Electric Motors and Loudspeakers

A current-carrying coil of wire rotates in a magnetic field

·       The diagram below shows a basic dc motor. Forces act on the side arms of a coil of wire that’s carrying a current.

·       These forces are just the usual forces which act on any current in a magnetic field.

·       Because the coil is on a spindle and the forces act one up and one down, it rotates

·       The spilt-ring commutator is a clever way of swapping the contacts every half turn to keep the motor rotating in the same direction

·       The direction of the motor can be reversed either by swapping the polarity of the dc supply (reversing the current) or swapping the magnetic poles over (reversing the field).

You can use Fleming’s left-hand rule to work out which way the coil will turn.

·       Draw in current arrows (from positive to negative)

·       Using Fleming’s left-hand rule on one branch

·       Point your first finger in the direction of the magnetic field (this is north to south)

·     Point your second finger in the direction of the current

·   Draw in the direction of motion (the direction your thumb in pointing in)

Loudspeakers work because of the motor effect

Loudspeakers and headphones both use electromagnets:

·       An alternating current(ac) is sent through a coil of wire attached to the base of a paper cone.

·       The coil surrounds one pole of a permanent magnet, and is surrounded by the other pole, so the current causes a force on the coil (which causes the cone to move)

·       When the current reverses, the force acts in the opposite direction, which causes the cone to move in the opposite direction too.

·       So, variations in the current make the cone vibrate, which makes the air around the cone vibrate and creates the variations in pressure that cause a sound wave.

·       The frequency of the sound wave is the same as the frequency of the ac, so by controlling the frequency of the ac you can alter the sound wave produced.