Induced potential, transformers and the National Grid (7.3) Flashcards
long description
What is the generator effect?
If an electrical conductor moves relative to a magnetic field (magnetic field fixed and conductor moving around it) or if there is a change in the magnetic field around a conductor, a potential difference is induced across the ends of the conductor.
If the conductor is part of a complete circuit, a current is induced in the (whole) conductor
An induced current generates what?
A magnetic field that opposes the original change, either the movement of the conductor or the change in magnetic field
What happens if a permanent magnet is move into a solenoid (or vice-versa)?
the induced magnetic field tends to repel the magnet back out of the coil
3 factors
What are the factors that affect the size of the induced potential difference/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 the factor that affects the direction of the induced potential difference/induced current?
The orientation of the poles of the magnet
2 ways
When will current reverse when using a magnet and solenoid?
- if the magnet is moved out of the coil
- if the other pole of the magnet is moved into the coil
Why is it harder to pull a magnet out of a solenoid?
As you take the magnet out of the solenoid, the pole of the solenoid becomes opposite to that of the magnet, so it is harder to pull out and therefore work is done
The generator effect is used in an alternator to generate…
ac
The generator effect is used in an dynamo to generate…
dc
Explain how a dynamo works
A direct current and pd are induced causing the coil to rotate due the force created.
The side of the coil moving up is connected to part A of the split-ring commutator and the side of the coil moving down is connected to part B.
When current is perpendicular to the magnetic field, pd at peak/maximum (positive).
However, when it completes a half rotation (pd is zero at parallel), the side now moving up is still connected to part A, and vice versa with part B.
Therefore, the current does not alternate as the direction of the current and potential difference do not reverse when the coil rotates.
A current, and therefore pd, will not be induced, if the wire is (…) to the magnetic field
parallel
What do split ring commutators do in a dynamo
A split ring commutator 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.
How does an alternator work?
- the coil moves through the magnetic field
- a potential difference is induced (across the coil)
- there is a complete circuit, so a current is induced (in the coil)
- However, the direction of pd reverses (up to down or down to up) every half turn, so (every half turn) the current changes direction; this produces an alternating current
at 0o zero pd, 90o max positive pd, 180o zero pd, 270o max negative pd
What does an oscilloscope look like for a dynamo?
Other than increasing number of turns or strength of mag field, how else can a greater pd be produced in alternators and dynamos?
by increasing the rate of rotation
Alternators have (…) commutators, dynamos have (…) commutators?
slit-ring
split-ring
What does an oscilloscope look like for an alternator?
Microphones use the generator effect to do what?
to convert the pressure variations in sound waves into variations in current in electrical circuits
4 marks
Explain how a moving-coil microphone works
- sound (waves) cause the diaphragm to vibrate
- the diaphragm causes the coil / wire to vibrate
- the coil / wire moves through the magnetic field
- a potential difference is induced (across the ends of the coil / wire)
A basic transformer consists of what?
a primary coil and a secondary coil wound on an iron core
Why is iron used in a transformer?
it is easily magnetised
The greater the number of coils, the (…) the potential difference induced
greater
Describe a step-up transformer
There is a greater number of coils and therefore greater voltage induced on the secondary coil compared to the primary coil
Describe a step-down transformer
There is a lesser number of coils and therefore less voltage induced on the secondary coil compared to the primary coil
What would happen if transformers were 100% efficient with regards to electrical power input/output?
the electrical power output would equal the electrical power input.
Describe how a transformer works?
- An alternating potential difference is supplied to the primary coil
- Which causes an alternating current to flow in the priamry coil
- So a magnetic field (which alternates/changes) is produced around the primary coil
- This induces an alternating magentic field in the iron core
- Which induces an alternating potential difference in the secondary coil
- If it is part of a complete cirucit, this causes currnet to flow
Why can’t current just flow from one coil top another directly in a coil through the iron core?`
Coils are insulated in plastic, which prevents current flowing through the iron core
The ratio of the potential differences across the primary and secondary coils of a transformer depends on what?
the ratio of the number of turns on each coil
this is more of a concept, it is ratio, not abundance you need to think about in the exam
Why is a larger current produced when the magnet leaves the solenoid, than when it enters it?
The magnet is moving faster when it leaves the solenoid
What is voltage caused by in magnetic field?
the change in the magnetic field
the stronger the change in the magnetic field, the greater the voltage produced
What is the difference between direct and alternating potential difference?
Alternating p.d. constantly changes direction. Direct p.d. always goes in same direction (one direction only).
Does a current induce voltage, or voltage induce current?
An induced voltage induces current (which in turn induces a magnetic field around the conductor/wire)
Explain
when moved vertically, the wire cuts through field lines
Which induces pd in wire
Why moved in other directions, less field lines are cut
When moved horizontally, no field lines are cut
Why are insulated wires used in transformers?
tp prevent current flowing through/entering the core
What is the use of a transformer being adjusted to have a different number of turn son the secondary coil?
to vary the (output) potential difference
so that you don’t need a different generator for each type of device
What is the purpose of slip rings (alternator)?
provides a continuous / moveable contact
Why is the handle of an alternator much easier to turn after it is disconnected from an electrical circuit?
there is no induced current
so no magnetic field produced around coil
to oppose the movement of the coil
