Electricity and Magnetism Flashcards
what effects the direction of induced current in the generator effect?
- it depends on the direction of movement of the magnet relative to the coil.
- The current is reversed when:
- the magnet is moved out of the coil
- the other pole of the magnet is moved into the coil
what is Lenz’ Law?
the direction of the induced current will oppose the change in flux that induced it
3 factors that increase induced p.d. or induced current
- the speed of movement is increased
- the magnetic field strength is increased
- the number of turns on the coil is increased
what is electromagnetic induction/ the generator effect?
The production of a potential difference (voltage) when a conductor, such as a wire, is moved through a magnetic field or exposed to a varying magnetic field. If the conductor is part of an electric circuit, an induced current will flow.
how is the generator effect used in an alternator to generate a.c.?
- as on side of the coil moves up through the magnetic field, a p.d./ current is induced in one direction.
- as the rotation continues and that side of the coil moves down, the induced potential difference reverses direction
- this means that the alternator produces a current that is constantly changing (AC)
how is the generator effect used in a dynamo to generate d.c.?
- a split ring commutator (reverses the direction of the current every half turn of the coil) changes the coil connections every half turn.
- as the induced potential difference is about to change direction, the connections are reversed
- this means that the current to the external circuit always flows in the same direction (DC)
what does a split ring commutator do?
reverses the direction of the current every half turn of the coil.
what does a p.d. against time graph look like for an alternator?
- curves up from 0 and then curves back down to 0
- curves below 0 then back up to 0
- repeats
what does a p.d. against time graph look like for a dynamo?
- curves up from 0 then curves back down to 0
- curves back up from 0 again and back down to 0 again etc.
interpret 5 stages of the p.d. against time graph of an alternator
- A - The coil is at 0°. The coil is moving parallel to the direction of the magnetic field, so no potential difference is induced.
- B - The coil is at 90°. The coil is moving at 90° to the direction of the magnetic field, so the induced potential difference is at its maximum.
- C - The coil is at 180°. The coil is moving parallel to the direction of the magnetic field, so no potential difference is induced.
- D - The coil is at 270°. The coil is moving at 90° to the direction of the magnetic field, so the induced potential difference is at its maximum. Here, the induced potential difference travels in the opposite direction to what it did at B.
- A - The coil is at 360°, ie it is back at its starting point, having done a full rotation. The coil is moving parallel to the direction of the magnetic field, so no potential difference is induced.
interpret 5 stages of the p.d. against time graph of a dynamo
- A - The coil is at 0°. The coil is moving parallel to the direction of the magnetic field, so no potential difference is induced.
- B - The coil is at 90°. The coil is moving at 90° to the direction of the magnetic field, so the induced potential difference is at its maximum.
- C - The coil is at 180°. The coil is moving parallel to the direction of the magnetic field, so no potential difference is induced.
- D - The coil is at 270°. The coil is moving at 90° to the direction of the magnetic field, so the induced potential difference is at its maximum. Here, the induced potential difference travels in the same direction as at B.
- A - The coil is at 360°, ie it is back at its starting point, having done a full rotation. The coil is moving parallel to the direction of the magnetic field, so no potential difference is induced.
how does a moving-coil microphone work?
- sound waves hit a flexible diaphragm that is attached to a coil of wire wrapped around a magnet
- this causes the coil of wire to move back and forth in the magnetic field
- this generates a current in the coil
- changing the volume or the pitch of the sound will vary the vibrations of the diaphragm, and therefore the size and frequency of the induced current
how does the effect of an alternating current in one coil induce a current in another in transformers?
- an alternating p.d. is applied to the primary coil
- this causes a current to flow in the primary coil that is also alternating
- an alternating magnetic field is created around the primary coil
- this induces an alternating magnetic field in the iron core
- this induces an alternating p.d. in the secondary coil
- causing an alternating current to flow in the secondary coil
explain what the ratio of potential difference across the two coils in a transformer depend on?
- it depends on the ratio of the number of turns on each
