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AQA Physics GCSE > Electeomagnetica > Flashcards

Electeomagnetica Flashcards

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1
Q

features of a permanent magnet

A
  • it produces its own magnetic field
  • the magnetic field cannot be turned on or off- it is always there
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2
Q

features of an induced magnet

A
  • only becomes a magnet when it is placed in a magnetic field
  • the induced magnetism is quickly lost when the magnet is removed from the magnetic field
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3
Q

what does induced magnetism always cause

A

a force of attraction

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4
Q

what happens if we take away a permanent magnet away from two induced magnets

A

the induced magnets lose most or all of their magnetism quickly

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5
Q

examples of permanent magnets

A

a horeshoe magnet
a bar magnet

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6
Q

what are the four types of magnetic material (MUST KNOW!)

A
  • iron
  • steel
  • cobalt
  • nickel
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7
Q

what is a magnetic field

A

a region around a magnet where a force acts on another magnet or on a magnetic material

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8
Q

wha would happen if we placed a magnetic material into a magnetic field

A

it will experience a force of attraction towards the magnet

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9
Q

what does the strength of a magnetic field depend on

A

the distance from the magnet

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10
Q

where is the magnetic field strongest at

A

the poles of the magnet (N and S)

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11
Q

how can the direction of a magnetic field be found

A

using a compass

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12
Q

how can we use a compass to map out the direction of a magnetic field

A
  1. place the plotting compass near the magnet on a piece of paper
  2. mark the direction that the compass needle points
  3. move the plotting compass to many different positions in the magnetic field, marking the needle direction each time
  4. join the points to show the field lines
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13
Q

in what directions do forces move in a magnet

A

from the North pole to South

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14
Q

what does it mean when the lines in a magnetic field are closer

A

the field is stronger

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15
Q

how can we prove that the Earth has its magnetic field

A
  • a compass contains a small bar magnet
  • if we hold the compass away from any magnets, the needle always points in the North - South direction
  • the earths core, which is made from iron and nickel, produces this magnetic field
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16
Q

what are key features of a magnetic field in a diagram

A
  • the magnetic field lines never cross each other
  • the closer the lines, the stronger the magnetic field
  • the lines have arrows to show the direction of the force exerted by a magnetic north pole
  • the arrows point from the north pole of the magnet to the south pole
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17
Q

what happens when you close the switch of an electric circuit

A

an electric current flows around the curcuit

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18
Q

what happens when a current flows through a conducting wire

A

a circular magnetic field is produced around the wire

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19
Q

how can we prove that there is a magnetic field around a wire

A
  • by using a compass
  • when the current is turned off, the compass needle lines up with the Earth’s magnetic field
  • when the cureent is turned on, the compass needle deflects
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20
Q

what does the strength of a magnetic field around a wire depend on

A
  • the size of the current
  • the distance from the wire
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21
Q

when is the magnetic field in a wire strongest💪🏾

A

closer to the wire

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22
Q

what happens if we change the direction of the current

A

we change the direction of the magnetic field

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23
Q

How can we increase the strength of a magnetic field

A
  • increase the number of turns on the coil
  • increase the size of the current
  • placing a piece of iron inside the solenoid (iron core)
  • decrease the length of the coil whilst keeping the number of turns the same
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24
Q

what is a solenoid

A

a coiled up wire

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25
Q

what happens when we turn on a current in a circuit which contains a solenoid

A

we get a strong and uniform magnetic field inside the solenoid

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26
Q

what is an electromagnet

A

a solenoid containing an iron core

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27
Q

why are electromagnets useful

A

because we can change the strength of the magnetic field by changing the size of the current

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28
Q

what does the direction of the field depend on

A

the direction of current

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29
Q

how can you tell the north and south pole on a solenoid

A

arrows going in are south, arrows going out are north

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30
Q

how would we reverse the direction of a magnetic field

A

by reversing the direction in which the current is flowing

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31
Q

why would an iron core increase the strength of an electromagnet

A

because it is a soft magnetic material, it will become an induced magnet when a solenoid is switched on, increasing the strength of the electromagnets magnetic field

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32
Q

what is the motor effect

A

the idea that a current carrying wire in the presence of a magnetic field will experience a force

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33
Q

how to calculate the strength of a force

A

f (force) = B (magnetic flux density) x I (current) x length of wire (m)

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34
Q

how do electric motors work

A

1.

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35
Q

how does a 90 turning coil work

A
  1. Current flows in from the positive terminal and out through the negative terminal
  2. Because the left hand side experiences an upwards force whilst the right hand side experiences a downwards, the coil will start to spin clockwise
  3. once the coil has rotated 90, the current flips, so does the force
  4. this means that the coil will now start turning anti clockwise and flip back to where it started
  5. this means if we left it like this, the coil will keep flipping back and forth as the forces keep flipping, and it will never turn 360, time
36
Q

how to fix a coil that only turns 90

A
  • change the direction of the current every half turn so that the direction of the forces will always be acting clockwise
37
Q

how does the electric motor work

A

-

38
Q

what does a split ring commutator do

A

it swaps the positive and negative connections every half turn so that the direction of the current also swaps every half turn, meaning the forces of the coil will always be acting in the same direction
- it breaks the current for a tiny fraction of a second

39
Q

how to increase the speed of rotations in an electric motor

A
  • increase the current passing through the wire
  • add more turns to the coil
  • increase the magnetic flux density by adding more powerful magnets
40
Q

how does the motor effect work in a loudspeaker

A
  • we have a cone which has a coil of wire wrapped around one end
  • the coil of wire is connected to an alternating current electrical supply
  • there is also a permanent magnet that goes inside the coil of the wire
  • as the current passes through the coil, it generates a magnetic field
  • the two magnetic fields either attract or repel each other
  • this produces a resultant force, which causes the cones to move
  • when the current switches direction, the direction of the force on the cone reverses
  • this causes the cone to vibrate, generating sound waves
41
Q

how to change the frequency that a cone vibrates

A

by changing the frequency of the AC supply
- a higher frequency will produce a higher pitched sound

42
Q

what would happen if we increase the size of the current in a loudspeaker

A

we increase the amplitude of the vibration, which increases volume of the sound

43
Q

what is the generator effect

A
  • if a conducting wire passes through a magnetic field, then a potential difference is induced in the wire
  • if the circuit is complete, then an induced current is formed
44
Q

what does the size of the induced pe or current depend on

A
  1. size of magnetic field
  2. Speed of wire moving
  3. number of turns on the coil
45
Q

how else can we switch the direction of the induced current

A

by switching the poles of the magnet

46
Q

what does an induced current do

A

it creates its own magnetic field, and it opposes the movement of the magnet, making it harder to push the magnet in, but when the magnets pulled out, the current attracts it, so its harder to pull out

47
Q

what is an alternator

A

a coil of wire rotating in a magnetic field, the coil is connected to two metal rings called commutators

48
Q

what do commutators do

A

they allow current to pass out of the coil

49
Q

when do we get the max potential difference in an alternator and why

A

when the coil is horizontal, because at this point the wire is sweeping directly through the magnetic field lines at the fastest possible rate

50
Q

what happens when the coil is vertical in an alternator and why

A

the old falls to 0, because the coil is now moving parallel to the field

51
Q

why does the pd in an alternator reverse direction

A

because the two sides of the coil are now moving in a different direction to before

52
Q

why does an alternator produce an alternating PD and current

A

because the two sides of the coil are attached to two different rings

53
Q

how to increase size of AC

A
  • increase strength of magnetic field
  • increase number of turns on a coil
  • increase area of the coil
  • increase the rotation speed of the coil
54
Q

features of a dynamo

A
  • produces a dc
  • it has a split ring commutator which has two sides separated by a gap
  • when the coil is vertical, pd is 0
  • direction of the pdand current do not reverse when the coil rotates, as the sides of the wire will still be connected to their original sides of the commutator
55
Q

why do we get two peaks for each full rotation in a dynamo

A

because each side of the coil passes through the magnetic field twice during each cycle of rotation(up and down)

56
Q

how does a moving coil microphone work

A
  1. When sound waves hit the diaphragm, they cause it to vibrate
  2. now the coil of wire moves in and out of the magnetic field
  3. this induces a pd across the ends of the wire
  4. the pd switches direction as the coil moves backwards and forwards through the magnetic field
  5. the changing pattern of pd is passed through an amplifier then into a moving coil loudspeaker
    - this increases the volume of the sound
57
Q

what is the relationship between the frequency of the changing PD and the frequency of the sound waves in a microphone

A

they are the same

58
Q

features of the primary coil

A
  • contains an iron core
    connected to an alternating current
  • as current flows through it, it generates a changing magnetic field
59
Q

how is the magnetic field transmitted from the primary to secondary coil

A

along the iron core

60
Q

what happens when the changing magnetic field passes through the secondary coil

A

it induces a pd

61
Q

why do transformers only work with an AC

A

because we need a changing magnetic field to induce a pd, a dc produces a constant magnetic field

62
Q

if the number of turns in the secondary coil is greater than the number of turns in the primary, what does it mean

A

the secondary coil has a greater induced pd

63
Q

what is a step up transformer

A

a transformer that has a larger pd in the secondary coil than the primary

64
Q

what is a step down transformer

A

one where the pd in the primary coil is more than the pd in the secondary

65
Q

why is a large current not suitable for transmitting a large amount of power

A

because power is wasted in transmission cables as heat, and the amount of power wasted as heat depends on the square of the current

66
Q

what do step up transformers do in the national grid

A

increases of to around 400000V

67
Q

what do step down transformers do in the national grid

A

reduces the pd to 230V

68
Q

what direction does the needle point to in a plotting compass

A

to the south pole of the magnet

69
Q

why are solenoids strong

A

The small magnetic fields caused by the current in each coil add together to make a stronger overall magnetic field

70
Q

what does an iron core do

A

increases the solenoid’s magnetic field strength

71
Q

describe the motor effect

A
  • a wire carrying a current creates a magnetic field
  • this magnetic field can interact with another, causing a force that pushes the wire at right angles
72
Q

how to increase the force on a given length of wire in a magnetic field

A
  • increase magnetic field strength
  • increase the current
73
Q

when is the force greatest at any given combination of current and magnetic field strength

A

when the direction of the current is 90 degrees to the direction of the magnetic field
- There is no motor effect force if the current and magnetic field are parallel to each other.

74
Q

what are the 4 ways we can increase an electromagnets strength

A
  1. increase the current
  2. Increase the number of turns
  3. decrease the length of the coil whilst keeping the number of turns the same
  4. add an iron core to the inside of the solenoid
75
Q

features of a relay

A
  • used to turn high voltage circuits on and off
  • contains a low voltage circuit containing an electromagnet
  • on the high voltage circuit, the switch is replaced with two metal contacts, one of the contacts is connected to a spring, which keeps the contacts apart
  • theres also an iron block next to the spring
76
Q

what happens if the low voltage circuit is turned off in a relay

A

no current is flowing through the magnet, there’s no electric field, contacts are apart

77
Q

why is the high voltage circuit turned off in a relay

A

because the contacts are not touching

78
Q

what happens if we switch on the low voltage circuit in a relay

A
  • a current flows around the circuit
  • meaning there is now a magnetic field around the electromagnet
  • the magnetic field now attracts the iron block in the high voltage circuit
  • this causes the contacts to close, and switches on the high voltage circuit
79
Q

how does a doorbell work

A
  • the switch is closed when the buzzer is pressed
  • this causes a current to flow through the circuit
  • a magnetic field is now produced by the electromagnet
  • the iron contact is now attracted towards the magnetic field
  • when the contact moves towards the magnetic field, the clapper now hits the bell
  • at the same time, this breaks the circuit
  • now there’s no current flowing, so there’s no magnetic field
  • the iron contact now springs back to its original position, closing the circuit, allowing a current to flow around the circuit and the process is repeated
80
Q

rules when using the motor effect

A
  • the f =bil rule only applied to a wire which is at right angles to the magnetic field
  • The magnetic flux density is a measure of the strength of the magnetic field
  • If the conductor parallel to the magnetic field, it will not experience a force
81
Q

what happens when we move a wire through a magnetic field

A
  • a potential difference is induced across the ends of the wire
82
Q

what is induced potential difference

A

when potential difference changes directions

83
Q

when does the direction of the current switch

A

when the direction of movement switches

84
Q

what other times can we get an induced pd

A

if we keep the wire still but move the magnetic field

85
Q

why is iron used in transformers

A

it’s easily magnetised

AQA Physics GCSE (38 decks)

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