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physics > Electromagnetic Induction > Flashcards

Electromagnetic Induction Flashcards

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1
Q

what is electromagnetic induction

A

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.

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2
Q

when does electromagnetic induction happen

A

when a conductor cuts through the magnetic field lines

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3
Q

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

A

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

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4
Q

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

A

the potential difference/current will be reversed

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5
Q

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

A

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

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6
Q

when is current induced

A

when a wire is moved in a magnetic field

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7
Q

how do you increase the induced potential difference

A

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

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8
Q

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

A

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

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9
Q

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

A

there will be more field lines that can be cut

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10
Q

how does having more turns increase the induced potential difference

A

there are more wires to cut the field lines

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11
Q

how is the second magnetic field produced

A

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

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12
Q

how does the magnetic field produced oppose the original change

A

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

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13
Q

what do alternators produce

A

alternating current

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14
Q

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

A

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

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15
Q

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

A

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

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16
Q

how do alternators produce AC current

A

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

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17
Q

what do dynamos generate

A

direct current / DC

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18
Q

how are dynamos and alternators different

A

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

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19
Q

how do dynamos produce DC current

A

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.

20
Q

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

A

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

21
Q

how do microphones generate current from sound waves

A

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

22
Q

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

A

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

23
Q

what are transformers

A

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

24
Q

how does a transformer work

A

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

25
Q

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

A

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

26
Q

what does the iron core do

A

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

27
Q

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

A

you would get nothing out of the secondary coil at all

28
Q

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

A

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.

29
Q

what is the ratio between the primary and secondary potential differences

A

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

30
Q

what happens in a step up transformer

A

the output pd is larger than the input pd

31
Q

what happens in a step down transformer

A

the output pd is smaller than the input pd

32
Q

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

A

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

33
Q

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

A

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

34
Q

pd across primary coil / pd across secondary coil =

A

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

35
Q

electrical power =

A

current x potential difference

36
Q

pd across primary coil x current in primary coil =

A

pd in secondary coil x current in secondary coil

37
Q

what is the national grid

A

a network of wires and transformers across the UK

38
Q

what does the national grid do

A

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

39
Q

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

A

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

40
Q

what is the problem with transmitting electricity at a high current

A

you lose loads of energy as the cables heat up

41
Q

why do cables heat up when transmitting electricity

A

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

42
Q

the lower the current through the cables …..

A

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

43
Q

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

A

use transformers

44
Q

what do step up transformers do at power stations

A

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

45
Q

what do step down transformers do in power stations

A

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

46
Q

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

A

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

physics (14 decks)

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