A wire carrying a current experiences a force in an external magnetic field. This is because the magnetic field around the wire due to the current interacts with the external magnetic field. This is known as the motor effect. The magnitude of the force is proportional to the size of the current I, the strength of the magnetic field B (magnetic flux density) and the length of the wire in the magnetic field l. The direction of the force can be found using Fleming's left hand rule and is at right angles to both the magnetic field and the current.