When a conductor carrying a current is placed in a magnetic field, the magnet producing the field and the conductor exert a force on each other. This is called the motor effect. Reversing the direction of the current, or reversing the direction of the magnetic field, reverses the direction of the force.
Fleming's left-hand rule gives the relative orientation of the force, the current and the field. Hold the thumb, the first finger and the second finger of your left hand at right angles to each other. The first finger points in the direction of the magnetic field (north to south), the second finger points in the direction of the current, and the thumb then points in the direction of the force, which is the direction the conductor tends to move.
The size of the force depends on the magnetic flux density, the current through the conductor and the length of conductor in the field. For a conductor at right angles to the field: force = magnetic flux density × current × length, or F = BIl. Force is in newtons (N), magnetic flux density in tesla (T), current in amperes (A) and length in metres (m). For example, a wire with 0.1 m in a field of 0.2 T carrying 5 A has a force of 0.2 × 5 × 0.1 = 0.1 N. For a given wire and field, the force is directly proportional to the current.
