EM Induction Flashcards
What are the 2 ways of generating electricity
Using an…
a) Alternator
b) Dynamo
Do alternators produce AC or DC
AC
What does an alternator consist of
A coil of wire rotating in a magnetic field created by a permanent magnet
Describe structure of alternator
Current flows in from external circuit
As the coil is rotated in the magnetic field a p.d is induced in one direction
As the rotation continues the p.d reverses direction
Slip rings are in constant contact with the same side of the coil
Carbon brushes make continuous contact with external circuit and slip rings
How can the maximum p.d or current in an both an alternator and a dynamo be increased?
Increasing rate of rotation
Increasing strength of magnetic field
Increasing number of turns on the coil
Does a dynamo produce ac or dc current
DC
What does a dynamo consist of
A coil of wire rotating in a magnetic field as an alternator does
HOWEVER it uses a split ring commutator rather than 2 slip rings
What happens 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.
What happens in a moving coil microphone
Pressure variations in sound waves are converted into a varying electric current
1. Pressure variations cause flexible diaphragm to vibrate
2. Diaphragm is connected to a coil of wire which is wrapped around a permanent magnet
3. The movement of the coil produces a current in the coil via electromagnetic induction
How does a loudspeaker work
A loudspeaker converts variations in an electric current into sound waves
1. A coil of wire inside the loudspeaker is wrapped around a permanent magnet
2. When a current flows through the coil there is a force that makes the diaphragm moves
3. Changes in the current causes the movement of the diaphragm which produces the sound waves
What is a transformer?
A device that can change the p.d of an alternating current
Does a step up transformer increase or decrease the p.d
Increase
Does a step down transformer increase or decrease the p.d
Decrease
What happens when a transformer is working
A primary pd drives an AC current through a primary coil
The primary coil produces a magnetic field which changes as current changes
Iron core increases the strength of the magnetic field
The changing magnetic field induces a changing pd in the secondary coil
The induced potential difference produces an AC current in the external circuit
What is another name for electromagnetic induction
The generator effect
Describe the structure of a transformer
A transformer is made from two coils of wire, a primary coil from the (ac) input and a secondary coil leading to the ac output.
The coils are not electrically connected.
Instead, they are wound around an iron core.
This is easily magnetised and can carry magnetic fields from the primary coil to the secondary coil.
What is the transformers equation
Vp Np
—- = —-
Vs. Ns
Equation for power in transformers
Ip x Vp = Is x Vs
What do the symbols used in the equations for transformers mean
N= number of coils
V= voltage
I = current
p = primary
s = secondary
Why do the national grid transmission lines use high voltages
Power = Voltage x current means that for a given power transfer, the higher the voltage used, the lower the current needed.
Heat energy wastage through electrical resistance is proportional to the square of the current, as given by the equation Power = Current^2 x resistance. Reducing the current can create huge reductions in energy lost to the surroundings through resistance.
Why are transformers used in the national grid
To ensure the transmission of electricity is efficient
They increase voltage in order to decrease current
Decreasing current means less energy is lost as heat
Voltage is then decrease for safer sue in our homes and industry
