When a wire carrying a current is placed in the magnetic field of a magnet, the wire experiences a force. This is called the motor effect. The force is largest when the wire is at right angles to the field. If the wire lies parallel to the field lines, the force is zero.
The magnet also experiences a force. It is equal in size and opposite in direction to the force on the wire. This is Newton's third law: when two objects interact, the forces they exert on each other are equal and opposite. Usually the magnet is heavy or held in place, so we only see the wire move.
The force on the wire gets bigger with a larger current or a stronger magnet. Reversing the current, or reversing the direction of the magnetic field, reverses the direction of the force on the wire.
