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LC Physics > magnetism > Flashcards

magnetism Flashcards

1
Q

magnetism

A

force of attraction between magnets and magnetic object (ferromagnetic objects)

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2
Q

ferromagnetic materials

A 
iron
steel
nickel
cobalt
alloys of these
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3
Q

characteristics of magnets

A
  • every magnet has 2 poles (exist in pairs, North and South)

- every magnet has a magnetic field around it

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4
Q

magnets and electricity

A
  • magnets used to generate/produce electricity
  • spinning coil of wire inside magnetic field produces an electric force between ends of coil
  • an electric current produces a magnetic field around it
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5
Q

induced magnetism

A
  • when magnet placed near ferromagnetic material, it makes that material magnetic (induced magnetism)
  • some materials lose this magnetism (temporary magnets), some keep it (permanent magnets)
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6
Q

to show magnetic effect of an electric current

A
  • align piece of wire north-south + place plotting compass underneath. Compass needle also lines up n-s due to Earth’s magnetic field acting on it
  • send steady current thru wire + compass needle will deflect. Direction it deflects depends on direction of current. Reverse direction of C + needle deflects in opposite direction
  • Switch off current, magnetic field due to current disappears + needle lines up n-s.
  • Concludes: every current-carrying conductor has a magnetic field around it caused by current
  • magnetic field due to single current-carrying wire is weak unless current is v. large
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7
Q

electromagnet

A

a temporary magnet made by passing electric current thru a solenoid coiled around an iron bar

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8
Q

use of electromagnet

A
  • lift scrap iron and steel (junkyard)
  • electric motors
  • electromagnetic relays
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9
Q

The Earth

A
  • circulating electric currents in core
  • south pole is in Northern hemisphere, and vice versa
  • angle between true north + magnetic north is called the magnetic declination or variation
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10
Q

the magnetic declination or variation

A

angle between true north + magnetic north

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11
Q

temporary magnets

A

materials that lose induced magnetism

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12
Q

permanent magnets

A

materials that keep induced magnetism

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13
Q

to plot magnetic fields due to bar magnet

A
  • place bar magnet on sheet of paper
  • place plotting compass next to one pole + mark w/ dots on paper both ends of compass needle
  • move compass as in diagram + mark other end of needle
  • repeat until you end up at other pole
  • mark each line w/ arrow head showing direction of magnetic field (pointing north to south)
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14
Q

field lines around bar magnet

A
  • field lines in space around bar magnet start at n pole, end at s pole
  • near poles - where magnetic field is strongest - lines close together.
  • further away - where field is weaker - lines far apart
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15
Q

plotting magnetic field around diff magnets

A

in hback

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16
Q

to plot the magnetic field due to current in a long straight wire

A
  • use equipment in diagram + send current through wire (2A is suitable)
  • place plotting compass near wire + mark w/ dots on paper both ends of compass needle
  • move compass so s pole at dot that marked n pole. Mark other end on needle w/ dot
  • repeated until you end up back at point you started at. Join dots w/ smooth curved line. Will be found to be circle
  • continue process drawing a no. of circles around wire
  • mark each circle w/ arrowhead showing direction
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17
Q

solenoid

A

like a coil, but length longer than radius

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18
Q

use of Earth’s magnetic field

A

-used in navigation on land + sea, compasses always point north if nothing interfering w/ them

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19
Q

is a magnetic field vector or scalar?

A

vector (magnitude + direction)

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20
Q

magnetic field

A

any region in space where magnetic forces can be felt
/
lines of force running from the north-seeking pole to the south-seeking pole of the magnet

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21
Q

magnetic field line

A

line drawn in a magnetic field so that the tangent to it at any point shows the direction of the magnetic field at that point

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22
Q

the right-hand grip rule

A

if the right hand clasps a conductor w/ thumb pointing in direction of current, then fingers give direction of magnetic field around conductor

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23
Q

current in a magnetic field

A
  • current carrying conductor has magnetic field due to current, when placed in another magnetic field it experiences a force
  • this force can move the conductor
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24
Q

to show the force on a current-carrying conductor in a magnetic field

A
  • set up as diagram
  • send current thru tinfoil (2A suitable)
  • foil seen to move forwards + backwards depending on which direction current is flowing
  • conclusion: current-carrying conductor in magnetic field experiences a force
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25
Q

Fleming’s left hand rule

A
  • hold thumb, first finger, second finger at right angles to each other (of left hand)
  • first finger points in direction of magnetic field, second finger in direction of current, thumb in direction of force
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26
Q

force on a current carrying coil

A

current carrying coil in a magnetic field will always experience a force unless it is parallel to magnetic field

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27
Q

force depends on

A

current
length of wire
strength of magnetic field

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28
Q

if a conductor of length l, carrying a current I is placed at right angles to a magnetic field of flux density B it experiences a force F

A

F = BIL

B = flux density
I = current
L = lnegth of conductor
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29
Q

magnetic flux density B

A

B at a point in a magnetic field is a vector whose magnitude is equal to the force that would be experienced by a conductor of length 1m carrying a current of 1A at right angles to the field at that point and whose direction is the direction of the force on a north pole placed at that point

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30
Q

magnetic flux density - vector

A

quantity

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31
Q

magnetic flux density - unit

A

Tesla

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32
Q

magnetic flux density - formula

A

B = F/IL

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33
Q

tesla

A

magnetic flux density at a point is 1 tesla if a conductor of length 1m carrying a current 1A experiences a force of 1N when placed perpendicular to the field

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34
Q

uses of the force

A

d.c. motor
moving coil loudspeaker
moving coil voltmeter
moving coil ohmmeter

35
Q

force on a moving charge in a magnetic field

A

if a charge q Coulombs is moving with a speed of v metres per second at right angles to a magnetic field of flux density B teslas then there is a force F on it given by:

F = qvB

36
Q

force on a moving charge in a magnetic field - formula

A

F = qvB

37
Q

to prove F = qvB

A

hardback

38
Q

force on a moving charge in a magnetic field

A

seen in a cathode ray tube (TV screen)

39
Q

charged particle moving at constant speed in a uniform magnetic field

A

if a charged particle moving at constant speed enters a uniform magnetic field and moves at right angles to the field, the particle will move in a circle

40
Q

forces between currents

A
  • if two parallel conductors are carrying current in opposite directions then there is a force of repulsion on each wire due to their magnetic fields. This forces them apart
  • f they carry current in the same direction, they are attracted to each other
41
Q

the ampere A

A

that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible cross section and placed 1 metre apart in a vacuum, would produce a force on each conductor of 2x10⁻⁷ Newtons per metre of length

42
Q

to show the magnetic forces between two current-carrying conductor

A
  • set up
  • pass current of about 4A thru parallel strips of tinfoil
  • foil strips seen to move away from each other
  • in conclusion, there is a force between current-carrying conductors due to their magnetic fields
43
Q

electromagnetic induction

A

whenever the magnetic field passing through a coil changes, an emf appears in the coil. This emf causes current to flow

  • no emf unless magnetic field is changing
  • emf induced by changing magnetic flux/magnetic field
44
Q

magnetic flux

A

magnetic flux through area A = magnetic flux density x area

Φ = BA
notation

45
Q

magnetic flux unit

A

weber (Wb)

1 WB = 1 Tm²

46
Q

weber

A

if the magnetic flux density over an area of 1m² is 1 tesla then flux through area is 1 weber

47
Q

Faraday’s Law of Electromagnetic induction

A

size of the induced emf is directly proportional to rate of change of the flux

E = -ΔΦ /Δt
+ notation

48
Q

induced emf formula (not in log table)

A

induced emf = (final flux - initial flux)/time

E = -N(dΦ/dt) where N = no. of turns, dΦ = change in magnetic flux, dt = change in time

Φ = BA (B = magnetic field, A = area)

49
Q

Lenz’s Law

A

the direction of an induced current i always such as to oppose the change producing it

50
Q

laws of electromagnetic induction

A

Faraday’s and Lenz’s laws are together known as the Laws of Electromagnetic Induction

-state these two laws when asked to state laws of electromagnetic induction

(magnitude of the) induced emf is proportional to rate of cutting flux induced current /emf in such a direction 3 as to oppose the change that causes it

51
Q

all the fucking diagrams and experiments

A

in hback!!!!!!!!!!!

52
Q

generators

A
  • electric generator based on electromagnetic induction
  • form of energy used to turn a turbine which rotates a coil in a magnetic field, this causes magnetic flux to change + emf to be produced: mechanical to electrical
53
Q

uses of generators

A

electricity power stations
alternator in a car
dynamo on a bicycle

54
Q

alternating current

A
  • in dc: electrons go in same direction

- in ac: electrons change direction regularly

55
Q

current at any instant formula

A

current at any instant = voltage at instant/resistance

i = V/R

when v large, i is also large. when v small, so is i. when v changes direction, so does i

56
Q

oscilloscope

A

if an ac is placed across an oscilloscope, an alternating current is displayed

-diagrams for ac and dc in hback

57
Q

rms

A

because ac changes all the time, we cannot say exact measure of current or voltage, instead we talk about its equivalent value, its rms value in dc

58
Q

rms formulas

A

in hback

59
Q

mutual induction

A

if a changing magnetic field in one coil causes an induced emf to appear in a nearby coil there is said to be mutual induction between the two coils

60
Q

the size of induced emf + hence the mutual induction may be increased by:

A
  • having coils nearer each other
  • winding coils on same soft iron core
  • increasing number of turns one either or both coils
61
Q

self induction

A

-whenever current passing thru coil changes, magnetic field surround coil changes
-this changing magnetic field induces an emf in the coil that opposes the changing current (a back emf).
This is called self induction

62
Q

where does mutual conduction occur

A

occurs in transformers

63
Q

where does self induction occur

A

occurs in inductors

64
Q

inductor

A

a coil which has the property of self induction is called an inductor

65
Q

transformers

A

a device used to change the value of an alternating voltage

66
Q

what transformers consist of

A

two coils of wire wound on a soft iron core

67
Q

transformers - info

A
  • charged capacitor blocks dc. A capacitor conducts ac since it charges + discharges as ac changes direction
  • an alternating voltage (input voltage) applied to primary coil
  • output voltage applied by transformer to secondary coil
68
Q

step up transformer

A

if Nₛ greater than Nₚ, then Vₒ greater than Vᵢ, called a step up transformer

69
Q

step down transformer

A

if Nₛ less than Nₚ, then Vₒ less than Vᵢ, called a step down transformer

70
Q

transformer equation

A

Vᵢ/Vₒ = Nₚ/Nₛ

Vᵢ -number of turns on primary coil
Vₒ - number of turns on secondary coil
Nₚ - input voltage
Nₛ - output voltage

71
Q

uses of a transformer

A
  • changes to voltages on national grid supply
  • computers, radios, tvs, chargers all contain transformers
  • tvs have both step up and step down transformers
72
Q

inductors and ohmic resistance

A
  • a coil (inductor) opposes the flow of direct current with its ohmic resistance
  • a coil opposes the flow of alternating current with its ohmic resistance + the back emf induced in it
73
Q

inductor uses

A
  • to smooth out slight variations in dc in power supply unit
  • in the tuning circuits of radios to tune to different stations
  • in dimmer switches used in stage lighting
74
Q

magnetic field about diff objects

A

in hardback

75
Q

magnetic field around earth

A

know how to draw

76
Q

why does a magnet that is free to rotate point towards the north?

A

like poles repel and unlike poles attract

77
Q

sketch magnetic field due to long straight current-carrying conductor

A

know magnetic fields of diff conductors?

78
Q

effective emf (not in log tables)

A

Effective emf = Applied Emf - Induced Emf

79
Q

calculating current thru coil

A

Find effective emf, then use that in R=V/I calculations where R is the effective resistance

80
Q

finding average current thru a wire

A

find effective emf, then use it to find current using the formula I = V/R

81
Q

how is the output voltage of a wind turbine changed to 230 V a.c?

A

transformer

82
Q

principle on which the definition of the ampere is based

A

force between (two) conductors carrying current

current-carrying conductor experiences a force in a magnetic field

83
Q

In some modern seismometers a magnet is attached to the mass and a coil of wire is attached to the frame. During an earthquake, there is relative motion between the magnet and the coil. Explain why an emf is generated in the coil.

A

magnetic field passing through the coil is changing

// coil cuts (magnetic) flux

LC Physics (25 decks)

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