motor

• the motor effect occurs when a conductor carrying a current is placed in a magnetic field (usually at a right angle)

  the magnetic field around the wire interacts with the magnetic field between the magnets

  = experiences a force , so will move
• if the two magnetic fields are parallel there will be no interaction between the two magnetic fields and therefore no force is produced
• for a conductor at right angles to a magnetic field and carrying current:

  force = magnetic flux density x current x length

  F = N

  B = T (measure of strength of magnetic field)

  I = A

  L = M
• increasing the force:

  • increasing the amount of current flowing through the wire = increase magnetic field around wire
  • using stronger magnets = increase magnetic fields between poles
  • placing the wire at a right angle to the direction of the magnetic field lines between the poles of the magnets = maximum interaction between the 2 magnetic fields
• to change the direction of force:

  • reverse current
  • reverse poles of the magnet
• flemings left hand rule:

  THUMB = THRUST

  FIRST finger = FIELD

  SECOND finger = CURRENT (conventional, + to -)
• decrease force acting on wire:

  • decrease current
  • decrease strength of magnets
  • rotate the magnets so the field is no longer perpendicular to wire
• electric motor: 1

  a coil of wire is positioned in a uniform magnetic field

  when the current is flowing in the coil at 90 degrees to the direction of the magnetic field the current creates a magnetic field around the coil

  this interacts with the magnetic field produced by the magnets = force exerted on the coil
• electric motor: 2

  the current will flow in opposite directions on each side of the coil, so the force produced from the interaction of magnetic fields causes one side up and one side down (moment on each side)

  = coil rotates
• electric motor: 3

  the coil rotates until it is in a vertical position, where there will be a force acting upwards and downwards

  the split ring commutator swaps the contacts of the coil, this reverses the direction the current is flowing in, so reversing the direction in which the forces are acting in = the coil continues to rotate
• the commutator reverses the direction of the current in the coil every half turn, keeping the coil rotating continuously as long as the current is flowing
• increase speed of electric motor by:

  • increasing the current
  • using a stronger magnet
• change the direction of rotation of coil in d.c. motor by:

  • reversing the direction of the current
  • reversing the direction of the magnetic field by reversing the poles of the magnet
• increase the force supplied by the motor by:

  • increasing the current in the coil
  • increasing the strength of the magnetic field
  • increasing the number of turns (in the coil)
• loudspeakers and headphones use the motor effect to convert variations in current in electrical circuits to pressure variations in sound waves
• loudspeakers 1:

  a loudspeaker consists of a coil of wire, of which an alternating current passes through, which is wrapped around one pole of a permanent magnet

  this generates a changing magnetic field around the coil

  as the current is constantly changing direction, the direction of the magnetic field will also be constantly changing
• loudspeakers 2:

  the magnetic field from the coil interacts with the magnetic field from the permanent magnet (either attract or repel)

  this interaction produces a (resultant) force causes the coil to move
• loudspeakers 3:

  when the current changes direction, the direction of the force on the coil reverses, making the coil oscillate, this causes the speaker cone to oscillate, which makes the air oscillate, therefore creating sound waves
• we can change the frequency that the speaker cone vibrates at by changing the frequency of the AC supply
• high frequency = high pitched sound

  low frequency = low pitched sound
• if we increase the size of the current in loudspeakers, then we increase the amplitude of the vibration, which increases the volume of the sound