The motor effect

A current flowing through a wire produces a magnetic field around the wire. The field forms concentric circles around a straight wire, and its direction can be found using the right-hand grip rule: point the thumb of your right hand along the direction of the current, and your curled fingers point in the direction of the magnetic field.

• •A larger current produces a stronger magnetic field.
• •The field gets weaker with increasing distance from the wire.

Placing a soft iron core inside a solenoid makes an electromagnet, significantly increasing the field's strength — magnetic field lines pass through iron much more easily than through air. The strength of a solenoid's field also depends on the size of the current, the number of turns of wire, and the coil's length and cross-sectional area.

Picture a solenoid as a coil of wire wrapped around a straight iron core, connected to a battery so current flows through every turn. Outside the coil, the field lines loop around from the end that behaves as north, back to the end that behaves as south — exactly like a bar magnet's field — while inside the coil the field runs straight and roughly uniform along the core's length. Reversing the battery's terminals reverses the current, which reverses which end is north and which is south.

This force is at its maximum when the wire is at right angles (perpendicular) to the magnetic field, and is zero if the wire runs parallel to the field.

A simple electric motor has a rectangular coil of current-carrying wire positioned between the poles of two permanent magnets (so the coil sits within their combined magnetic field), mounted so it can spin around a central axis.

Because the current flows in opposite directions along the two long sides of the coil, the motor effect pushes one side of the coil upward while pushing the opposite side downward at the same time (which direction can be checked with Fleming's left-hand rule on each side). These two opposite forces, acting on opposite sides of the coil, create a turning effect that makes the coil rotate.

A loudspeaker uses the motor effect to convert an electrical signal back into sound. It has a fixed permanent magnet at its centre, surrounded by a coil of wire attached to a paper or fabric cone, free to move back and forth.

As the electrical signal (an alternating current representing the sound) flows through the coil, the interaction between the coil's magnetic field and the fixed magnet's field pushes the coil — and the attached cone — backward and forward, following the pattern of the signal. This movement of the cone creates pressure variations in the air, which we hear as sound.