Electric Motors and Loudspeakers

• Current-Carrying Coil of Wire Rotates in a Magnetic Field:

  1. basic dc motor. Forces act on the two side arms of a coil of wire that's carrying a current
  2. these forces are just the usual forces which act on any current in a magnetic field
  3. because the coil is on a spindle and the forces act one up and one down, it rotates
  4. the split-ring commutator is a clever way of swapping the contacts every half turn to keep the motor rotating in the same direction
  5. direction of motor can be reversed by swapping the polarity of the dc supply or swapping the magnetic poles over
• How do loudspeakers work?

  • an alternating current is sent through a coil of wire attached to the base of a paper cone
  • coil surrounds one pole of a permanent magnet, and is surrounded by the other pole, so the current causes a force on the coil
  • when 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 sound wave is the same as the frequency of the a.c, so by controlling the frequency of the a.c you can alter the sound wave produced