Electromagnetic Induction Flashcards
How can an electric current be produced on a small scale?
Spinning a coil of wire in between two permanent magnets will cause a current to form in the wire, which can be shown by a sensitive ammeter.
How can an electric current be produced on a large scale?
In a thermal power station, water heats up and evaporates to form steam. 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 a strong magnetic field (the generator).
Current in generated in the coil by the spinning motion of the coil through the field.
Outline three factors that affect the size and direction of the voltage/current produced
- Number of coils of wire
- Speed of rotation
- Magnetic field strength
Explain current 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. The direction of this new field is in the opposite direction to the first field and opposes the original change.
Explain how an alternator works?
Every half turn, the current switches direction as the wire will be in the opposite orientation compared to its starting position.
Use the left-hand rule to show the direction of the current induced switches as it moves up or down.
This produces an alternating current.
Explain how a dynamo works?
Same set up as an alternator.
At end of the coil, there is a ‘commutator’. This is a metal ring that reverses the sign of the current that flows from the coil ensuring current output remains positive. Every half-turn the commutator switches the sign of the current, so it remains positive.
This produces a direct current.
Explain how an alternating current in one circuit can induce a current in another circuit in a transformer
The alternating current in the first coil creates a changing magnetic field which cuts through the secondary coil.
This induces a voltage and then this induces a current in the secondary coil (an alternating current).
If the primary current is DC, the magnetic field produced will be constant, not inducing anything in the secondary coil.
Explain the action of the microphone in converting the pressure variations in sound waves into variations in current in electrical circuits
Produce a current which is proportional to the sound signal.
Fixed magnet is at the centre, and the coil of the wire around the magnet is free to move.
Pressure variations I the sound waves cause the coil to move, and as bit moves, current is induced in the coil because it passes through the magnetic field.
This current is then sent to a loudspeaker.
The loudspeaker set up is identical.
Current flows into the coil.
The magnetic field from the magnet and from the current interact, causing the coil to move.
The cone therefore moves.
Producing pressure variations, making sound.
Explain the national grid
Electricity is produced in power stations by a generator that has a voltage that has stepped up by a step up transformer.
The step up transformer produces an alternating current.
The pylons carry the voltage at 400KV.
There is power loss in the cables, carrying the electricity (as heat energy).
P =I2R
In order to go to houses, a step down transformer is needed on the other side of the pylons.
Our houses, need to be safe at 230V maximum.
Transformer summary
Step-up transformers increase the voltage.
Step-down transformers decrease the voltage.
Power for a transformer with 100% efficiency:
Power of Primary circuit = Power in Secondary circuit
