Magnetism and electromagnetism Flashcards
(86 cards)
1
Q
What are the two poles all magnets have?
A
North
South
2
Q
What do all magnets produce?
A
A magnetic field.
3
Q
Define the term magnetic field.
A
A region where other magnets or magnetic material experience a force. This is a non contact force similar to the force in an electric field.
4
Q
Give four examples of magentic materials.
A
Iron,steel,nickel and cobalt.
5
Q
How do you show a magnetic field?
A
Drawing magnetic field lines.
6
Q
When drawing a magnetic field how do you draw them?
A
North to south, and they show which way a force would act on a north pole if it was put at that point in the field.
7
Q
What do close magnetic field lines show?
A
Stronger field.
8
Q
What do far apart magentic field lines show?
A
Weaker field.
9
Q
Where is the magnetic field strongest?
A
Poles of a magnet.
So the magnetic FORCES are also strongest at the poles.
10
Q
Describe the force between a magnet and a magnetic material.
A
Always attractive, no matter the pole.
11
Q
If two poles of a magnet are put near each other what happens?
A
They will exert a force upon each other.
12
Q
What can forces be?
A
Attractive or repulsive.
13
Q
What are two poles that are the same called and what force do they have?
A
“like poles”
they will repel each other.
14
Q
What are two poles that are the different called and what force do they have?
A
Unlike poles, they attract each other.
15
Q
What is inside a compass?
A
A tiny bar magnet.
The north pole of magnet is attracted to south pole of any other magnet it is near.
So the compass points in the direction of the magnetic field it is.
16
Q
How to see the magnetic field of a compass?
A
Move a compass around a magnet and trace its position on some paper to build a picture of what the magnetic field looks like.
17
Q
In a compass what happens when the poles are not near a magnet?
A
Compass always points north.
Because the Earth generates it own magnetic field which shows the core of the Earth must be magnetic.
18
Q
Give the two types of magnet.
A
Permanent magnets
Induced magnets
19
Q
Define the term permanent magnet.
A
They produce their own magnetic field.
20
Q
Define the term induced magnet.
A
Magnetic materials that turn into a magnet when theyr’e put into a magnetic field.
21
Q
What is the force between permanent and induced magnet called?
A
Attractive.
22
Q
What happens when you take away the magnetic field from the induced magnet?
A
They lose their magnetism, or most of it so stop producing a magnetic field.
23
Q
When a current flows through a wire what is created?
A
A magnetic field around the wire.
24
Q
What is magnetic field created by a moving charge made up of?
A
Concentric circles, perpendicular to the wire, with the wire in the centre.
25
Give the method of hwo you can see the concentric circles in the magentic field of a moving charge.
Place a compass near a wire that is carrying current.
| As you move the compass it will trace the direction of the magnetic field.
26
What does changing the direction of a current in magnetic field do?
Changes the direction of the magnetic field.
27
What is the right hand rule for?
To see the direction of the magnetic field.
28
What changes the strength of the magnetic field?
It changes with the current and the distance from the wire
29
What makes the magnetic field stronger in the moving charge?
The larger the current through the wire, or the closer to the wire you are the stronger the field is.
30
How does the right hand thumb rule work?
Use right hand
Point thumb in direction of current.
Curl your fingers
The direction of your fingers is the direction of the field.
31
Give an other way to increase the strength of the magnetic field that a wire produces?
Wrapping the wire into a coil called a solenoid.
32
Why does a solenoid increase the strength of the magentic field?
Field lines around each loop of wire line up with each other.
This results in lots of field lines pointing in the same direction that are close to each other, closer together field lines are the stronger the field is.
33
Describe the magnetic field in the solenoid, what is it like?
Strong and uniform, same strength and direction at every point in that region.
34
What is the magnetic field outside the solenoid coil?
Like the one round a bar magnet.
35
How to increase the field strength of the solenoid even more? -and how does this work?
Put a blokc of iron in the centre of the coil, it becomes an induced magnet whenever current is flowing.
36
What happens if the current id stopped in a solenoid?
The magnetic field dissapears.
37
Define the term electromagnet.
A solenoid with an iron core (a magnet whose magentic field can be turned on and off with an electric current)
38
Why are electromagnets useful?
Switch on and off easily, as they can create a varying force.
39
How are electromagnets used in cranes?
To attract and pick up things made from magnetic materials like steel and iron eg:scrap yards
The magnet can be turned on when you want to pick up something and switched off when you want to drop it.
40
How can electromagnets be used within other circuits?
Act as switches. eg:electric starters of motors
When switch in circuit one is closed it turns on the electromagnet which attracts the iron contact on the rocker.
The rocker pivots and closes the contacts,completing circuit two and turing on the motor.
41
How to draw diagram for a current carrying solenoid.
Draw solenoid
lines from right to left arrows going to left on each side
Around the solenoid = above, draw two rings attatched to either end and the arrows should go right
The rings below again attatched to solenoid, have arrows going left too.
42
How to draw a current carrying wire?
black dot in middle
3 rings around
each ring around quarter past position has an arrow pointing upwards.
43
Define the term motor effect.
When a current carrying wire or any other conductior is but between magnetic poles, magnetic field around the wire interacts with magentic field it has been placed in.
Casues magnet and conductor to exert a force on each other and this can cause the wire to move.
44
In the motor effect how do you experience the full force?
Wire has to be at 90 degrees to magnetic field.
45
In the motor effect when will you experience no force?
if wire runs parralel to magnetic field it will not experience force.
At angles in between it will feel some force.
46
In the motor effect where does the force always act?
Right angles to the magnetic field of the magnets and the direction of the current in the wire.
47
What is a good way to show the direction of the force?
Apply a current to the set of rails inside a horseshoe magnet.
A bar is placed on the rail which completes the circuit,
This generates a force that rolls the bars along the rails.
Magnitude of force increases with the strength of the magnetic field.
Force increases too with the amount of current passing thorugh the conductor.
48
What does the force acting on a conductor in a magnetic field depend on?
The magnetic flux density- how many field (flux) line there are in a region, this shows the strength of the magnetic field.
Size of current through the conductor.
Length of conductor in the magnetic field.
49
When the current is at 90 degrees to the magnetic field it is in how can the force acting on it be found?
```
force = magnetic flux density x current x length
force=N
MFD=Tesla (T)
Current = A
Length = m
```
50
What can you find out with Flemings left hand rule?
Direction of the force.
| If either the current or magnetic force is reversed then direction of force will also be reversed.
51
How does flemings left hand rule work?
Use left hand point first finger in direction of field from north to south
point second finger in direction of current from + to -
your thumb will then point in direction of the force (motion)
52
Where do forces act on a DC motor?
On the two side arms of a coil of the wire that is carrying current.
53
What are they type of forces acting on a DC motor?
Usual forces which act on any current in a magnetic field.
54
What is the coil on in a DC motor?
A spindle, and the forces act one up and one down so it rotates.
55
What does the split ring commutator do in a DC motor?
Swaps the contacts every half turn to keep motor rotating in the same direction.
56
In a DC motor how can the direction of the motor be reversed?
Swapping polarity of DC supply (reversing the current)
| Swap magnetic poles over (reverse field.)
57
What are loud speakers and headphones?
Tiny loud speakers.
58
How do loudspeakers and headphones use electromagnets?
Alternating current is sent through a coil of wire attatched to base of a paper cone.
Coil surrounds one pole of a permanent magnet, and is surrounded by the other pole so current causes a force on the coil this causes it to move.
When current reverses force acts in opposite direction which causes cone to move in opposite direction too.
Variations in current make cone vibrate this makes air around cone vibrate too and creates variations in pressure that causes a sound wave.
Frequency of sound wave is same as frequency of ac, so by controlling frequency of ac you can alter sound wave produced.
59
Define the generator effect.
The induction of potential difference and current if their is a complete circuit in a wire which is moving relative to a magnetic field or experiencing a change in magnetic field.
60
What does the generator create?
A potential difference in a conductor, and a current if the conductor is part of a complete circuit.
61
One way that the generator effect creates a potential difference in a conductor and a current if the conductor is part of a whole circuit?
Move magnet in a coil of wire
OR
move a conductor in a magnetic field.
Shifting magnet from side to side creates a blip of current if the conductor is part of a complete circuit
If you move magnet or conductor in opposite direction then the pd/current will be reversed.
Like wise if polarity is reversed then pd/current reverses too.
If you keep magnet or coil moving backwards and forwards you create a pd that keeps swapping direction which is an alternating current
62
How to create the generator effect with a different method.
As you turn magnet magnetic field through the coil changes.
This change in magnetic field induces a potential difference which can make a current flow in a wire.
When you jave turned magnet through a half turn direction of magnetic field through coil reverses.
When this happens p.d reverses so current flows in opposite direction around coil of wire.
If you keep turning magnet in same direction say clockwise then p.d will keep reversing every half turn and you will get an alternating current.
63
What does the magnetic field created by the induced current always act against in generator effect?
The change that is made.
whether a movement of wire or change in field.
So its trying to return to how things were.
So induced current always opposes the change that is made.
64
How to change the size of the induced pd and induced current in generator effect
increase speed of movement by cutting more magnetic field in a short amount of time
increase strength of magnetic field so there are more field lines to be cut.
65
Tell me how a generator produces an alernating potential difference.
Generators rotate a coil in a magnetic field.
Construction is like a motor.
As coil or magnet spins a current is induced in the coil.
Current changes direction every half turn.
Instead of split ring commutator ac generators have slip rings and brushes so contacts don't swap every half turn.
So they produce an alternating potential difference.
66
Tell me how dynamos create direct current.
Work in same way as alternators apart from one difference.
They have slip ring commutators instead of slip rings.
This swaps connection every half turn to keep current flowing in the same direction.
67
What do oscillioscopes show?
How potential difference generated in the coil changes over time.
68
What does AC oscillioscope look like?
Line goes up and down crossing horizontal axes so goes up and down inbetweeen x axis.
69
What does DC oscillioscope look like?
Above the axis as pd is always positive so its still direct current.
looks like little hill above the axis
70
What does the height of the line on an oscillioscope show?
generated potential difference at that time.
71
What does increasing revolutions oscillioscope look like?
increases overall pd but also creates more peaks
| so it is slim hills all over the graph
72
What are microphones?
loud speakers but in reverse
73
How do microphones generate current from sound waves?
Sound waves hit a flexible diaphram that is attatched to a coil of wire wrapped around a magnet.
Causes coil of wire to move in the magnetic field which generates current.
Movement of coil and generated current depends on properties of the sound wave, louder sounds make the diaphram move further.
This is how microphones convert pressure variations of a sound wave into variations in current in an electric circuit.
74
What do transformers do?
change the size of the potential difference of an alternating current.
75
What are transformers made from?
Two coils of wire the primary and secondary and are joined with an iron core.
76
In a transformer what happens when a alternating pd is applied across primary coil?
Iron core magnetises and demagnetises quckly.
| This changing magentic field induces an alternating pd on the second coil.
77
What happens if the second coil in a transformer has a complete circuit?
A current is induced.
78
What is ratio between primary and secondary pd equal to?
Ratio between number of turns on primary and secondary coils.
79
Why is iron used in a transformer?
it is easily magnetised
80
What is a step up transformer?
steps p.d up so they have more turns on the secondary coil than the primary coil
81
What is a step down transformer?
steps down the p.d so they have more turns on the primary coil than secondary.
82
Formula for cumber of turns needed to increase pd and decrease current to right levels.
```
vp/vs = np/ns
VP=input pd
VS=output pd
NP=number of turns on primary coil
NS=number of turns on secondary coil.
Then rearange
```
83
Are transformeres efficient?
Almost 100% if you assume they are then input power is equal to output
84
How to work out power?
P=IV
85
Give formula for potential difference and current for a transformer.
```
Vs Is = Vp Ip
Vs=pd across secondary coil
Is = current through secondary
Vp= pd across primary coil
Ip=current across primary coil
```
86
How is the national grid efficient?
low current means less enegy is being wasted heating wires and surroundings making national gird an effective way of transmitting power.
VsIs=VpIs shows why a high pd is needed for a low current.
