A wire carrying a current in a magnetic field can feel a force. This is the motor effect. The size of the force on a straight conductor at right angles to the field depends on three things: the strength of the field, the size of the current, and the length of wire that lies in the field.
The strength of a magnetic field is measured by its magnetic flux density, B. Its unit is the tesla (T), which is the same as one newton per ampere metre (N/A m). A stronger field has a larger flux density. The equation is:
force = magnetic flux density × current × length
F = B × I × l
Force F is in newtons (N), flux density B in tesla (T), current I in amperes (A) and length l in metres (m). The equation only works when the wire is at right angles to the field. If the current, the field or the length is doubled, the force doubles.
Worked example: a wire of length 0.20 m carries a current of 2.0 A at right angles to a field of flux density 0.60 T. F = 0.60 × 2.0 × 0.20 = 0.24 N. Always convert lengths to metres first, so 20 cm must be written as 0.20 m.