Passing an electric current through a wire induces a magnetic field around the wire, which goes clockwise around it if looking in the direction of current
With multiple wires, the induced field superpose
In a solenoid, the resultant field goes straight through it (it effectively becomes a bar magnet) and if looking in its direction, the current goes clockwise
The strength of the magnetic field is inversely proportional to the distance from the wire and directly proportional to the magnitude of the current, the number of coils and the presence of an iron core
Force on a current-carrying conductor
In a magnetic field, a current-carrying conductor perpendicular to the direction of the field experiences a force perpendicular to both, namely the direction in which the magnetic effect of the current counteracts the field
d.c. motor
In a magnetic field, a current-carrying coil pointing perpendicular to the direction of the field experiences a turning effect in which the axis of rotation is perpendicular to the direction of the field & coil
The magnitude of the turning effect is directly proportional to the magnitude of the current and the number of turns on the coil
This phenomenon can be utilised to create a motor, using slip rings or commutators to provide current from an a.c. or d.c. e.m.f. respectively; commutators are a set of contacts that touch the ends of motor, so that as the motor turns, the ends advance to a different contact and the current in the motor switches direction, allowing it to generate the correct torque to keep turning