The force on a straight conductor carrying a current at right angles to a magnetic field can be calculated using: force (N) = magnetic flux density (T) × current (A) × length (m), or F = B × I × l. The force is measured in newtons (N). Magnetic flux density measures how strong the field is and is measured in tesla (T). The current is measured in amperes (A) and the length is the length of wire inside the field, in metres (m).
Worked example: a wire 0.50 m long carries 3.0 A at right angles to a field of flux density 0.20 T. F = 0.20 × 3.0 × 0.50 = 0.30 N.
The equation can be rearranged. For example, to find the current, I = F ÷ (B × l). The equation only works when the wire is at right angles to the field. Doubling the current, the length in the field or the flux density doubles the force.