Science - Physics - Magnetism and electromagnetism - P7 Flashcards
1
Q
what does a magnet have around it ?
A
a magnetic field
2
Q
what does a magnetic field cause in a magnet ?
A
magnetic objects to be attracted or repelled by a magnet
3
Q
what is a magnetic field ?
A
a region where magnets, magnetic materials and also wires carrying a currents experience a force acting on them
4
Q
what kind of force is a magnetic force ?
A
non - contact
5
Q
how can magnetic fields be represented ?
A
by a field diagram
6
Q
what is a field diagram ?
A
a series of lines that show where a magnetic field exits and its direction
7
Q
in magnets, where are fields the strongest ?
A
at the poles
8
Q
what are the 2 poles in magnets ?
A
north
south
9
Q
what are the direction in magnetic field lines always going in ?
A
north to south
10
Q
what do the direction of the field lines show ?
A
the direction of the force a north pole would feel if it was placed in that direction
11
Q
if the magnetic field is stronger, what happens to the field lines ?
A
they are closer together
12
Q
what kind of force is always between the magnet and a magnetic material ?
A
attractive
13
Q
what are the 2 forces that can occur between 2 magnet which are placed close to each other ?
A
attractive
repulsive
14
Q
what will happen if like poles are put together ?
A
they repel
15
Q
what will happen if opposite poles are put together ?
A
they attract
16
Q
you know what they say…
A
opposites attract ;)
17
Q
how do compasses work ?
A
- inside a compass is a tiny bar magnet
- the north pole of this magnet is attracted to the south pole of any other magnet it is near
- the compass points in the direction of the magnetic field it is in
18
Q
how can you use a compass to build up an image of a magnetic field ?
A
by moving a compass around a magnet and tracing its position on paper to build up a picture of what the magnetic field looks like
19
Q
when they’re not near a magnet, what direction do compasses always point ?
A
north
20
Q
why do compasses always point north when not near a magnet ?
A
the earth generates its own magnetic field which shows that the earths core must be magnetic
21
Q
what are the 2 types of magnet ?
A
permanent
induced
22
Q
what are permanent magnets ?
A
magnets which produce their own magnetic fields
23
Q
what are induced magnets ?
A
magnetic materials that turn into a magnet when they are in a magnetic field
24
Q
what is the force between permanent and induced magnets always ?
A
attractive
25
when you take away the magnetic field from induced magnets, what often happens ?
magnets lose their magnetism
26
when a current flows through a wire, what happens ?
a magnetic field is created around the wire
27
what is the field made up around the wire made up of ?
concentric circles ( circles which share the same centre) perpendicular to the wire with the wire in the centre
28
how can you see the magnetic field of a wire ?
by placing a compass near a wire which is carrying a current , as you move the compass, it will trace the direction of the magnetic field
29
what does changing the direction of the current do to the magnetic field ?
change its direction
30
how do you work out which way the magnetic field goes when you have changed the direction of the current ?
the right hand rule
31
what is the right hand rule ?
using your right hand point your thumb in the direction of the current, and curl your fingers. The direction of your fingers is the direction of the field
32
what does the strength of the magnetic field around a current depend on ?
the current
the distance from the wire
33
the larger the current, or closer you are to the wire, the what ?
stronger the field is
34
how can you increase the strength of a magnetic field that a wire produces ?
by creating a solenoid
35
how do you create a solenoid ?
wrap the wire into a coil
36
how does the magnetic field get stronger when using a solenoid ?
the field lines around each loop of wire line up with each other, and form a stronger magnetic field
the result is lots of field lines pointing in the same direction that are very close to each other . closer together the field lines = stronger the field
37
what kind of magnetic field is inside a solenoid ?
strong
uniform
38
how do you increase the strength of a magnetic field of a solenoid ?
- add an iron core - becomes an induced magnet whenever current is flowing
- increase number of coils
39
if you stop the current with a solenoid with an iron core what happens ?
it becomes an electromagnet
40
what is an electromagnet ?
a magnet whose magnetic field can be turned on and off by an electric current
41
why are electromagnets useful ?
they're quick to turn on and off
- they can create a varying force
42
give examples of uses of electromagnets ?
- they can pick up things made up of magnetic materials and release them , e.g in scrap yards/steel works
- relay switches
43
what do relay switches do ?
link together 2 circuits so turning on 1 circuit causes the other to turn on too
44
what are relay switches used for ?
to turn on very high current circuits using a lower current circuit as they stop the user from having to come into contact with the high current
45
give an example of when a relay switch is used ?
in a cars motor
46
how do relay switches work ?
1. when the switch in the 2 circuit current is closed, current flows through the electromagnet producing a magnetic field
2. the electromagnet attracts the iron contact on the rocker, which pivots and closes the contacts in the high current circuit
3. current flows through the starter motor and the motor spins
4. when the low current switch is opened the electromagnet stops pulling, the rocker returns, and the high current circuit is broken again
47
what happens if you put a wire into another magnetic field (the wire already has a magnetic field)
the 2 magnetic field combine - the wire and the magnet exert a force on each other
48
what is the motor effect ?
when a current-carrying wire in a magnetic field experiences a force
49
what will happen to the force if you increase the strength of the magnetic field or the size of the current flowing through the wire ?
it will increase the size of the force
50
how does the wire experience the full force (motor effect)
the wire has to be at 90 degrees to the magnetic field
51
when will the wire experience no force ?
if it runs parallel to the magnetic field
52
what is the motor effect useful for ?
producing movement
53
give an example of how the motor effect is used ?
- horseshoe magnet is sued to provide a magnetic field
- a conducting wire is placed on 2 conducting rails at 90 degrees to the magnetic field
- if a current is applied to the rails the current will flow through the bar and the motor effect will cause the bar to experience a force
- the bar is free to move so it will roll in the direction of the force
54
what does the force acting on a conductor in a magnetic field depend on ?
- magnetic flux density
- size of the current through the conductor
- length of the conductor that's in the magnetic field
55
what is magnetic flux density ?
how many field lines there are in a region
56
what does magnetic flux density show ?
the strength of the magnetic field
57
what is magnetic flux density measured in ?
tesla
58
how can you tell which way the force acting on the conductor due to the motor effect is acting if a current is flowing at 90 degrees to a magnetic field ?
Flemings left hand rule
59
describe Flemings left hand rule
1. using your left hand, point your First finger in the direction of the Field
2. point your seCond finger in the direction of the Current
3. stick your thuMb out so its at 90 degrees to the other fingers, it will then point in the direction of the force (Motion)
60
if the direction of the current or magnetic field is reversed, what will happen to the force ?
the direction of the force is also reversed
61
how do electric motors use the motor effect ?
to produce rotation
62
how, step by step, do electric motors work ?
1. a loop of wire that's free to rotate about an axis is placed in a magnetic field
2. when a direct current flows through the loop, the 2 side arms ( which are 90 degrees to the field) each experience a force due to the motor effect
3. these arms experience forces in opposite directions because the direction of the current in each arm is opposite
4. the loop will start to rotate around its axis because the forces act one up and one down
5. when the loop wire reaches a vertical position the forces will still be acting on up and one down on the same arms of the loop so the loop gets stuck
6. for the motor to keep rotating in the same direction the forces acting on the arms of the loop need to swap direction
7. reversing the direction of the current reverses the direction of the force - often used with a split ring communicator
8. by linking the end of each loop to one half of a split ring communicator, you change the electrical contacts of the loop every half turn
9. this means the force acting on each arm of the loop will swap every half turn allowing rotation to continue in the same direction
63
what is a split ring communicator ?
a conducting ring with a gap between the 2 halves, as it rotates the part of the communicator that is touching each contact changes every half turn
64
how does a split ring communicator help create an electric motor ?
it swaps the contacts of the loop every half turn, reversing the current
65
how can the speed of an electric motor be increased ?
- increasing the current
- increasing the strength of the magnetic field
66
why does increasing the current and increasing the strength of the magnetic field increase the speed of an electric motor ?
they increase the force experienced by the wire due to the motor effect so it rotates faster
67
how will using a coil of wire instead of a single loop increase the force caused by the motor effect ?
- the sides of each individual loop experience a force, so the more loops you have, the larger the total force on the sides of the coil
68
how do you calculate the force on a coil of wire in a magnetic field ?
first calculate the force on a single loop then multiply it by the number of loops in the coil
69
how can the direction of a simple electric motor be reversed ?
- swapping the polarity of the direct current
- swapping magnetic poles over
70
how can you tell whether the electric motor is moving anti-clockwise or clockwise ?
1. draw the magnetic field lines from north to south and the direction of the current positive to negative
2. use Flemings left hand rule on one side of the loop to work out the direction of force
3. draw the direction of the force then see if its clockwise or ant-clockwise
71
how do loudspeakers work ?
motor effect
72
describe step by step how the motor effect allows loudspeakers to work
- an alternating current is sent through a coil of wire attached to the base of a a paper cone
- the coil surrounds one pole of a permanent magnet and is surrounded by the other pole so the current causes a force on the coil
- when the current reverses the force acts in the opposite direction which causes the cone to move in the opposite direction too
- variations in the current makes the cone vibrate which makes the air around the cone vibrate and creates variations in the air pressure that form a sound wave
- the frequency of the sound wave produced is the same as the frequency of the alternating current
73
how by controlling frequency of the alternating current can you change the sound wave ?
the frequency of the sound wave produced is the same as the frequency of the alternating current
74
what can an electrical conductor which moves relative to a magnetic field induce ?
a potential difference across the conductor
75
how can a potential difference be induced ?
if there is a change in an external magnetic field around a conductor , if the conductor is part of a complete circuit current will flow
76
what is the generator effect ?
the induction of potential difference across a conductor which is experiencing a change in an external magnetic field
77
when does the generator effect happen ?
when a conductor 'cuts' through the magnetic field lines
78
how can the potential difference across the ends of a conductor be induced ?
- moving the electrical conductor in a magnetic field
- moving or changing a magnetic field relative to the electrical conductor
79
give an example of why potential difference is induced ?
- moving wire in a magnetic field will cause pd to be induced across ends of the wire
- moving a bar magnet through a coil of wire will induce a pd across the ends of the wire
- if you connect each end of the coil to a bulb you'll see the bulb light up as the magnet moves
80
what will happen if you move the magnet in opposite direction ( generator effect ) ?
the pd/current will be reversed
81
if you keep the magnetic field moving backwards and forwards what happens to the potential difference ?
it keeps swapping = creates an alternating current
82
how else can you create an alternate current ?
- turning a magnet end to end in a coil
- turning a coil inside a magnetic field
83
what happens when you turn a magnet end to end in a coil or turn a coil inside a magnetic field ?
- magnetic field through the coil changes
- this induces a pd which can make a current flow in the wire
- when you've turned the magnet through half a turn, the direction of the magnetic field through the coil reverses
- when this happens the pd reverses so the current flows in the opposite direction around the coil of wire
- if you keep turning the magnet in the same direction the potential difference will keep reversing every half turn = alternating current
84
when the pd reverses what happens to the current ?
it flows in the opposite direction around the coil of wire
85
how can induced potential difference be increased ?
- increasing the speed of movement
- increasing the strength of the magnetic field
86
how can you induce a current in a wire ?
by changing a magnetic field
87
what happens when a current is induced in a wire ?
when a current flows through the wire a magnetic field is created around the wire
so you get a second magnetic field, different to the one whose field lines are being cut in the first place
88
what way does the magnetic field created by the induced current act against ?
the field that made it
89
what does the induced current always oppose ?
the change that made it
90
how do generators use the generator effect to induce a current ?
they rotate a magnetic field
as the coil spins a current is induced in the coil
whether they generate an ac current or dc current depends on the device
91
what type of current do alternators generate ?
an alternating current
92
how do alternators work ?
as the coil spins, a current is induced in the coil
every half turn this current changes direction
93
what do alternators use to maintain the alternating pd which generates an ac ?
slip rings and bushes so the contacts don't swap every half turn
94
what do dynamos generate ?
direct current
95
what is the difference between dynamos and alternators ?
they use a split ring communicator as their electrical contact
96
why do dynamos use a split ring communicator as their electrical contact ?
to swap the connection every half turn to keep the current flowing in the same direction
97
what do oscilloscopes do to see generated pd ?
show how the pd generated in the coil changes over time
98
describe the oscilloscope trace of an alternating pd
positive and negative ends keep changing s the trace cycles between positive and negative peaks crossing the horizontal axis
99
describe the oscilloscope trace of a direct pd
pd doesn't alternate so the current flows in the same direction, the trace never crosses the horizontal axis and you get peaks above the horizontal axis due to the dynamo reversing direction every half turn
100
what does increasing the frequency of rotations in the generator do to oscilloscope traces ?
give higher peaks and more of them, stretched vertically but squashed horizontally - because increasing frequency increases overall pd
101
how do microphones generate current ?
from sound waves
102
how is a current created in a microphone ?
they 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
creating a current
103
what does the movement of coil in a sound wave dependent on ?
the properties of the sound wave
104
how do microphones convert the pressure variations of a sound wave into variations in current in an electrical circuit ?
the movement of the coil
105
what are transformers ?
devices that change potential difference of an electrical supply using the generator effect
106
what type of current do transformers only work on ?
alternating current
107
what does a transformer consist of ?
2 coils - primary and secondary wrapped around an iron core
108
why is iron used in a core of a transformer ?
it can magnetise and demagnetise quickly
109
describe how a transformer works
1. when an ac flows through the primary coil in a transformer it produces a magnetic field which magnetises the iron core
2. because there is ac in the primary coil, the magnetic field in the iron core is alternating too
3. this constantly changing magnetic field cuts through the secondary coil
4. the changing field induces an alternating pd across the secondary coil by the generator effect
5. if the secondary coil is part of a complete circuit this pd causes an ac to flow and has the same frequency as the ac in the primary coil
110
what does the size of potential difference induced across the secondary coil dependent on in transformers ?
- size of the pd across the primary coil
- number of turns on each coil
111
what does the iron core in transformers do ?
transfer the changing magnetic field from the primary coil to the secondary
112
if you supplied direct current to the primary coil of a transformer what would happen ?
you'd get nothing out of the secondary coil because you need a changing field to induce a pd
113
what are the 2 types of transformer ?
step up transformer
step down transformer
114
what do step up and step down transformers do ?
change an electricity supply so that the size of the output pd is not the same as the input pd
115
in step up and step down transformers what is the ratio between the primary and secondary pds the same as ?
the ratio between the number of turns on the primary coil and the number of turns on the secondary coil
116
what is a step up transformer ?
where the number of turns in a secondary coil and the size of the pd across it are greater than the number of turns on the primary coil
117
what is a step down transformer ?
the number of turns on the secondary coil and the size of the pd across it is smaller than than across the primary coil
118
where are step up and step down transformers used ?
in the national grid
119
what do step up transformers do in the national grid ?
increase the pd of the supply for transmission around the country via power lines
120
why are step up transformers useful in the national grid ?
for a given power, increasing pd decreases the current so less energy is lost by heating
121
what do step down transformers do in the national grid ?
reduce the pd supply before it reaches our homes making it safer to use
122
where are transformers used in homes ?
to reduce the pd of an appliance to a suitable level
123
what is a transformers efficiency ?
nearly 100%
124
what is the law for electrical power ?
electrical power output = electrical power input
