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On the wild side > Magnetism > Flashcards

Magnetism Flashcards

1
Q

What does a magnet have around it?

A

a magnetic field

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2
Q

How can a magnetic field be created around a wire?

A

Pass a current through it

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3
Q

What is a magnetically hard material?

A

Steel - retains its magnetism once magnetised

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4
Q

What is a magnetically soft material?

A

Iron - loses its magnetism easily and is therefore a useful temporary magnet

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5
Q

How can magnetism be induced in a magnetic material?

A

leave it in a magnetic field

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6
Q

What is a magnetic field?

A

The volume of space around a magnet where magnetism can be detected

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7
Q

How can we show the shape of a magnetic field?

A

Using iron fillings or compasses which will align with the direction of the magnetic field

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8
Q

What happens when two magnets are placed near each other?

A

Their magnetic fields affect each other

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9
Q

What is electromagnetism?

A

A phenomenon where a magnetic field is created around a wire that has current passing through it

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10
Q

What is the shape of the field around a current carrying wire?

A

Circular

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11
Q

How can you increase the strength of a magnetic field around a current carrying wire?

A

increase the current in the wire

wrap the wire into a coil or a solenoid (a long coil)

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12
Q

What happens when the direction of current flowing through a solenoid is reversed?

A

Its poles are also reversed

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13
Q

How can you increase the strength of a field around a solenoid?

A

increase the current flowing through

increase the number of turns of the solenoid

wrap the solenoid around a magnetically soft core such as iron

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14
Q

What is an electromagnet?

A

the combination of soft iron core and solenoid

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15
Q

How does an electric bell work?

A

Bell push pressed; circuit complete and current flows

Soft core of electromagnet is magnetised and attracts the iron armature

Armature moves so hammer strikes bell

Gap created at contact screw

Circuit incomplete and current stops flowing

Electromagnet turned off so spring’s armature returns to original position

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16
Q

How does a circuit breaker work?

A

If current is too high the electromagnet is strong enough to pull the iron catch out of position so that the contacts open

Circuit breaks

But can be reset with reset button

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17
Q

What is the purpose of a relay switch?

A

In circuits with a large current there is a danger of receiving a large electric shock.

A relay switch uses a small current in one circuit to turn on a second that may have a much large current

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18
Q

How does a relay switch work?

A

Switch of small circuit closed so current flows

As current passes through coil C the iron core is magnetised and attracts the iron armature

The armature is pivoted so its lower end pushes the contacts of the second circuit together

Now the second circuit is complete and current flows.

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19
Q

How does a relay switch turn off the second circuit?

A

If switch S is opened the electromagnet is turned off

Iron armature moves back to its original position and contacts spring apart

Circuit off b/c current can no longer flow

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20
Q

What happens to a charged particle moving in a magnetic field?

A

It experiences a force

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21
Q

When will a charged particle moving in a magnetic field not experience a force?

A

If its motion is not parallel to the field

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22
Q

How to make a wire move?

A

Pass a current through a wire held at right angles to the mag.field of a magnet.

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23
Q

Why does a current carrying wire in a field experience a force?

A

The two magnetic field lines interact around the wire and the magnet.

In certain places, the fields are in the same direction and so reinforce each other giving a strong magnetic field.

Where the fields are in opposite directions weaker fields are produced.

The wire experiences a force pushing it from the strong to the weak field. MOTOR EFFECT

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24
Q

How can a stronger force be produced to move the wire?

A

stronger mag.field

increased current

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25
What happens if the current or direction of the mag.field is reversed?
The wire experiences a force in the opposite direction
26
How does a moving-coil loudspeaker use the MOTOR EFFECT?
Signals from amplifier fed into coil as a.c The coil now has a changing magnetic field that interact with those of the magnet creating varying forces on the wires of the coil. This makes the speaker cone vibrate The vibrations create the sounds waves we hear
27
How does a split ring help a motor work?
Lets the connections to the supply through the brushed swap over so current continues to flow in the same direction and the motor turns consistently
28
What is the effect of the arrangement of the brushes and the split ring?
Changes the direction of the current every half turn which means that the rotation can be continuous
29
How can you increase the rate at which the motor turns?
increase the number of turn of wire COIL increase the strength of mag.field increase the current flowing
30
How are practical motors different from lab ones? 1 - CURVING
Curved electromagnets which are capable of producing strong mag.fields
31
How are practical motors different from lab ones? 2 - COILS
Single loop is replaced with several coils of wire wrapped on the same axis; makes the motor powerful allows it to run more smoothly
32
How are practical motors different from lab ones? 3 - LAMINATED
Coils are wrapped on a laminated soft iron core to make motor more efficient and powerful
33
What do motors do vs generators?
Motors use electricity to produce movement....generators movement to produce electricity
34
How is the mains supply produced?
By large generators in power statioms
35
What is the generator effect?
When a wire is moved across a magnetic field at right angles, a voltage is induced in the wire.
36
How can current flow from a generator?
If the wire is part of a complete circuit
37
How can you increase the size of the induce voltage?
move the wire more quickly use a stronger magnet wrap the wire into a coil (move wires move in the mag.field)
38
What is the other version of the generator effect?
a voltage can be generated by pushing a magnet into a coil
39
How can you increase the the size of the induced voltage using the other version of the generator effect?
move the magnet more quickly use a stronger magnet using a coil with more turns using a coil with a large cross sectional area
40
What does the size of induced voltage depend on?
the rate at which the magnetic lines of flux are being cut
41
What is directly prop. relationship in electromagnetic induction?
The size induced voltage is dir.prop to the rate at which the flux lines are cut
42
What happens to the direction of induced current if motion direction is reversed?
it too, reverses
43
How does a dynamo work?
As the cyclist pedals the wheel rotates and a magnet in the dynamo spins around. As it turns its field lines cut through the surrounding coil inducing a current in it This current is used to work the cyclist's lights
44
Why is the induced voltage from a generator greater than that of a dynamo?
Uses: STRONGER MAGNETS MORE COILS OF WIRE SPINS COIL FASTER
45
With generators, the wire alternates consistently due to slip rings. What does this mean?
ALTERNATING CURRENT IS PRODUCED
46
What is generator than produces alternating current called?
ALTERNATOR
47
What is the frequency of mains supply?
50 Hz
48
How does a transformer work?
AC current passed through coil Its magnetic field is changing; as current increases field grows and visa versa If a second coil placed near first, changing mag.field of first coil interacts with the second's coil inducing and a.c. current Size and direction of current induced changes depending on current provided to first coil
49
What is the name for the combination of two magnetically linked coils?
TRANSFORMER
50
Equation for V and turns on coil?
input V/output V = turns on primary coil/turns on secondary coil
51
What is true if a transformer is 100% efficient?
p1 = p2 v1 x I1 = v2 x I2
52
What happens in the national grid as current is transported?
energy is lost in the form of heat
53
Why is energy lost?
The wires on pylons are long and have a high resistance
54
How are heat loses reduced?
By transporting at low I and high V
55
What happens to current after it is generated in alternators?
Step-up transformer to increase V and decrease I
56
Why is current transported high off ground?
High voltages in the cables is v.dangerous
57
What happens to current when it is going into homes?
STEP DOWN TRANSFORMER - increase current
58
How does current get to our homes?
via underground cables
59
Why is iron used in transformers?
it can magnetise and demagnetise 50 times/second because it is magnetically soft
60
At what angle does a current-carrying wire need to cut a magnetic field of a magnet to experience max.F?
90 degrees
61
What direction do we use for the motor effect for motion of electrons?
conventional current (+ to -)
62
How to we say the current is flowing in a motor?
from positive to negative
63
What is electromagnetic induction?
creation of a voltage (maybe current) in a wire experiencing a change in mag.field
64
What is the dynamo effect?
Using electromagnetic induction to transfer kinetic energy to electrical energy
65
How can you test if a current has been induced in the conductor?
Attach an ammeter to it; when moving it through the magnetic field the ammeter will shows the magnitude and direction of the induced current
66
In a generator what do faster revolutions generate?
Higher overall voltage and more peaks (higher frequency)
67
When will current be generated from a generator?
If the conductor is connected to a circuit
68
Why do step-down transformers reduce the voltage?
So that it is more useful and safer to use
69
At what V is electricity transported?
400 000V
70
At what voltage is electricity received at factories?
33000 V
71
At what voltage does electricity run at before entering a step up transformer?
25000 V
72
At what voltage is electricity supplied to our homes?
230V
73
What are the advantages of circuit breakers?
respond more rapidly to current surges than fuses do more reliable more sensitive can be reset (no need to replace)
74
How can a circuit breaker be made more sensitive and be made to respond quicker to current surges?
wrap more coils around the iron core to create a stronger electromagnet that can attract the iron armature quicker and with more force
75
How does a loudspeaker work in terms of a moving coil?
Wire from amplifier carries ac which makes the coil move backwards and forwards at the same frequency as the changing current. The coils mag.field and that of the magnet interact producing a force which is alternating due to the a.c current and thus changing mag.field of the amplifier wire. The paper cone is then forced to move which creates sound.
76
Why is current reduced when transported electricity along pylons?
P = VI The heating effect of a high current would cause too much heat loss so current is reduced and if P is constant, then V must increase
77
How is an electromagnet created?
Wrap wire around an iron core. When current is passed through the wire, a mag.field is created. This mag.field means that the wire magnetizes the iron core strongly.
78
How will a compass be positioned around a current carrying wire?
Compass always points perpendicular to the wire (flow of current)
79
Advantages of an electromagnet?
Can be made stronger Field strength can be varied Can be turned on/off
80
What would happen to the motor if the split ring didn't have a split?
The motor wouldn't spin as the current would short circuit across the ring and back to the power supply.
81
What is the purpose of brush contacts in a motor?
Made of carbon; soft and good electricity conductor Reduced friction between it and split ring commutator.
82
In an experiment checking the strength of an electromagnet how can you increase its strength?
use a larger battery with a greater voltage add another battery in series thicker iron nail more coils of wire
83
How can you measure the strength of an electromagnet?
how much compass deviates mass of iron filings picked up no of paperclips it can pick up
84
What might have caused a systematic error that made the strength of the electromagnet to start above 0?
Experiment already carried out before and the iron still slightly magnetized She did not zero the balance when measuring the mass of iron filings picked up
85
What would happen if a motor didn't have a split ring?
It would rotate to vertical position and vibrate until it came to rest
86
What does the split ring do?
Changes the direction of current flowing through the coil after every half turn by swapping the contacts at the DC supply
87
What happens when a magnet moving in a conductor stops moving?
There is no induced voltage and no current flows b/c the flux lines are no longer being cut
88
Why is a N pole created at the end of a coil when the N pole of a magnet is brought near?
It generated a mag.field to try and oppose that that is induced by the current in order to oppose the motion of the bar magnet's mag.field

On the wild side (35 decks)

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