Topic 13 - Electromagnetic Induction Flashcards
What is Electromagnetic induction?
The production of current in a conductor as it moves through a magnetic field.
How can you produce an electric current on a small scale in a lab?
You can do this by moving or rotating a magnet in a coil of wire OR a conductor in a magnetic field (“cutting” magnetic field lines). If you keep the magnet (or the coil) moving backwards and forwards, or keep it rotating in the same direction, you produce an alternating current.
How can you produce an electric current in a large-scale generation of electrical energy?
A transformer can be used to change the potential difference of an alternating electricity supply. When an alternating current flows through the primary coil, it produces a changing magnetic field in the iron core. This magnetic field induces an alternating potential difference across the secondary coil. The alternating potential difference in the secondary coil leads to an alternating current when the circuit is completed.
Describe how an alternator uses electromagnetic induction to create current.
As one side of the coil moves up through the magnetic field, a potential difference 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. This is alternating current or AC.
Describe how a dynamo uses electromagnetic induction to create current.
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 - Direct current or DC
Describe how a microphone works. (6)
1) Pressure variations in sound waves cause the flexible diaphragm to vibrate
2) The vibrations of the diaphragm cause vibrations in the coil
3) The coil moves relative to a permanent magnet, so a potential difference is induced in the coil
4) The coil is part of a complete circuit, so the induced potential difference causes a current to flow around the circuit
5) The changing size and direction of the induced current matches the vibrations of the coil
6) The electrical signals generated match the pressure variations in the sound waves
Describe how a loudspeaker works. (7)
1) A current in the coil creates a magnetic field
2) The magnetic field interacts with the permanent magnet generating a force, which pushes the cone outwards
3) The current is made to flow in the opposite direction
4) The direction of the magnetic field reverses
5) The force on the cone now pulls it back in
6) Repeatedly alternating the current direction makes the cone vibrate in and out
7) The cone vibrations cause pressure variations in the air - which are sound waves
Explain how an alternating current in one circuit can induce a current in another circuit in a transformer
An alternating current passes through a primary coil wrapped around a soft iron core. The changing current produces a changing magnetic field. This induces an alternating voltage in the secondary coil. This induces an alternating current (AC) in the circuit connected to the secondary coil.
What is the turns ratio equation for transformers to calculate either the missing voltage or the missing number of turns:
Potential difference across primary coil / Potential difference across secondary coil =
Number of turns on primary coil/ Number of turns on secondary coil
Why is electrical energy transferred at high voltages from power stations and transferred at low voltages for domestic uses?
Electrical energy is transferred at high voltage (400,000v) to reduce the current which means that less heat is lost and thinner wires can be use. This reduces cost. When the energy reaches the area of domestic use, the voltage is reduced for safe use in homes or industries.
Explain where and why step-up and step-down transformers are used in the transmission of electricity in the national grid
Step-up transformers are used at power stations to produce the very high voltages needed to transmit electricity through the National Grid power lines. These high voltages are too dangerous to use in the home, so step-down transformers are used locally to reduce the voltage to safe levels.
How can you calculate the power output from the transformer, assuming that a transformer is 100% efficient?
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)
Vp x Ip = Vs x Is
Advantages of power transmission in high voltage cable
With increase in the transmission voltage size of the conductors is reduced (Cross section of the conductors reduce as current required to carry reduces).
As the reduction in current carrying requirement losses reduces results in better efficiency
Due to low current voltage drop will be less so voltage regulation improves
