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chapter 15 Flashcards

1
Q

what is a magnetic compass?

A

a tiny magnetic needle pivoted at its centre

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2
Q

what are the two ends of a magnet?

A

one end of the compass always points north (north-seeking pole) and the other end always points south (south-seeking pole)

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3
Q

what happens if you put two of the same poles together?

A

they repel

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4
Q

what happens if you put opposite poles together?

A

they attract

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5
Q

what kind of specific materials can be magnetised and _______?

A

demagnetised (if already magnetised)
iron, steel, cobalt, nickel

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6
Q

what are permanent magnets made from, and why?

A

steel
because magnetised steel does not easily loose its magnetism

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7
Q

what is a magnetic field?

A

the space around a magnet or a current-carrying wire

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8
Q

what is the direction of a line of force?

A

from north pole of a magnet to south pole

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9
Q

what happens if an object that is attracted is further from the magnet?

A

the further the attracted object is from the magnet, the less effect the magnet has on the object, this is because of the greater distance between, the weaker the strength of the magnetic field

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10
Q

how do you magnetise an unmagnetised material?

A

place it into a magnetic field, the magnetic field is said to induce magnetism in the material.
e.g. an unmagnetised iron rod is placed in line with a bar magnet becomes a magnet with poles at each end, the nearest poles of the rod and the bar magnet always have opposite polarity

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11
Q

what does induced magnetism cause?

A

a force of attraction between any unmagnetised magnetic material placed near one end of a bar magnet. the force is always an attractive force whichever end of the bar magnet is nearest to the material.

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12
Q

what happens when an electric current passes along a wire?

A

a magnetic field is set up along the wire

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13
Q

how can you find the pattern of a magnetic field along a long straight wire?

A

using a plotting compass, the lines of force caused by a straight current-carrying wire are a series of concentric circles, these circles are centred on the wire in a plane that is perpendicular to the wire. To eliminate magnetism caused by nearby iron objects, use a wooden stand to support the cardboard sheet so its horizontal.

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14
Q

what is the corkscrew rule?

A

The corkscrew rule helps visualize current and magnetic field directions.

Turn a corkscrew to mimic current flow.
The screw’s motion shows the magnetic field direction.
It links rotation (current) to linear movement (field lines).

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15
Q

what is a solenoid?

A

long coil of insulated wire, used in lots of devices where a strong magnetic field needs to be produced. The magnetic field is produced in and around the solenoid

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16
Q

what happens to the magnetic field when a currents passes through a wire?

A

increases the strength if the current is increased
reverses its direction if the current is reversed

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17
Q

what happens inside a solenoid?

A

magnetic field is much stronger than if the wire was straight, field lines parallel to axis of solenoid & all in the same direction (uniform).

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18
Q

what happens outside a solenoid?

A

magnetic field lines bend around from one end of the solenoid to the other end, magnetic field lines outside like the field of a bar magnet, except each field line is a full loop as it passes through inside of solenoid

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19
Q

how do you find the polarity of each end of a solenoid?

A

s-pole is the end where the current is clockwise, n-pole is end where current is anticlockwise

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20
Q

what is an electromagnet?

A

a solenoid in which the insulated wire is wrapped around an iron bar (core). when a current is passed along a wire, a magnetic field is created around the wire, because of this, the magnetic field of the wire magnetises the iron bar when current is switched off-bar looses most of its magnetism.

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21
Q

how are electromagnets used in scrapyard cranes?

A

scrap vehicles lifted in scrap yard using powerful electromagnets attached to cranes, steel frame of vehicles sticks to the electromagnet when current passes through the coil of electromagnet. when switched off the frame of the vehicle falls off it.

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22
Q

how are electromagnets used in circuit breakers?

A

circuit breaker is a switch in series with an electromagnet, the switch is held closed by spring, when current is too big switch is pulled open by the electromagnet and stays open until manually reset.

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23
Q

how are electromagnets used in electric bells?

A

when an elsectric bell connected to battery, iron armature is pulled onto elctromagnet. this opens the make-and-break switch, electromagnet is switched off. therefore the armature springs back and the make-and-break switch recloses so cycle repeats.

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24
Q

how are electromagnets used in relays?

A

relay used to switch an elctrical machine on or off, small current through coil of electromagnet magnetises the iron core, then pulling the armature onto electromagnet. this closes the switch gap and switches machine on. in this way a small current in coil switches on a machine with a much bigger current.

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25
what are examples of uses of electric motors?
hairdryers, electric shaver, refrigerator pump, computer hard drive
26
how does an electrical motor work?
a force can act on a wire (or any other conductor) in a magnetic field when a current is passed throught the wire, this is called motor effect
27
how do you increase force size?
increase current, use stronger magnet
28
what does the size of force depend on? describe the force?
the angle between the wire and the magnetic field lines the force is: greatest when the wire is perpendicular to the magnetic field zero when the wire is parallel to the magnetic field lines
29
what is the direction of the force and how can you tell?
the direction of the force is always at right angles to the wire and the field lines, also the direction of the force is reversed if either the direction of the current or magnetic field lines is reversed. use FLEMING'S LEFT-HAND RULE, which teells you how these directions are related to eachother
30
how do you do fleming's left hand rule?
First finger = Field seCond finger = Current thuMb = Movement hold fingers at right angles to eachother, use this to work out direction of the force (movement) of the wire.
31
what is magnetic flux density?
magnetic flux density of a magnet is the measure of the strength of the magnetic field. the symbol B is used for magnetic flux density and the unit is tesla (T)
32
in an experiment used to investigate the motor effect the size of the force on the conductor depends on what?
the current in the conductor, the length of the conductor, the magnetic flux density of the magnetic field
33
what does an electrical motor use?
the motor effect
34
how do you control the speed of an electrical motor?
change the current, also you can reverse the direction the motor turns in by reversing the current
35
explain a simple motor...
has a rectangular coil of insulated wire (the armature coil) that is forced to rotate, the coil is connected to the battery by two metal graphite brushes, the brushes press onto a metal split-ring commutator fixed to the coil, graphite is a form of carbon that conducts electricity and is slippery, causes little friction when in contact with rotating commutator.
36
when a current is passed through a coil, why does it spin?
a force acts on each side of the coil due to motor effect, the force on one side is in the opposite direction to the force on the other side.
37
what does the split-ring commutator do?
reverses the current around the coil every half-turn of the coil. because the sides swap over each half-turn, the coil is pushed in the same direction every half-turn.
38
why does a hospital have its own electricity generator?
it is always on stand by in case of a power cut, patients lives would be put at risk if the mains electricity failed and there was no generator.
39
a generator contains coil of wire that...
spin in a magnetic field
40
what is electromagnetic induction
a potential difference, or voltage, is created, or induced, across the ends of the wire when it crosses through the magnetic field lines, this happens when any conductor crosses through the magnetic field lines.
41
what is the generator effect?
if the conductor is part of a complete circuit, the induced potential difference makes an electric current pass around the circuit
42
what is a generator test?
a coild of insulated wire is connected to a centre-reading ammeter, when one end of a bar magnet is pushed into the coil the ammeter pointer deflects. This is because: the movement of the bar magnet causes an induced potential difference in the coil. the induced potential difference causes a current because coil is part of complete circuit.
43
what happens if a stronger magnet is used in a generator test?
stronger magnet used = both the induced potential difference and current will be bigger
44
what happens if the bar magnet in a generator test is withdrawn from the coil?
the ammeter pointer deflects in the opposite direction. This is because the induced potential difference acts in the opposite direction, so the induced current is in the opposite direction
45
what happens when an induced current generates a magentic field?
it generates a magnetic field in and around the coil but only when the magnet is moving, this induce magnetic field always opposes the original change. so work is done by the person moving the magnet
46
what is the electricity generated in an induced current?
the electricity generated is the result of the work done by the person moving the magnet
47
how do you work out the direction of the induced current?
using the solenoid rule to see if the end A is like the north or south pole of the bar magnet
48
what is an alternator?
an alternating-current generator
49
what is an alternator made up of?
a rectangular coil that is forced to spin in a uniform magnetic field. The coil is connected to a centre-reading meter by metal brushes that press on two metal slip rings. The slip rings and brushed give a continuous connection between the coil and meter.
50
What happens in an alternator when the coil turns steadily in one direction?
the meter pointer deflects first one way, then the opposite way, and then back again. This carries on as long as the coil keeps turning in the same direction.
51
when is the size of induced potential greatest?
when the plane of the coil is parallel to the direction of the magnetic field. At this position, the sides of the coil cross directly though the magnetic field lines so the induced potential difference is at its peak value.
52
The faster the coil rotates in an alternator:
the bigger the frequency of the alternating current. because each full cycle of alternating potential difference takes the same time as one full rotation of the coil the bigger the peak value of alternating current. This is because sides of coil move faster so cross through field lines at a faster rate.
52
when is the size of induced potential zero?
when the plane of the coil is perpendicular to the magnetic field lines. At this position the sides of the coil move parallel to the field lines and DO NOT cross through them.
53
how can you also increase peak value?
by using a magnet with a stronger magnetic field and using a coil with a bigger area and with more turns of wire on.
54
how can you show alternating potential difference?
displayed on an oscilloscope screen. If generator is rotated faster, the screen display will show more and taller waves on the screen
55
what is a dynamo?
a direct current generator. A simple dynamo is the same as an alternator just the dynamo has a split-ring commutator instead of two seperate slip rings
56
what happens as the coil spins in a dynamo?
the split-ring commutator reconnects the coil the opposite way aroujnd in the circuit every half-turn this happens every time the coil is perpendicular to the magnetic field lines.
57
how is induced potential different in a dynamo to an alternator?
induced potential difference does not reverse it direction in the dynamo unlike the alternator. it varies from zero to a max value twice each cycle and never changes polarity
58
what is a moving coil microphone?
generates an alternating potential difference as sound waves make the coil vibrate. coil is attatched to small diaphram and is between the poles of a cylindrical magnet. the pressure vibrations of sound waves on diaphram make it vibrate so coil vibrates magnetic field.
59
what is the same in a moving coil microphone?
alternating potential difference induced in the coil has the same frequency as the sound waves
60
what does the moving coil of a loud speaker create?
sound waves when an alternating potential difference is applied to its coil. The coil is in a magenetic field the current in the coil causes force on coil (motor effect) because current alternates force is repeatedly reversed direction and makes the coil and diaphram vibrate this creates sound waves of the same frequency as alternating potential difference
61
What is a transformer?
electrical device used to change an alternating voltage.
62
what is a step up transformer
used to increase the size of alternating potential difference
63
what is a step down transformer
used to decrease the size of alternating potential difference
64
how could a transformer be used for daily use?
to charge a low-voltage device like laptop or phone
65
how does a transformer work?
two coils of insulated wire both wound around the same iron core . Iron is used for the core as it easily magnetised and demagnetised. coils are called primary (connected to a.c supply) and secondary coil. When alternating current passes through primary an alternating potential difference is induced in the secondary.this means a bulb can be connected on the secondary and light up.
66
what happens when an alternating current is passed through the primary coil?
alternating magnetic field in the iron core is generated
67
what passes through the secondary coil
the lines of alternating magnetic field
68
what does the magnetic field in the secondary coil induce?
an alternating potential difference between terminals of the secondary coil
69
how do you make a model of a transformer?
wrap a coil of insulated wire around iron core of a model transformer as the primary coil. connect coil to 1 V a.c. supply then connect a second length of insulated wire to a 1.5v torch bulb. when you wrap enough turns of the second wire around the bulb should light up. test if cores with different materials affect transformer
70
what is a practical transformer?
primary and secondary coils are both wrapped around the same part of the iron core
71
whats the difference between coils in step-up and step-down transformers?
step-up=more turns in secondary coil than primary step-down=less turns in secondary than primary
72
are transformers efficient?
almost 100% efficient, when device is connected to secondary coil almost all electrical power supplied to transformer is delivered to the device
73
what does a step-up transformer do in the national grid?
electrical power supplied to any appliance depends on its current and its potential difference. To suppley a specific amount of power the current is lowered if potential difference is raised
74
what is the heating effect?
in a resistor of a wire. is proportional to the square of the current, by increasing grid potential difference current through grid cables is reduced so heating effect is smaller and less power is wasted

Physics (4 decks)

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