Electromagnetic Induction

Question 1

what is electromagnetic induction

Answer 1

the induction of a potential difference (and current if there is a complete circuit) across a conductor which is experiencing a change in an external magnetic field.

Question 2

when does electromagnetic induction happen

Answer 2

when a conductor cuts through the magnetic field lines

Question 3

how to induce a potential difference across the ends of a conductor

Answer 3

by moving the electrical conductor in a magnetic field

by moving or changing a magnetic field (e.g. moving a magnet) relative to the electrical conductor

Question 4

what happens to the pd/current if you move the magnet in the opposite direction

Answer 4

the potential difference/current will be reversed

Question 5

what happens to the pd/current if you keep the magnetic field moving backwards and forwards

Answer 5

you produce a potential difference/current that keeps swapping direction which is called alternating current

Question 6

when is current induced

Answer 6

when a wire is moved in a magnetic field

Question 7

how do you increase the induced potential difference

Answer 7

increasing the speed of the movement
increasing the strength of the magnetic field
having more turns per unit length on the coil of wire

Question 8

how does increasing the speed of movement increase the induced potential difference

Answer 8

the field through the coil is changing quicker, so more field lines are being cut in a given time

Question 9

how does increasing the strength of the magnetic field increase the induced potential difference

Answer 9

there will be more field lines that can be cut

Question 10

how does having more turns increase the induced potential difference

Answer 10

there are more wires to cut the field lines

Question 11

how is the second magnetic field produced

Answer 11

when a current flows through a wire, a magnetic field is produced around the wires

Question 12

how does the magnetic field produced oppose the original change

Answer 12

the magnetic field created by an induced current always acts against the change that made it, whether it is the movement of a wire or a change in the field its in to return things to the way they were

Question 13

what do alternators produce

Answer 13

alternating current

Question 14

how do you produce an electric current by the relative movement of a magnet on a small scale

Answer 14

an electrical conductor that is moving relative to a magnetic field can induce a pd across the conductor

Question 15

how do you produce an electric current by a conductor on a small scale

Answer 15

if there is a change in an external magnetic field around a conductor and if the conductor is part fo a complete circuit, current will flow

Question 16

how do alternators produce AC current

Answer 16

as the coils spins, a pd and a current is induced in the coil

every half turn, these change direction

Alternators use slip rings and brushes so that the contacts don’t swap every half turn - the positive and negative ends keep swapping

this maintains the alternating potential difference

Question 17

what do dynamos generate

Answer 17

direct current / DC

Question 18

how are dynamos and alternators different

Answer 18

dynamos use a split ring commutator as their electrical contact but alternators use brushes

Question 19

how do dynamos produce DC current

Answer 19

As the coil rotates, it cuts through the magnetic field lines, inducing a current in the coil.

A split ring commutator then ensures that the current flows in the same direction, as it swaps the connection every half turn resulting in a DC output.

Question 20

how do you produce an electric current by the relative movement of a magnet on a large scale

Answer 20

the boilers heat water to convert it to steam, which turns a turbine

the turbine is connected to a powerful magnet inside a generator

as the turbine spins, the magnet spins with it

this induces a large pd and AC in the coils of the generator

Question 21

how do microphones generate current from sound waves

Answer 21

sound waves hit a flexible diaphragm that is attached to a coil of wire, wrapped around a magnet.

This causes the coil of wire to move back and forth in the magnetic field

this generates a current

Question 22

how do microphones convert the pressure variations of a sound wave into variations in current

Answer 22

the movement of coil depends on the properties of the sound wave

Question 23

what are transformers

Answer 23

devices that can change the ac/potential difference (and so current) of an electrical supply using electromagnetic induction

Question 24

how does a transformer work

Answer 24

when an alternating current flows through the primary coil in a transformer it produces a magnetic field which magnetises the iron core

because there is alternating current in the primary coil, the magnetic field in the iron core is alternating too

this constantly changing magnetic field cuts through the secondary coil

the changing field induces an alternating potential difference across the ends of the secondary coil

if the secondary coil is part of a complete circuit, this pd causes an alternating current to flow

Question 25

what does the size of the potential difference induced across the secondary coil depend on

Answer 25

the size of the potential difference across the primary coil and the number of turns on each coil

Question 26

what does the iron core do

Answer 26

it is purely for transferring the changing magnetic field from the primary coil to the secondary. NO electricity flows round the iron core

Question 27

what would happen if you supplied DC to the primary coil instead of AC

Answer 27

you would get nothing out of the secondary coil at all

Question 28

why would there be nothing out of the secondary coil if we used DC

Answer 28

there would still be a magnetic field in the iron core but it wouldnt be constantly changing, so there would be no induction in the secondary coil because you need a changing field to induce a potential difference.

Question 29

what is the ratio between the primary and secondary potential differences

Answer 29

the same as the ratio between the number of turns on the primary and secondary coil

Question 30

what happens in a step up transformer

Answer 30

the output pd is larger than the input pd

Question 31

what happens in a step down transformer

Answer 31

the output pd is smaller than the input pd

Question 32

what happens to the number of turns in a step up tranformer

Answer 32

number of turns on the secondary coil and size of pd across it are greater than across the primary pd

Question 33

what happens to the number of turns and pd in a step down transformer

Answer 33

the number of turns on the secondary coil and size of pd across it are smaller than across the primary coil

Question 34

pd across primary coil / pd across secondary coil =

Answer 34

number of turns on primary coil / number of turns on secondary coil

Question 35

electrical power =

Answer 35

current x potential difference

Question 36

pd across primary coil x current in primary coil =

Answer 36

pd in secondary coil x current in secondary coil

Question 37

what is the national grid

Answer 37

a network of wires and transformers across the UK

Question 38

what does the national grid do

Answer 38

it transfers electrical energy from power station anywhere on the grid to anywhere else on the grid where it is needed

Question 39

what do you need to do to transmit the huge amount of electrical power needed

Answer 39

you need to transmit the electricity at either a high voltage or high current

Question 40

what is the problem with transmitting electricity at a high current

Answer 40

you lose loads of energy as the cables heat up

Question 41

why do cables heat up when transmitting electricity

Answer 41

because the electrical charges have to do work against the resistance of the cables and doing work causes energy to be transferred to the thermal energy stores of the cables and surroundings

Question 42

the lower the current through the cables …..

Answer 42

the lower the resistance of the cables so less energy is wasted

Question 43

how do you make the national grid more efficient, high voltage and low resistance

Answer 43

use transformers

Question 44

what do step up transformers do at power stations

Answer 44

they boost the pd up really high and keep the current low which decreases the energy lost by heating

Question 45

what do step down transformers do in power stations

Answer 45

they bring the pd back down to safe,usable levels at the consumers end

Question 46

what does the equation vp x ip = vs x is show

Answer 46

it shows that increasing the pd across the secondary coil decreases the current through the secondary coil so the pd must be increased before transmission to lower the current