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PHYSICS > CHAPTER 15 > Flashcards

CHAPTER 15 Flashcards

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AP® Physics flashcards Physics 101 flashcards
1
Q

general rule for magnetism

A

like poles repel

opposite poles attract

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2
Q

what are permanent magnets made from and why

A

steel

it doesn’t loose its magnetism easily

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3
Q

what is a magnetic field

A

the region around the magnet

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4
Q

why are magnetic field lines concentrated at the poles

A

the magnetic fields are strongest and the poles

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5
Q

where will the line of direction pf a force from a magnet always go

A

from north to south

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6
Q

why does a magnet have a smaller effect on a compass when it is further away

A

the magnetic force is weaker

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7
Q

what is induced magnetism

A

when an unmagnetised material becomes magnetised by placing it in a magnetic field

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8
Q

what happens when a induced magnet is held near a bar magnet

A

there is always an attractive force

no matter what end of the magnet is

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9
Q

what is a solenoid

A

a long coil of insulated wire

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10
Q

what happens to the magnetic field in the solenoid when the current is increased

A

the strength of the magnetism increases

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11
Q

what happens to the magnetic field in a solenoid when the current is reversed

A

the direction of the magnetic field reverses

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12
Q

what is an electromagnet and how is one formed q

A

a solenoid is wrapped around an iron bat which acts as the core when the current is switched on it travels along the solenoid and creates magnetic field around the wire

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13
Q

what is the magnetic field of a solenoid like

A

the magnetic field of a bar magnet but this one is a complete loop

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14
Q

what is the magnetic field like inside a solenoid

A

strong and uniform

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15
Q

which direction do the field lines of a solenoid go in and what are they like in comparison to the axis

A

parallel to axis all in same direction

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16
Q

what is a magnetic field

A

a region surrounding a magnet where other magnetic objects experience a force

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17
Q

what do compasses always point north

A

the earth generates its own magnetic field

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18
Q

how does a compass work

A

there is a tiny magnet inside the North Pole is attracted to the South Pole of any other magnet
the compass points in the direction of the magnetic field

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19
Q

what is the difference between a permanent and induced magnet

A

a permanent magnet generates its own magnetic field

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20
Q

what do the magnetic field lines of a wire look like

A

concentric circle perpendicular to the wire

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21
Q

how does the right hand thumb work

A

point the thumb in the direction of the current flowing

curl fingers direction of fingers is direction of field

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22
Q

what is an electromagnet

A

a solenoid wrapped around in an iron core which can be turned on and off

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23
Q

how do electromagnetic work in a crane

A

when current passes through the wire it becomes magnetised
it sticks to the metal to pick it up
when the current is switched off it drops

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24
Q

4 examples of electromagnets

A

electric bell
circuit breaker
crane
relay

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25
Q

how do electromagnets work in an electric bell

A

when the switch is pressed the circuit is complete
current flows
this magnetises the iron core attracting the iron
the arm hits the bell and makes it ring
the make and break switch is opened and the current and so the electromagnet is switched off
the armature swings back the make and break switch closes and the whole cycle is ready to repeat itself

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26
Q

how does an electromagnet work in replay

A

when the circuit is complete
current flows round the coil and the iron is magnetised
this pulls the armature onto the magnet
this action makes the pivot swing and hit the poles
this closes the gap between poles and the circuit is complete so the current flows

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27
Q

what is the motor effect

A

a current passes through a wire in an electric field and it moves

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28
Q

how can the size of the force between a wire and magnet be increased

A

increasing the current using a stronger magnet

29
Q

what happens to the size of the force between a wire and magnet when the wire is perpendicular to the field

A

it is greatest

30
Q

what happens to the size of the force between a wire and magnet when the wire is parallel to the field lines

A

it is 0

31
Q

what does flemings left hand rule go

A

shows how current motion and field interact

32
Q

what does the first finger stand for in FLHR

A

magnetic field

33
Q

what does the second finger stand for in FLHR

A

current of wire

34
Q

what does the thumb in FLHR stand for

A

motion

35
Q

what is the magnetic flux density of a magnetic field

A

the strength of a magnetic field

36
Q

what does the force on a conductor the wire depend on

A

current and length of conductor magnetic flux density

37
Q

equation for force on a conductor n

A

magnetic flux density x length x current

38
Q

why is graphic used as brushes in a motor

A

it conducts electricity but creates very little friction

39
Q

how does an electric motor work

A

when the current flows the negatives side of the wire will rise USE FLHR
the positive side will move down
because of FLHR when the contacts reach the gaps of the split ring commutator no current flows and it continues turning out of its own inertia
the wires have now swapped sides this process repeats

40
Q

importance of split ring commutator

A

it swaps the contact every half turn to keep the motor rotating in the same direction

41
Q

how does a loudspeaker work

A

when the alternating current is switched on, it flows around the wire and magnetises the middle pole, turning it into an electromagnet because the current is constantly alternating this means it alternatively attracts and repulses the cone
the cone vibrates because of this and produces csound waves

42
Q

what is electromagnetic induction

A

when the coils of wire spinning in a magnetic field create a voltage by crossing through magnetic field lines

43
Q

what is the pd of mains electricity

A

230 v

44
Q

what do transformers do

A

they change the size of pd in an ac current

45
Q

what does a step up transformer do

A

increases the size of an alternating potential difference

46
Q

what does a step down transformer do

A

decreases the size of an alternating potential difference

47
Q

why are iron cores used in transformers

A

it is easily magnetised

48
Q

where are step up transformers used

A

step pd up from power stations to the national grid

49
Q

what pd is found on the national Grid

A

132,000 v

50
Q

hat pd is found in power stations

A

25 Kv

51
Q

where are step down transformers used

A

step pd down from national grid to consumers

52
Q

why dont transformers work with a dc

A

there is no changing magnetic field

53
Q

what is the transformer equation

A

pd across primary coil / pd across secondary coil

number of turns on primary coil / number go turns on a secondary coil

54
Q

what is the efficiency of most transformers

A

almost 100%

55
Q

equation for transformer efficiency

A

primary pd x primary current

secondary pd x secondary current

56
Q

why does a step up transformer increase the pd

A

this means the current is reduced so the heating effect is similar this means less energy is waster

57
Q

what is the heating effect in a wire proportional to

A

the square of the current

58
Q

how does a transformer work

A

the primary coil is connected to an ac supply
when the ac passes through this coil the core. magnetises and demagnetises quickly
this results in a changing magnetic field
this induced an alternating pd in the seoncdary coil
is part of a complete circuit a current is induced

59
Q

how to create a generator

A

move a magnet or a coil of wire in a magnetic field

60
Q

what is created if you keep moving an magnetic back and forth through a magnetic field

A

an ac current

61
Q

why does a generator work

A

the movement of the bar magnet induces a pd in the coi

this pd creates a current because the coil is part of a complete circuit

62
Q

how to make the current created by a stronger generator

A

use a stronger magnet

63
Q

how does a microphone work

A

like a loudspeaker in reverse
sound waves hit flexible diaphragm
that is attached to a coil of wire wrapped around a magnet this causes the coil or wire to move in and out of the magnetic field = current
louder sound makes the diaphragm move further
microphones can convert the pressure variations in sound waves into variations in current in the circuit

64
Q

what is an alternator

A

an ac generator

65
Q

how to increase the size of induced pd 2

A

increase the speed at which magnetic field lines are cut increase the strength of the magnetic field

66
Q

how to reverse the direction of the current created in a generator 2

A

move the magnet in the opposite direction

reverse the polarity of the magnet

67
Q

what is a dynamo

A

a direct current generator

68
Q

different between dynamo and generator

A

dynamos have a split ring commutator instead of split rings

69
Q

why does an induced current always oppose the charge that made it

A

a magnetic field can induce a current in a wire but when a current flows through a wire a magnetic field is induced
the induced magnetic field is trying to return things ti the way it was - it always acts against the charge that it made

PHYSICS (16 decks)

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