induced potential

• if an electrical conductor (i.e. wire) moves relative to a magnetic field of there is a change in the magnetic field around the conductor, a potential difference is induced across the ends of a conductor (aka wire)

  if the conductor is part of a complete circuit, a current is induced in the conductor.

  = GENERATOR EFFECT
• if the conductor is part of a complete circuit, a current is induced.

  The induced current generates a magnetic field that opposes the original change:

  • movement of the conductor
  • OR change in magnetic field
• the direction of current switches when the direction of movement switches

  if the movement stops then the current stops
• to induce a pd an electrical conductor must:

  • move up and down in the magnetic field
  • the magnetic field moves around and the conductor is still
• we only see the generator effect if the wire passes through a magnetic field

  if it moves along the magnetic field = no induced pd or current
• in increases the induced pd and current:

  • stronger magnetic field
  • increase speed of movement
  • shape wire into coil OR increase number of turns of coil
• the direction of induced current depends on:

  • direction of movement of the magnet (relative to the coil)
  • current is reversed: magnet moves out of coil or other pole of magnet moved into coil
• the induced current creates its own magnetic field

  which opposes the movement of the magnet (i.e. magnet comes in, the induced magnetic field repels the magnet to go out of the coil)

  so if pull N pole out, that end of coil becomes S pole, attracting, harder to pull out
• the induced current makes it harder to move the magnet, so work is done

  transferring energy from movement of magnet into the movement of the current
• an alternator is a coil of wire rotating in a magnetic field to produce a pd, when the wire passes through the magnetic field

  the coil Is connected to 2 metal rings, called commutators, which allows the current to pass out of the coil
• • max pd is when the coil is horizontal , wire sweeps directly through the magnetic field lines at the fastest possible rate, red down, orange up

  
• • when the coil is vertical, the pd falls to zero. this is because the coil is moving parallel to the field, therefore not cutting through the magnetic field lines

  
• • the pd has reversed direction, the 2 sides of coil are moving at different directions (red up orange down)

  
• an alternator produces an alternating current and pd because 2 sides of the coil are attached to 2 different rings
• increase size of AC:

  • stronger magnetic field
  • increase the number of turns in the coil
  • increase area of the coil
  • increasing the rotation speed of the coil increases the size and frequency of AC
• a dynamo produces a direct current

  it consists of a coil of wire rotating in a magnetic field that produces a pd

  it uses a split ring commutator , which has 2 sides
• • when the coil is vertical it moves parallel to the magnetic field so the pd = 0
  • the direction of the pd and current do not reverse when the coil rotates = DIRECT current
  • 2 peaks = 1 full rotation

  
• a split ring commutator changes he coil connections every half turn.

  as the induced pd is about the change direction, the connections are reversed

  this means the current always flows in 1 direction = DC
• how does a moving-coil mic work?

  • when sound waves hit the diaphragm, it vibrates
  • the coil of wire vibrates
  • so the coil of wire moves in and out of the magnetic field
  • this induces a pd as the coil cuts the magnetic field lines
  • the pd is alternating as the coil is continually changing direction due to vibrations of diaphragm
• increasing the amplitude increases the volume of the sound
• alternators = slip rings and brushes

  dynamos=split-ring commutator