Topic 13 - Electromagnetic induction Flashcards
Describe induction
Current is induced if a wire is moved in a magnetic field. The conductor (wire) forms a potential difference (electrons move to one side of the conductor as the field changes). If the conductor is connected in a circuit, a current will flow.
(This current will produce its own magnetic field which is in the opposite direction to the first field.)
Explain how to produce an electric current by the relative movement of a magnet and a conductor on a small scale in the laboratory
Spinning a coil of wire in between 2 permanent magnets will cause a current to flow in the wire, which can be shown by a sensitive ammeter (only milliamps will be generated). Passing a wire through a field will also show a deflection in an ammeter (a reading).
Explain how to produce an electric current by the relative movement of a magnet and a conductor in the large-scale generation of electrical energy
In a thermal power station, water heats up and evaporates to form steam due to the combustion of fuels or nuclear fission.
This steam is put under pressure and forced into a turbine. This causes the turbine to rotate, which is connected to a massive coil of wire in as strong magnetic field (the generator).
Current is generated in the coil by the spinning motion of the coil through the field.
Recall the factors that affect the size and direction of an induced potential difference
-Number of coils of wire
-Speed of rotation
-Magnetic field strength
Induced current will increases as these do
-Direction of movement. As a magnet moves into a coil of wire positive current flows. As it moves out negative current flows.
Explain how electromagnetic induction is used in alternators
Every half turn, the current switches direction, as the wire will be in the opposite orientation compared to it starting position. This produces a.c (alternating current).
Describe Fleming’s left hand rules
The left hand rule shows the direction in which the current induced switches act in alternators.
The forefinger is lined up with magnetic field lines pointing from north to south.
The second (middle) finger is lined up with the current pointing from positive to negative.
The thumb shows the direction of the motor effect force on the conductor carrying the current.
Explain how electromagnetic induction is used in dynamos
It has the same set up as an alternator.
At the end of the coil, there is a commutator. This is a metal ring that, every half-turn, reverses the sign of the current that flows from the coil, ensuring current output remains positive.
This produces d.c. (direct current).
Explain the action of a microphone
It converts the pressure variations in sound waves into variations in current in electrical circuits.
In a moving-coil microphone:
-Pressure variations in sound waves cause the flexible diaphragm to vibrate
-The vibrations of the diaphragm cause vibrations in the coil
-The coil moves relative to a permanent magnet, so a
potential difference/current
is induced in the coil
-The coil is part of a complete circuit, so the induced potential difference causes a current to flow around the circuit
-The changing size and direction of the induced current matches the vibrations of the coil
-The electrical signals generated match the pressure variations in the sound waves
Explain the action of loudspeakers and headphones
-Alternating current produces a changing magnetic field around the coil.
-The varying current in the magnetic field produces a force. -The force on the coil continuously changes direction.
-The coil vibrates, which causes pressure variations in the air which are sound waves.
This is the reverse effect of microphones.
Describe how a transformer works
-A primary voltage drives an
alternating current through the primary coil.
-The primary coil current produces a magnetic field, which changes as the current changes.
-The iron core increases the strength of the magnetic field.
-The magnetic field passes through (or cuts) the secondary coil.
-The changing magnetic field induces a changing potential difference in the secondary coil.
-The induced potential difference produces an alternating current in the external circuit.
How can a transformer effect voltage?
-If a transformer has more coils on the primary coil it is a step down transformer which decreases voltage. This is because a smaller potential difference forms on the secondary coil.
-If a transformer has more coils on the secondary coil it is a step up transformer which increases voltage. This is because a changing field will cut through more of the secondary wire, inducing a larger potential difference
What is the power equation for transformers with 100% efficiency?
potential difference across primary coil (volt, V) × current in primary coil (ampere, A) = potential difference across
secondary coil (volt, V) × current in secondary coil (ampere, A)
What is the turns ratio equation for transformers?
number of coils on primary/number of coils on secondary = potential difference of primary/potential difference of secondary
This can be used to calculate the missing voltage or the missing number of turns
Describe the national grid
-Long transmission wires have resistance reduced potential difference at the destination
-Thermal energy is dissipated in the transmission wires which creates a power loss
-Transformers are used to step up to a high voltage for transmission. This means a low current so power loss is smaller, efficiency is improved.
-Transformers are used to step down to a safer voltage for consumers/in localities.
-Consumer wire are shorter and so power loss is less of an issue.
