). One tesla is the same as one newton per ampere metre (N/A m).
Worked example: a wire 0.20 m long carries 4.0 A at right angles to a field of 0.50 T. F = 0.50 × 4.0 × 0.20 = 0.40 N. If the current is doubled to 8.0 A, the force doubles to 0.80 N. Doubling the length of wire in the field also doubles the force.
The equation can be rearranged to give B = F ÷ (I × l). A wire 0.50 m long carrying 3.0 A feels a force of 1.5 N, so B = 1.5 ÷ (3.0 × 0.50) = 1.0 T.
The equation only works when the current and the field are at right angles. If the wire lies parallel to the field, the force is zero.