electromagnetic induction:alternators, dynamos & microphones

Cards (18)

  • 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.