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Physics > Magnetism and Electromagnetism > Flashcards

Magnetism and Electromagnetism Flashcards

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AP® Physics flashcards Physics 101 flashcards
1
Q

features of magnets

A

north and south poles
same poles repel
opposite poles attract

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2
Q

what are the poles of a magnet

A

where the magnetic forces are strongest

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3
Q

what happens when two magnets are brought close together

A

they exert a force on each other

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4
Q

features of a permanent magnet

A

always magnetic
always have poles

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5
Q

features of induced magnets

A

materials that are magnetic but don’t have fixed poles
made into temporary magnets by stroking them with a permanent magnet

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6
Q

how does stroking an induced magnet with a permanent magnet work

A

aligns the domains in the material in all the same direction creating a temporary magnet

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7
Q

examples of induced magnets

A

iron
nickel
cobalt

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8
Q

which direction do field lines go

A

north to south

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9
Q

what happens to strength of the magnet with distance

A

strength decreases with distance from the magnet

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10
Q

what happens when induced magnets are removed from the magnetic field

A

it loses all or most of its magnetism quickly

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11
Q

what is a magnetic field

A

the region around a magnet where a force acts on another magnet or on a magnetic field

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12
Q

what is the direction f the magnetic field at any point given by

A

the direction of the force that would act on another north pole placed at that point

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13
Q

what does a magnetic compass contain

A

a small bar magnet

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14
Q

how to map out magnetic fields using a plotting compass

A
  1. place the plotting compass near the magnet on a piece of paper
  2. mark the direction the compass needle points
  3. move the plotting compass to many different positions in the magnetic field, marking the needle direction each time
  4. join the points to show the field lines
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15
Q

where does the needle of a plotting compass point

A

to the south pole of the magnet

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16
Q

how does the behaviour of a compass show the earth has a magnetic field

A

the earths core is made of iron and nickel and produces a magnetic field

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17
Q

what happens when a current flows through a conducting wire

A

a magnetic field is produced around the wire

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18
Q

what does the strength of the magnetic field produced by a current depend on

A

the current through the wire and the distance from the wire

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19
Q

what does a solenoid do

A

increases the strength of the magnetic field created by a current through the wire

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20
Q

how is the direction of a current shown

A

the right hand grip rule

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21
Q

how does the right hand grip rule work

A

by gripping the object the fingers give the direction of the field and the thumb points in the direction of the current

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22
Q

features of a magnetic field around bar magnet drawing

A

the magnetic field lines never cross eachother
the closer the lines the stronger the magnetic field
the lines have arrow heads to show the direction of the force exerted by the magnetic north pole
the arrowheads point from the north pole of the magnet to its south pole

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23
Q

how to check if a magnet is a permanent magnet

A

it repels another magnet

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24
Q

how can you show the direction of a current

A

right hand grip rule
plotting compasses on a piece of paper when a wire is pierced

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25
Q

what is a solenoid

A

wire coiled into a spiral

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26
Q

why do solenoids enhance the magnetic effect

A

makes magnetic field similar to field of a bar magnet
strong and uniform field
small magnetic fields caused by the current in each coil add together to make a stronger overall magnetic field

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27
Q

what does the iron core in an electromagnet do

A

increases its magnetic field strength

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28
Q

what is an electromagnet

A

a solenoid with an iron core

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29
Q

which factors of a solenoid affect strength

A

size of current
length
cross sectional area
number of turns (coils)
using a soft iron core

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30
Q

what is the motor effect

A

a wire carrying a current creates a magnetic field
which can interact with another magnetic field
causing a force that pushes the wire at right angles

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31
Q

when does the force on a given length of wire in a magnetic field increase

A

the current in the wire increases
the strength of the magnetic field increases

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32
Q

when is the force greatest on a given length of wire

A

when the direction of the current is 90 degrees to the direction of the magnetic field

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33
Q

when is there no motor effect force

A

if the current and magnetic field are parallel to eachother

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34
Q

how can the direction of a motor effect force be found

A

flemings left hand rule

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35
Q

how to use flemings left hand rule

A

hold your thumb, forefinger and second finger at right angles to eachother
the forefinger is lined up with the magnetic field lines pointing north to south
the second finger is lined up with the current pointing from positive to negative
the thumb shows the direction of the motor effect force on the conductor carrying the current

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36
Q

which effect makes an electric motor

A

a coil of wire carrying a current in a magnetic field experiencing a force that makes it rotate

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37
Q

what causes the coil in a dc motor to rotate anticlockwise

A

current in the left hand part of the coil causes a downward force and current in the right hand part of the coil causes an upward force

38
Q

what happens when the coil in a dc motor is vertical

A

it moves parallel to the magnetic field producing no force

39
Q

why does the motor not stop when the coil is vertical

A

the momentum of the motor carries it on round a little
a split ring commutator changes the current direction every half turn

40
Q

what happens once the conducting brushes reconnect with the commutator after a half turn

A

current flows in the opposite direction through the wire in the coil
each side of the coil is now near the magnetic pole

41
Q

equation for a conductor at right angles to a magnetic field and carrying a current

A

force = magnetic flux density x current x length

42
Q

what is magnetic flux density measured in

A

tesla (T)

43
Q

how do loudspeakers and headphones use the motor effect

A

to convert variations in current in electrical circuits to the pressure variations in sound waves

44
Q

how do loudspeakers create sound waves

A
  • a current in the coil creates an electromagnetic field
    -the electromagnetic field interacts with the permanent magnet generating a force, which pushes the cone outwards
    -the current is made to flow in the opposite direction
  • the direction of the electromagnetic field reverses
    -the force on the cone now pulls it back in
    -repeatedly alternating the current direction makes the cone vibrate in and out
    -the cone vibrations cause pressure variations in the air which are sound waves
45
Q

what is needed for a loudspeaker cone to vibrate correctly

A

the electric current must vary in the same way as the desired sound

46
Q

what do headphones contain

A

small loudspeakers

47
Q

how does electromagnetic induction happen

A

a coil of wire is moved in a magnetic field
a magnet is moved in a coil of wire
as a potential difference is needed to make electric current flow in a circuit

48
Q

what is electromagnetic induction also referred to as

A

the generator effect

49
Q

what are the results of the induced voltage

A

an induced current is produced if the conductor is connected in a complete circuit
the induced current creates a magnetic field around itself that opposes the original charge

50
Q

example of a magnet moving into a coil of wire

A

induced magnetic field repels the magnet back out of the coil

51
Q

what does the direction of the induced current depend on

A

the direction of movement of the magnet relative to the coil

52
Q

when is the current reversed in the generator effect

A

the magnet is moved out of the coil
the other pole of the magnet is moved into the coil

53
Q

ammeter reading of a magnet moving into a coil of wire

A

positive

54
Q

ammeter reading if a magnet is stationary in the coil of wire

A

no current flow

55
Q

ammeter reading when a magnet moves out of the coil of wire

A

negative current flow

56
Q

what factors increase the induced potential difference or current

A

speed of movement increased
magnetic field strength increased
the number of turns on the coil is increased

57
Q

what is the generator effect used in

A

an alternator to generate ac
a dynamo to generate dc

58
Q

what is an ac generator

A

a device that produces a potential difference

59
Q

why do cars use an alternator

A

to keep the battery charged
to run the electrical system while the engine is working

60
Q

features of a simple alternator

A

the coil is rotated in the magnetic field
current is induced in the rotating coil
slip rings connected to the coil
brushes make continuous contact between the external circuit and the slip rings
current flows in external circuit

61
Q

what do slip rings do in an alternator

A

maintain constant contact with the same sides of the coil

62
Q

what happens as one side of the coil moves up through the magnetic field in an alternator

A

a potential difference is induced in one direction

63
Q

how does the alternator cause an ac

A

as one side of the coil moves up through the magnetic field a potential difference is induced in one direction
as the rotation continues and that side of the coil moves down the induced potential difference reverses direction
this means the current is constantly changing

64
Q

how can the output of an alternator be represented

A

potential difference time graph
voltage on y axis
time on x axis

65
Q

what does the graph of an alternator look like

A

alternating sine curve

66
Q

how can the maximum potential difference or current be increase in an alternator

A

increasing the rate of rotation
increasing the strength of the magnetic field
increasing the number of turns on the coil

67
Q

how is a dc generator different to an ac generator

A

it uses a split ring commutator rather than two slip rings

68
Q

how do bike lights use dc generators

A

use a dynamo
to run the lamps while the wheels are turning

69
Q

features of a dynamo in a bike

A

wheel of dynamo rubs against tyre to make magnet turn
magnet is surrounded by coils of wire and wire is connected to lamp

70
Q

how does the current flow in only one direction in a dynamo

A

a split ring commutator changes the coil connections every half turn
as the induced potential difference is about to change direction the connections are reversed

71
Q

how can the output of a rotating dynamo be shown

A

potential difference time graph

72
Q

what does the graph of a dynamo look like

A

a sine curve that stays in the same direction all the time

73
Q

how can the maximum potential difference or current in a dynamo be increased

A

increasing the rate of rotation
increasing the strength of the magnetic field
increasing the number of turns on the coil

74
Q

what is a microphone

A

a device that converts sound waves into electrical signals

75
Q

how does a microphone use the generator effect

A

to induce a changing current from the pressure variations of sound waves

76
Q

moving coil microphone process

A

-pressure variations in sound waves cause the flexible diaphragm to vibrate
-the vibrations of the diaphragm cause vibrations in the coil
-the coil moves relative to a permanent magnet so a potential difference is induced in the coil
-the coil is part of a complete circuit so the induced potential difference causes a current to flow around the circuit
-the changing size and direction of the induced current matches the vibrations of the coil
-the electrical signals generated match the pressure variations in the sound waves

77
Q

what is a transformer

A

a device that can change the potential difference or voltage of an alternating current

78
Q

what does a step up transformer do

A

increases the voltage

79
Q

what does a step down transformer do

A

reduces the voltage

80
Q

what is a basic transformer made of

A

primary coil from the ac input
secondary coil leading to ac output
wound around an iron core

81
Q

why are the coils wrapped around an iron core

A

it is easily magnetised and an carry magnetic fields from the primary coil to the secondary coil

82
Q

how does a transformer work

A

a primary voltage drives an ac through the primary coil
the primary coil current produces a magnetic field which changes as the current changes
the iron core increases the strength of the magnetic field
the changing magnetic field induces a changing pd in the secondary coil
the induced potential difference produces an alternating current in the external circuit

83
Q

what do transformers need to work

A

an alternating current

84
Q

ratio of voltage to no of turns on a transformer

A

primary voltage / secondary voltage

number of turns on p coil/number of turns on s coil

85
Q

electrical power equation

A

power = potential difference x current

86
Q

power output from transformer equation

A

potential difference across primary coil x current in primary coil

potential difference across secondary coil x current in secondary coil

87
Q

what does the national grid do

A

carries electricity around britain

88
Q

what does a higher current in a cable mean

A

the greater the energy transferred to the surroundings by heating
so high currents waste more energy than low currents

89
Q

how does the national grid reduce energy transfers to the environment

A

step up transformers are used to increase voltage to power stations to thousands of volts
lowering the current in the transmission cables

90
Q

how are step down transformers used in the national grid

A

to decrease the voltage from the transmission cables so it is safer to distribute to homes and factories

Physics (19 decks)

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