Brainscape's Knowledge GenomeTM

Browse over 1 million classes created by top students, professors, publishers, and experts.


See full index

Physics > Chapter 13 - Electromagnetic > Flashcards

Chapter 13 - Electromagnetic Flashcards

You may prefer our related Brainscape-certified flashcards:
Physics 101 flashcards AP® Physics flashcards
1
Q

What happens at the poles of a magnet?
What happens when 2 magnets are brought close together?
When does repulsion/attraction occur?

A

Poles are the places where magnetic forces are strongest
When 2 magnets are brought close together, they exert a force on each other - like poles repel each other, unlike poles attract each other

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

Attraction and repulsion between 2 magnetic poles are examples of?

A

Non contact force

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

A permanent magnet….

A

Produces it’s own magnetic field

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

What is an induced magnet?
What does induced magnetism cause?
What happens when it’s removed?

A

A material that becomes a magnet when placed in a magnetic field
Induced magnetism always causes a force of attraction
When removed from magnetic field, it loses most/all of its magnetism quickly

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

What is a magnetic field?

A

The region around a magnet where a force acts on another magnet/magnetic material
Picture of battery with arrows going inwards at S end, out at N end, field around anticlockwise

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

What materials are magnetic?

A

Iron, cobalt, nickel, steel (an alloy)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

The force between a magnet and a magnetic material is always one of….

A

Attraction

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

What does the strength of the magnetic field depend on?
Where is the field strongest?

A

Depends on distance from magnet
Field is strongest at the poles of the magnet
Magnetic force is strongest where the lines are close together

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

The direction of the magnetic field at any point is given by….

A

The direction of the force that would act on another North Pole placed at that point

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

What is the direction of a magnetic field line?

A

From the north (seeking) pole of a magnet to the south (seeking) pole

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

Mnemonic for direction of magnetic field

A

ONIS field lines point out to the north and into the south

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

What does a magnetic compass contain?
Where does it point?

A

Contains a small bar magnet
Compass needle points in direction of the earth’s magnetic field

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

How to plot the magnetic field pattern of a magnet using a compass?

A

Draw around a magnet and make a dot at one end
Line a compass needle with the dot and draw a dot at other end of the needle
Move needle so it points to the new dot and draw another dot
Repeat until there is a chain of dots, then join them up
Repeat for other points along the magnet
The direction of the field line is the same as direction of the plotting compass

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

How else can we find out the shape of a magnetic field?

A

Sprinkle iron fillings near a magnet

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

Features of electromagnets

A

They can be switched on and off

Their strength can be varied

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

When a current flows through a conducting wire….

A

A magnetic field is produced around the wire

Picture of upright bar (red arrow up) with circles around middle going anti clockwise

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
17
Q

The strength of the magnetic field depends on….

A

The current through the wire and the distance from the wire

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
18
Q

The magnetic field lines form ______ _______ around the wire, which spread out (get weaker) as the distance from the wire _________.

A

The magnetic field lines form concentric circles around the wire, which spread out (get weaker) as the distance from the wire increases

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
19
Q

Shaping a wire to form a solenoid _______ the strength of the magnetic field created by a current through the wire

A

Increases

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
20
Q

What is a solenoid?

A

A tight coil of wire

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
21
Q

The magnetic field inside a solenoid is ______ and ________

A

Strong and uniform

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
22
Q

An electromagnet is…..

A

A solenoid with an iron core

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
23
Q

The magnetic field around a solenoid has a similar shape to that of a ______ _________, how is it similar?

A

A bar magnet

one end of the solenoid behaves like the North Pole of a magnet, the other side behaves like the South Pole
Inside the solenoid the field lines from the individual turns add up to form a strong uniform field
Outside the solenoid the fields from the individual turns cancel out, resulting in a much weaker magnetic field

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
24
Q

What is a uniform field?

A

A field which has the same strength and direction at all points

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
25
Q

How can we increase the strength of the magnetic field of a solenoid?

A

Insert an iron core
Increase the current flowing through the wire
Coil the wire

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
26
Q

When trying to figure out how an electromagnetic device works, what should you do?

A

Look for a coil/solenoid - this is going to act as an electromagnet

Look for a piece of iron - this will be attracted to the solenoid

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
27
Q

Eg Electromagnetism in an electric bell

A

When the button K is pressed
Current passes through the electromagnet E creating a magnetic field
This attracts the iron armature A, causing hammer to strike bell B
Movement of the armature breaks the circuit at T
This stops current, destroying the magnetic field, and so the armature returns to its previous position
This re-establishes the circuit, and the whole process starts again

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
28
Q

Describe what happens in an electromagnetic bell?

A

When the switch is closed, the current passes through the wire
With the wire coiled around the iron core, this will create an electromagnet with a magnetic field
The soft iron armature will be attracted to the electromagnet
This will then break the circuit, which will stop electromagnet working and the iron armature will fall back
This will complete circuit and process can be repeated

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
29
Q

What is the motor effect?

A

When a conductor carrying a current is placed in a magnetic field, the magnet producing the field and the conductor exert a force on each other

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
30
Q

Fleming’s left hand rule?

A

A rule that gives the direction of the force on a current-carrying wire in a magnetic field according to the directions of the current and the field (think FBI?)
ThuMb Movement
Forefinger field (N to S)
SeCond finger Current (+ to -)
Diagram showing hand with 3 fingers (including thumb stuck out like a K)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
31
Q

The direction of the force can be reversed by (magnetic fields)….

A

Changing the direction of the current (connect the battery the other way round) or the magnetic field

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
32
Q

In what circumstance will no force act on a conductor carrying an electric current and in a magnetic field?

A

If the conductor is parallel to the magnetic field

If the lines of the magnetic force and path of electricity do not cross

33
Q

Why is a force exerted when a current passes through a wire in a magnetic field?

A

The current creates a magnetic field around wire

This field interacts with the external magnetic field

Exerting a force on the wire

In accordance with the LH rule

34
Q

The strength of the force can be increased by (magnetic fields)…

A

Increasing the current or using a stronger magnet

35
Q

The magnetic field and current must act at _________ _________ to each other

A

Right angles

36
Q

When can the equation to calculate the magnitude of a force acting on a wire be used?

A

For a conductor at right angles to a magnetic field and carrying a current

37
Q

Equation for calculating the magnitude of the force acting on a wire?

A

Force = magnetic flux density x current x length
F = B I I

Force = force (Newtons, N)
B = magnetic flux density (Tesla, T)
C = current (Amps, A)
L= length (metres, m)

38
Q

What is magnetic flux density?

A

The amount of magnetic flux (ie field lines) which pass through a surface OR the strength of the magnetic field

39
Q

A coil of wire carrying a ______ in a ______ field tends to ________. This is the basis of an electric __________

A

A coil of wire carrying a current in a magnetic field tends to rotate. This is the basis of an electric motor.

40
Q

DC Motor - explain

A

Consists of a coil in a uniform magnetic field
Split ring commutator swaps the contacts every half turn to keep the motor rotating in the same direction.

Diagram showing split ring commutator, electrical contacts touching split ring, +ve on S, -ve on N side, force up and down arrows, axis through middle

41
Q

What happens when a current passes through the coil ( motors)?

A

Current creates a magnetic field around coil
Which interacts with field of the magnets
Exerting a force on the coil
(In accordance with LH rule)
This pushes one side of the coil UP and the other side DOWN, causing it to spin

42
Q

What does the commutator do?

A

Reverses the direction of the current in the coil every half turn

This reverses the direction of the forces, which keeps coil spinning

43
Q

PPQ: in a simple electric motor, when there is a current in the coil, the coil rotates continuously. Explain why (4 marks)

A

The sides of the coil (parallel to the magnet) experience a force (in opposite directions)
The moments cause the coil to rotate clockwise/anticlockwise
Each half turn, the two halves of the commutator swap from one carbon brush to the other
And so it reverses the current in the coil

44
Q

What can be done to reverses the direction of a motor?

A

Reverse direction of current
Reverse direction of magnetic field (switch the poles)

45
Q

What can be done to increase the turning force of a motor?

A

Increase current in the coil
Increase strength of magnetic field
Add more turns to the coil
Add more wires

46
Q

Loudspeakers (and headphones) work in the opposite way to a microphone - how?

A

They turn electrical signals into sound, rather than turning sound into electrical signals

47
Q

Loudspeakers and headphones use the _______ effect to convert variations in current in _________ circuits to the pressure variations in __________ waves

A

Loudspeakers and headphones use the motor effect to convert variations in current in electrical circuits to the pressure variations in sound waves

48
Q

Explain how a moving-coil loud speaker works?

A

An ac passes through coil of the loud speaker
This creates a changing magnetic field around coil
which interacts with the field from the permanent magnet
Exerting a force on the coil, in accordance with LH rule
This makes the coil and speaker cone oscillate, which makes the air oscillate, creating sound waves.

Diagram with magnet, coil and speaker cone shown

49
Q

Electromagnetic induction is the ____________ of the motor effect - why?

A

Opposite

Instead of using electricity to create motion, motion is being used to create electricity

50
Q

If an electrical conductor moves relative to a magnetic field or if there is a change in the magnetic field around a conductor, a ________ ___________ is ___________ across the ends of the conductor. If the conductor is part of a complete circuit, a current is ____________ in the conductor. This is called the __________ effect.

A

Potential difference, induced, induced, generator

If an electrical conductor moves relative to a magnetic field or if there is a change in the magnetic field around a conductor, a potential difference is induced across the ends of the conductor. If the conductor is part of a complete circuit, a current is induced in the conductor. This is called the generator effect.

51
Q

An induced current generates a magnetic field that ________ the original change, either the movement of the conductor or the change in magnetic field.

A

Opposes

52
Q

When a conductor (eg a wire) is moved through a magnetic field….

A

The wire cuts through the field lines, inducing a potential difference in the wire.

53
Q

If a magnet is pushed into (or taken out of) a coil….

A

The field lines cut through the turns on the coil
This induces a potential difference in the coil

54
Q

The size of the induced potential difference/induced current can be increased by….

A

Turning the coil faster (faster motion)
Increasing the strength of magnetic field (stronger magnet)
Adding more turns to the coil
Use a larger area of coil

55
Q

The direction of the induced potential difference/induced current can be changed by…

A

Reversing direction in which wire/coil/magnet is moved

Turning the poles of the magnet around (so field is in opposite direction)

56
Q

Uses of generator effect?

A

Alternators and Dynamos

57
Q

PPQ: the figure shows a coil and a magnet. An ammeter is connected to the coil. The ammeter has a centre zero scale, so that values of current going in either direction through the coil can be measured. A teacher moves the magnet slowly towards the coil. Explain why there is a reading on the ammeter?

A

When moved towards coil, a magnetic field is created around the magnet
The magnetic field moves and cuts through the coil
This induces a pd across the coil
This forms a complete circuit
So a current is induced

58
Q

What are alternators?

A

Generates an alternating current (ac)
Doesn’t contain a split ring commutator
Because of this, direction of current reverses every half turn

Diagram with split rings and brushes, induced ac out, force and axis with S and N either side

59
Q

What happens in alternators?

A

As the coil/magnet spins, a current is induced in the coil. This current changes direction every half turn
Instead of a split ring commutator, they have split rings and brushes, so contacts dont swap every half turn
This means an alternating potential difference is produced

60
Q

Alternator Graph

A

T shaped graph (T on side), start at baseline, rising in curve and falling below line then rising again in a curve up to the baseline

61
Q

What is a dynamo?

A

a direct current (dc) generator
It’s the reverse of a dc motor
Direction of current does not change

62
Q

What happens in dynamos?

A

The split ring commutator swaps the connections every half rotation, so that the induced current is always in same direction (dc output)

If the coil is a complete circuit then a current is induced

63
Q

Dynamo graph

A

T shaped graph (T on side) current upright, time length
Line starts at baseline and shows 2 upward bounces, ending back at baseline

64
Q

Microphones use the ________ effect to convert the pressure variations in ________ waves into variations in _______ in electrical circuits.

A

Generator, sound, current

Microphones use the generator effect to convert the pressure variations in sound waves into variations in current in electrical circuits.

65
Q

How does a moving coil microphone work?

A

When sound waves reach the microphone, the pressure variations cause diaphragm to vibrate
This causes coil to move back and forth through the magnetic field
The coil cuts across the field lines of the magnet
This induces a potential difference in the coil

Diagram showing diaphragm, magnet, coil and sound waves

66
Q

In microphones, what does the movement of the coil (+ the generated current) depend on and why?

A

Depends on the properties of the sound wave

Louder sounds make the diaphragm move further

67
Q

What is a transformer?

A

An electrical device that can be used to increase or decrease the potential difference of an alternating current

68
Q

A basic transformer consists of…..

A

A primary and secondary coil wrapped around an iron core.

69
Q

Why is iron used in transformers?

A

It magnetises and demagnetises quickly

70
Q

Why do transformers only work with ac?

A

A changing magnetic field is necessary to induce ac in the secondary coil.

71
Q

What happens in transformers?

A

When ac is supplied to primary coil, a changing magnetic field is produced in the iron core.
The alternating magnetic field lines cut through secondary coil.
This induces an alternating pd across secondary coil.

72
Q

What is the role of transformers?

A

Increase the pd of electricity before it’s transmitted across national grid
Lower the high voltage electricity used in power lines to be used in homes
Used in adapters to lower mains voltage to be used by electronic devices.

73
Q

What is a step-up transformer?

A

Increases the alternating potential difference of a power source
Has more turns on the secondary coil than on the primary coils

Step up more turns secondary

74
Q

What is step-down transformer?

A

Decreases the alternating potential difference of a power source
Has more turns on the primary coil than on the secondary coil

Step down more turns primary

75
Q

The ratio of the potential differences across the primary and secondary coils of a transformer Vp and Vs depends on …..

A

The ratio of the number of turns on each coil, np and ns

76
Q

Transformer and power equation

A

Input power = output power

(Assumes the transformer is 100% efficient)

77
Q

Where is the transformer-power equation derived from?

A

P=IV

78
Q

Why is electricity transmitted at a high voltage?

A

When electricity is transmitted over large distances, current in the wire heats them, causing energy loss
By raising the voltage, the same amount of power can be transmitted using a similar current P=IV)
So less heat is produced in the wire and less energy loss

Diagram showing power station - step-up transformer, high voltage / low current - step-down transformer - home

Physics (9 decks)

Brainscape helps you reach your goals faster, through stronger study habits.
© 2024 Bold Learning Solutions. Terms and Conditions