Electeomagnetica Flashcards
(85 cards)
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
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
3
Q
what does induced magnetism always cause
A
a force of attraction
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
5
Q
examples of permanent magnets
A
a horeshoe magnet
a bar magnet
6
Q
what are the four types of magnetic material (MUST KNOW!)
A
- iron
- steel
- cobalt
- nickel
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
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
9
Q
what does the strength of a magnetic field depend on
A
the distance from the magnet
10
Q
where is the magnetic field strongest at
A
the poles of the magnet (N and S)
11
Q
how can the direction of a magnetic field be found
A
using a compass
12
Q
how can we use a compass to map out the direction of a magnetic field
A
- place the plotting compass near the magnet on a piece of paper
- mark the direction that the compass needle points
- move the plotting compass to many different positions in the magnetic field, marking the needle direction each time
- join the points to show the field lines
13
Q
in what directions do forces move in a magnet
A
from the North pole to South
14
Q
what does it mean when the lines in a magnetic field are closer
A
the field is stronger
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
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
17
Q
what happens when you close the switch of an electric circuit
A
an electric current flows around the curcuit
18
Q
what happens when a current flows through a conducting wire
A
a circular magnetic field is produced around the wire
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
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
21
Q
when is the magnetic field in a wire strongest💪🏾
A
closer to the wire
22
Q
what happens if we change the direction of the current
A
we change the direction of the magnetic field
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
24
Q
what is a solenoid
A
a coiled up wire
25
what happens when we turn on a current in a circuit which contains a solenoid
we get a strong and uniform magnetic field inside the solenoid
26
what is an electromagnet
a solenoid containing an iron core
27
why are electromagnets useful
because we can change the strength of the magnetic field by changing the size of the current
28
what does the direction of the field depend on
the direction of current
29
how can you tell the north and south pole on a solenoid
arrows going in are south, arrows going out are north
30
how would we reverse the direction of a magnetic field
by reversing the direction in which the current is flowing
31
why would an iron core increase the strength of an electromagnet
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
32
what is the motor effect
the idea that a current carrying wire in the presence of a magnetic field will experience a force
33
how to calculate the strength of a force
f (force) = B (magnetic flux density) x I (current) x length of wire (m)
34
how do electric motors work
1.
35
how does a 90 turning coil work
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
how to fix a coil that only turns 90
- change the direction of the current every half turn so that the direction of the forces will always be acting clockwise
37
how does the electric motor work
-
38
what does a split ring commutator do
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
how to increase the speed of rotations in an electric motor
- increase the current passing through the wire
- add more turns to the coil
- increase the magnetic flux density by adding more powerful magnets
40
how does the motor effect work in a loudspeaker
- 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
how to change the frequency that a cone vibrates
by changing the frequency of the AC supply
- a higher frequency will produce a higher pitched sound
42
what would happen if we increase the size of the current in a loudspeaker
we increase the amplitude of the vibration, which increases volume of the sound
43
what is the generator effect
- 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
what does the size of the induced pe or current depend on
1. size of magnetic field
2. Speed of wire moving
3. number of turns on the coil
45
how else can we switch the direction of the induced current
by switching the poles of the magnet
46
what does an induced current do
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
what is an alternator
a coil of wire rotating in a magnetic field, the coil is connected to two metal rings called commutators
48
what do commutators do
they allow current to pass out of the coil
49
when do we get the max potential difference in an alternator and why
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
what happens when the coil is vertical in an alternator and why
the old falls to 0, because the coil is now moving parallel to the field
51
why does the pd in an alternator reverse direction
because the two sides of the coil are now moving in a different direction to before
52
why does an alternator produce an alternating PD and current
because the two sides of the coil are attached to two different rings
53
how to increase size of AC
- 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
features of a dynamo
- 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
why do we get two peaks for each full rotation in a dynamo
because each side of the coil passes through the magnetic field twice during each cycle of rotation(up and down)
56
how does a moving coil microphone work
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
what is the relationship between the frequency of the changing PD and the frequency of the sound waves in a microphone
they are the same
58
features of the primary coil
- contains an iron core
connected to an alternating current
- as current flows through it, it generates a changing magnetic field
59
how is the magnetic field transmitted from the primary to secondary coil
along the iron core
60
what happens when the changing magnetic field passes through the secondary coil
it induces a pd
61
why do transformers only work with an AC
because we need a changing magnetic field to induce a pd, a dc produces a constant magnetic field
62
if the number of turns in the secondary coil is greater than the number of turns in the primary, what does it mean
the secondary coil has a greater induced pd
63
what is a step up transformer
a transformer that has a larger pd in the secondary coil than the primary
64
what is a step down transformer
one where the pd in the primary coil is more than the pd in the secondary
65
why is a large current not suitable for transmitting a large amount of power
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
what do step up transformers do in the national grid
increases of to around 400000V
67
what do step down transformers do in the national grid
reduces the pd to 230V
68
what direction does the needle point to in a plotting compass
to the south pole of the magnet
69
why are solenoids strong
The small magnetic fields caused by the current in each coil add together to make a stronger overall magnetic field
70
what does an iron core do
increases the solenoid's magnetic field strength
71
describe the motor effect
- 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
how to increase the force on a given length of wire in a magnetic field
- increase magnetic field strength
- increase the current
73
when is the force greatest at any given combination of current and magnetic field strength
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
what are the 4 ways we can increase an electromagnets strength
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
features of a relay
- 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
what happens if the low voltage circuit is turned off in a relay
no current is flowing through the magnet, there’s no electric field, contacts are apart
77
why is the high voltage circuit turned off in a relay
because the contacts are not touching
78
what happens if we switch on the low voltage circuit in a relay
- 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
how does a doorbell work
- 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
rules when using the motor effect
- 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
what happens when we move a wire through a magnetic field
- a potential difference is induced across the ends of the wire
82
what is induced potential difference
when potential difference changes directions
83
when does the direction of the current switch
when the direction of movement switches
84
what other times can we get an induced pd
if we keep the wire still but move the magnetic field
85
why is iron used in transformers
it’s easily magnetised
