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Physics > Chapter 13 - Electromagnetic > Flashcards

Chapter 13 - Electromagnetic Flashcards

(78 cards)

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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

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2
Q

Attraction and repulsion between 2 magnetic poles are examples of?

A

Non contact force

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3
Q

A permanent magnet….

A

Produces it’s own magnetic field

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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

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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

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6
Q

What materials are magnetic?

A

Iron, cobalt, nickel, steel (an alloy)

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7
Q

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

A

Attraction

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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

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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

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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

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11
Q

Mnemonic for direction of magnetic field

A

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

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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

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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

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14
Q

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

A

Sprinkle iron fillings near a magnet

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15
Q

Features of electromagnets

A

They can be switched on and off

Their strength can be varied

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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

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17
Q

The strength of the magnetic field depends on….

A

The current through the wire and the distance from the wire

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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

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19
Q

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

A

Increases

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20
Q

What is a solenoid?

A

A tight coil of wire

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21
Q

The magnetic field inside a solenoid is ______ and ________

A

Strong and uniform

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22
Q

An electromagnet is…..

A

A solenoid with an iron core

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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

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24
Q

What is a uniform field?

A

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

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25
How can we increase the strength of the magnetic field of a solenoid?
Insert an iron core Increase the current flowing through the wire Coil the wire
26
When trying to figure out how an electromagnetic device works, what should you do?
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
27
Eg Electromagnetism in an electric bell
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
28
Describe what happens in an electromagnetic bell?
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
29
What is the motor effect?
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
30
Fleming’s left hand rule?
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)
31
The direction of the force can be reversed by (magnetic fields)….
Changing the direction of the current (connect the battery the other way round) or the magnetic field
32
In what circumstance will no force act on a conductor carrying an electric current and in a magnetic field?
If the conductor is parallel to the magnetic field If the lines of the magnetic force and path of electricity do not cross
33
Why is a force exerted when a current passes through a wire in a magnetic field?
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
The strength of the force can be increased by (magnetic fields)…
Increasing the current or using a stronger magnet
35
The magnetic field and current must act at _________ _________ to each other
Right angles
36
When can the equation to calculate the magnitude of a force acting on a wire be used?
For a conductor at right angles to a magnetic field and carrying a current
37
Equation for calculating the magnitude of the force acting on a wire?
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
What is magnetic flux density?
The amount of magnetic flux (ie field lines) which pass through a surface OR the strength of the magnetic field
39
A coil of wire carrying a ______ in a ______ field tends to ________. This is the basis of an electric __________
A coil of wire carrying a current in a magnetic field tends to rotate. This is the basis of an electric motor.
40
DC Motor - explain
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
What happens when a current passes through the coil ( motors)?
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
What does the commutator do?
Reverses the direction of the current in the coil every half turn This reverses the direction of the forces, which keeps coil spinning
43
PPQ: in a simple electric motor, when there is a current in the coil, the coil rotates continuously. Explain why (4 marks)
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
What can be done to reverses the direction of a motor?
Reverse direction of current Reverse direction of magnetic field (switch the poles)
45
What can be done to increase the turning force of a motor?
Increase current in the coil Increase strength of magnetic field Add more turns to the coil Add more wires
46
Loudspeakers (and headphones) work in the opposite way to a microphone - how?
They turn electrical signals into sound, rather than turning sound into electrical signals
47
Loudspeakers and headphones use the _______ effect to convert variations in current in _________ circuits to the pressure variations in __________ waves
Loudspeakers and headphones use the motor effect to convert variations in current in electrical circuits to the pressure variations in sound waves
48
Explain how a moving-coil loud speaker works?
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
Electromagnetic induction is the ____________ of the motor effect - why?
Opposite Instead of using electricity to create motion, motion is being used to create electricity
50
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.
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
An induced current generates a magnetic field that ________ the original change, either the movement of the conductor or the change in magnetic field.
Opposes
52
When a conductor (eg a wire) is moved through a magnetic field….
The wire cuts through the field lines, inducing a potential difference in the wire.
53
If a magnet is pushed into (or taken out of) a coil….
The field lines cut through the turns on the coil This induces a potential difference in the coil
54
The size of the induced potential difference/induced current can be increased by….
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
The direction of the induced potential difference/induced current can be changed by…
Reversing direction in which wire/coil/magnet is moved Turning the poles of the magnet around (so field is in opposite direction)
56
Uses of generator effect?
Alternators and Dynamos
57
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?
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
What are alternators?
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
What happens in alternators?
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
Alternator Graph
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
What is a dynamo?
a direct current (dc) generator It’s the reverse of a dc motor Direction of current does not change
62
What happens in dynamos?
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
Dynamo graph
T shaped graph (T on side) current upright, time length Line starts at baseline and shows 2 upward bounces, ending back at baseline
64
Microphones use the ________ effect to convert the pressure variations in ________ waves into variations in _______ in electrical circuits.
Generator, sound, current Microphones use the generator effect to convert the pressure variations in sound waves into variations in current in electrical circuits.
65
How does a moving coil microphone work?
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
In microphones, what does the movement of the coil (+ the generated current) depend on and why?
Depends on the properties of the sound wave Louder sounds make the diaphragm move further
67
What is a transformer?
An electrical device that can be used to increase or decrease the potential difference of an alternating current
68
A basic transformer consists of…..
A primary and secondary coil wrapped around an iron core.
69
Why is iron used in transformers?
It magnetises and demagnetises quickly
70
Why do transformers only work with ac?
A changing magnetic field is necessary to induce ac in the secondary coil.
71
What happens in transformers?
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
What is the role of transformers?
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
What is a step-up transformer?
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
What is step-down transformer?
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
The ratio of the potential differences across the primary and secondary coils of a transformer Vp and Vs depends on …..
The ratio of the number of turns on each coil, np and ns
76
Transformer and power equation
Input power = output power (Assumes the transformer is 100% efficient)
77
Where is the transformer-power equation derived from?
P=IV
78
Why is electricity transmitted at a high voltage?
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

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