Electricity

Question 1

How does electric motor work

Answer 1

• current passes through a coil placed in between the poles of a magnet, there is a force on each side of the coil.
• FLH shows that the forces on each side of the coil are in opposite directios so the coil will move
• one side of the coil moves up and the other moves down
• the coil will spin
• a DC motor has a split ring commutator which rotates with the coil in between the two carbon brushes.
• commutator allows the motor to spin without changing its direction.

Question 2

what is the motor effect?

Answer 2

• when a straight wire with current passing through it, is placed in between the poles of a magnet
• magnetic field of wire combines with magnetic field of magnet.
• makes a resultant magnetic force stronger in one area and weaker in another.
• force is produced on the wire, so the wire will move.
• direction of force and motion is perpendicular to the magnetic field and current.

Question 3

what is the generator effect?

Answer 3

• when a wire is held stationary between two poles of a magnet, there is no current in the wire.
• If the wire moves, it cuts the magnetic field lines
• a potential difference is induced across the ends of the wire
• if wire is part of a circuit, induced current passes along the wire.

Question 4

Alternating current alternator

Answer 4

• when coil is vertical it is moving parallel to the magnetic field, so no magnetic field lines are cut.
• the potential difference is 0
• as coil rotates, side coming up is now going down- induced potential difference is negative.
• as each side of the coil moves down and then up through the magnetic field
• current passes in one direction then the other

Question 5

Dynamo

Answer 5

direct current uses generator effect

• each half turn of commutator, sides of the coil connect to opposite sides of circuit.
• direction of potential difference reverses when coil cuts through the magnetic field lines in opposite directions.
• direction of induced current does not change.

Question 6

how does a transformer work?

Answer 6

• a transformer changes the size of an alternating Pd.
• consists of 2 coils of wire, wound separately on an iron core.
• an alternating Pd is applied to PC, produces an alternating current in PC.
• This causes an alternating magnetic field in the iron core.
  -strength of magnetic field increases as iron is easily magnetised.
• alternating magnetic field in iron core induces a Pd in SC
• this produces an alternating current in circuit connected to SC

Question 7

how does a moving coil loudspeaker work?

Answer 7

• cylindrical magnet produces a strong magnetic field.
• at right angle to the wire in the coil
• current that varies flows through the coil.
• changes in current cause change in magnetic force acting on coil
• changing force causes coil to move in and out
• as current varies, the coil moves
• as coil is attached to the cone, cone also moves, setting up vibrations in the air.

Question 8

how does moving coil microphone work?

Answer 8

• coil connected to a diaphragm
• edges of diaphragm are fixed
• coil surrounds a permanent magnet
• sound waves make diaphragm vibrate
• vibrations make coil move backwards and forwards in the gap
• this induces varying current in the coil