P15

?
What material can always be magnetised and demagnetised?
Any steel or iron object
1 of 83
What other materials can be magnetised and demagnetised?
Cobalt and nickel
2 of 83
Why are permanent magnets made of steel?
Magnetised steel does not lose its magnetism easily
3 of 83
Where is the magnetic field most strongest?
At the poles
4 of 83
What is the rule between distance and magnetic field?
The further away the magnet is the weaker the strength of the magnetic field
5 of 83
What is induced magnetism?
An magnetised magnetic material placed in a magnetic field
6 of 83
What will induced magnetism cause?
A force of attraction between any magnetised magnetic material placed near one end of the bar magnet
7 of 83
What force is it always?
An attractive one
8 of 83
What happens when an electric current passes along a wire?
A magnetic field is set up around the wire
9 of 83
If you reverse the direction of the magnetic field, what else do you reverse?
The direction of the magnetic field
10 of 83
What is a solenoid?
A long coil of insulated wire
11 of 83
When is magnetic field produced in a solenoid?
When a current is passed through the wire
12 of 83
When does the magnetic field increase in strength?
When the current is increased
13 of 83
When does the magnetic field reverse its direction?
If the current is reversed
14 of 83
How are the field lines to the axis of the solenoid?
Parallel
15 of 83
Why is each field line a loop in a solenoid?
They pass through the inside of the solenoid
16 of 83
How to increase the magnetic field produced by a solenoid ?
Increase current, increase coils of wire, iron inside solenoid
17 of 83
What is an electromagnet?
A solenoid in which the insulated wire is wrapped around an iron bar
18 of 83
What happens when the magnetic field is produced from the electric current?
Magnetises the iron bar and when current off no magentise
19 of 83
What are some uses of electromagnets?
A scrapyard crane, circuit breaker, electric bell and a relay
20 of 83
How does the scrapyard crane work?
Steel frame of car sticks to electromagnet when current passes through
21 of 83
How does a circuit breaker work?
Switch held shut by a spring when current is too large switch pulled open by electromagnet
22 of 83
How does the electric bell work?*
Bell connected to a battery, the iron armature pulled by the electromagnet. This opens the make and break switch so electromagnet is off which means spring goes back and repeats
23 of 83
What is a relay used for?
To switch an electrical machine on or off
24 of 83
How does the relay work?*
Small current through electromagnet magnetises iron core, pulls armature onto electromagnet, closes switch gap and turns machine on
25 of 83
Why does the electric motor work?
Force can act on a wire in a magnetic field when a current is passed through the wire this is known as motor effect
26 of 83
How can you increase the size of the force?
Increase current, stronger magnet
27 of 83
When is the force greatest?
When wire is perpendicular to magnetic field
28 of 83
When is the force zero?
When wire is parallel to magnetic field
29 of 83
What is magnetic flux density?
The measure of the strength of magnetic field
30 of 83
What is the unit for magnetic flux density?
Tesla
31 of 83
How can you control the speed of an electric motor?
Changing the current
32 of 83
How can you reverse the direction the motor turns in?
Reverse the current
33 of 83
Describe how an electric motor works?
Rectangular coil of insulated wire which is forced to rotate.Coil is connected to a battery by two graphite brushes. Brushes press onto split ring commutator. Graphite causes little friction
34 of 83
When a current is passed through the coil, the coil spins because?
A force acts on each side of coil due to motor effect and a force on one side is in the opposite direction to the forced on other side
35 of 83
What does the spilt ring commutator do?
Reverses the current around the coil every half turn of the coil
36 of 83
Why does this happen?
The sides swap over each half turn so coil is pushed in same direction each half turn
37 of 83
What does a generator contain ?
Coils of wire that spin in a magnetic field. A Pd is created across ends of wire
38 of 83
What is electromagnetic induction?
Any conductor passes through magnetic field lines?
39 of 83
What is the generator effect?
When the induced Pd makes an electric current pass around the circuit
40 of 83
When one end of the bar magnet is pushed into the coil the ammeter deflect because?
The movement of the bar magnet causes an induced potential difference in coil and this causes a current because coil is part of a complete circuit
41 of 83
How can we make a bigger current?
A stronger magnet used which means bigger Pd which means bigger current
42 of 83
If the bar magnet is withdrawn from ammeter what happens?
Ammeter points in opposite direction
43 of 83
Why does this happen?
The induced Pd acts in the opposite direction so current goes opposite direction
44 of 83
When is the magnetic field created?
When the magnet is moving
45 of 83
What is a simple alternator made up of?
Rectangular coil that is forced to spin in a uniform magnetic field
46 of 83
What is the coil connected to?
Centre-reading meter by metal brushes that press on two metal slip rings
47 of 83
What do the metal slip rings and brushes do?
Provide a continuous connection between the coil and the meter
48 of 83
What happens when the coil turns steadily in one direction?
The meter pointer deflects first one way, then the opposite way and then back again
49 of 83
How long does this carry on for?
As long as the coil keeps turning in the same direction
50 of 83
Why does the current in the circuit repeatedly change direction through the meter?
Because the induced potential difference in the coil repeatedly changes direction
51 of 83
When is the size of the induced potential difference at its greatest? *
When the plane of the coil is parallel to the direction of the magnetic field lines
52 of 83
When is the size of the induced potential difference is zero?
When the plane of the coil is perpendicular to the magnetic field lines, the sides of the coil move parallel to the field lines, does not cross through them
53 of 83
The faster the coil rates the what?
The bigger the freq and the bigger the peak value
54 of 83
How can you increase the peak value?
Using a stronger magnet, using a coil with more area and more turns of wire
55 of 83
What is a dynamo?
A direct current generator
56 of 83
What is the difference between a dynamo and an alternator?
The dynamo has a split ring commutator instead of two separate spilt rings
57 of 83
What happens as the coil spins?
The spilt ring commutator reconnects the coil the opposite way around in the circuit every half turn
58 of 83
When does this happen?
Each time the coil is perpendicular to the magnetic field lines
59 of 83
What is the result of this?
The induced potential difference does not reverse its direction as it does in an alternator
60 of 83
What does the induced potential difference vary from?
Zero to the maximum value twice each cycle and never changes polarity
61 of 83
What does the moving coil microphone generate?
An alternating potential difference as sound waves makes the coil vibrate
62 of 83
What is the coil attached to?
A small diaphragm and is between the poles of a cylindrical magnet
63 of 83
What do the pressure variations of the sound waves on the diaphragm do what?
Make it vibrate so the coil vibrates in the magnetic field
64 of 83
The alternating potential difference induced in the coil has the same freq as what?
Sound waves
65 of 83
When does the moving coil loudspeaker creates sound waves?
When an alternating potential difference is applied to its coil
66 of 83
What causes the force on the coil due to the motor effect?
The current
67 of 83
Because the current alternates the force repeatedly reverses direction and makes the coils and diaphragm vibrate what does this create?
Sound waves of the same freq as the alternating Pd.
68 of 83
What are step up transformers?
Transformers that increase the size of an alternating Pd
69 of 83
What are step down transformers?
Used to decrease the size of an alternating Pd.
70 of 83
What does a transformer have?
Two coils of insulated wire, both wound around the same iron core.
71 of 83
Why is iron used as the core?
Because iron is easily magnetised and demagnetised
72 of 83
What is the primary coil connected to?
An a.c. supply
73 of 83
What happens when the alternating current passes through the primary coil?
Alternating Pd is induced in the secondary coil
74 of 83
Why does this happen? (First reason)
Alternating current passing through the primary coil generates an alternating magnetic field in the iron core
75 of 83
Why does this happen? (Second reason?
The lines of alternating magnetic field in the iron core pass through the secondary coil
76 of 83
Why does this happen? (Third reason)
The magnetic field in the secondary coil induces an alternating Pd between the terminals of the secondary coil
77 of 83
Why dont transformers work with direct current?
No changing magnetic field so secondary Pd is 0
78 of 83
What does a step up transformer have?
More turns in the secondary coil than the primary
79 of 83
What does a step down transformer have?
Fewer turns in the secondary coil than primary
80 of 83
What does a step up transformer do at a power station in grid?
Current lowered if Pd raised
81 of 83
What is the heating effect in a resistor or wire proportional to?
The square of the current
82 of 83
What happens if you increase the grid potential difference?
Current through cables is reduced so heating effect is smaller and less power is wasted
83 of 83

Other cards in this set

Card 2

Front

What other materials can be magnetised and demagnetised?

Back

Cobalt and nickel

Card 3

Front

Why are permanent magnets made of steel?

Back

Preview of the front of card 3

Card 4

Front

Where is the magnetic field most strongest?

Back

Preview of the front of card 4

Card 5

Front

What is the rule between distance and magnetic field?

Back

Preview of the front of card 5
View more cards

Comments

No comments have yet been made

Similar Physics resources:

See all Physics resources »See all Electromagnetism resources »