electromagnetic induction:alternators, dynamos & microphones

• what is electromagnetic induction

  when a potential difference is induced across a conductor which is experiencing a change in its external magnetic field
• when does a conductor experience a change in its external magnetic field

  when it passes through magnetic field lines
• if the conductor is part of its own complete circuit , the induced p.d. will result in a current in the circuit . If the current produces its own magnetic field too , the magnetic field will oppose the original change in the external magnetic field
• two different situations where you get electromagnetic induction:

  • an electrical conductor and a magnetic field move relative to eachother
  • the magnetic field through an electrical conductor changes ( gets bigger , smaller or reverses )
• generators are the opposite of motors - they use the relative motion of a conductor and a magnetic field to induce a p.d. and a current
• generators:

  • if the direction of motion is reversed , then the direction of the induced p.d. / current reverses too
  • if the current induced will be greater if there are more turns on the coil , the magnetic flux density is increased or if the speed of rotation is increased
• two types of generators:

  • alternators
  • dynamos
• alternators

  • some alternators rotate a magnet in a coil of wire
  • as the magnet spins , an alternating p.d. is induced across the ends of the coil . The p.d. changes direction every half turn because the direction of the field changes as the magnet rotates
  • this produces an a.c. if the coil is part or a complete circuit
  • you can also generate a.c. by rotating a coil in a magnetic field
  • slip rings at the ends of the coil remain in contact with brushes that are connected to the rest of the circuit . This means the contacts don’t swap every half turn , so they also produce a.c.
• dynamos generate direct current

  • rotate a coil in a magnetic field
  • the output p.d. and current change direction with every half rotation of the coil , producing a.c. . The coil is part of a complete circuit
  • a split-ring commutator swaps the connection every half turn to keep the current flowing in the same direction - so it changes a.c. to d.c.
• microphones work due to electromagnetic induction

  • a dynamic microphones structure is like a loudspeaker , but the cone is replaced by a diaphragm
  • sound waves cause the diaphragm to move back and forth when hit by them
  • as the diaphragm moves , the coil of wire moves , inducing a p.d. across the ends of the coil of wire
  • the coil is part of a circuit , so the induced p.d. means variations in current in the electrical circuit
• state what is different about the current produced by alternators and dynamos

  an alternator produces a.c. and the dynamo produces d.c.
• if a wire cuts through magnetic field lines , potential difference is induced across the ends of the wire . If the wire is part of a complete circuit , a current is induced in the wire
• if the wire moves parallel to the field lines , zero p.d. is induced
• to increase the size of the current , you could:

  • move the wire ( or magnet ) faster
  • increase the magnetic field strength
  • coil the wire so more wire is in the field
• alternator pt.2

  • rotating coil in a magnetic field
  • produces alternating p.d. and alternating current
• dynamo pt.2

  • (d.c.) generator
  • the potential difference that drives a current does not change direction
• to increase the voltage of an alternator we can:

  • use more turns on the coil
  • use a stronger magnetic field
  • rotate the coil faster
• what would happen if we used an electric motor in reverse?

  • the split-ring commutator would reverse the current every half turn
  • would produce d.c. instead of a.c.