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Magnetism > Magnetism > Flashcards

Magnetism Flashcards

1
Q

What is a permanent magnet? And what is an example

A

Permanently magnetic forever/Made from iron/cobalt/nickle

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2
Q

What is a temporary magnet? And what’s an example

A

Only magnetic when in a strong magnetic field. When magnetic field removed they are no longer magnetic. Paper clips/soft metals

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3
Q

What are the 3 magnet types

A

Permanent,temporary, electromagnetic

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4
Q

What is electro magnet

A

Magnetic field provided by electrical current to a coil wire (copper)

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5
Q

What cannot be magnetised

A

Copper, aluminium, brass, silver , gold, titanium

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6
Q

What does velocity mean

A

Speed

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7
Q

3 ways to demagnetize

A

Heat, AC Current, Vibration/tapping

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8
Q

3 ways to magnetise

A

Tapping,stroking,DC current

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9
Q

Magnetic flux is directly proportional to magnetic motive force, true or false

A

True

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10
Q

What is EMF

A

Electro motive force

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11
Q

Are turns on coil and voltage directly proportional? True or false

A

True

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12
Q

What direction are flux lines

A

North Pole to South Pole/ south to north through magnet

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13
Q

What direction are flux lines flowing inside magnet

A

South Pole to North Pole

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14
Q

Explain angle of declination

A

Angular difference between true north and magnetic north

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15
Q

Magnetic flux is strongest at poles. True or false

A

True

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16
Q

Magnetic flux and magnetic field are same thing? True or false

A

True

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17
Q

Explain reluctance

A

The opposition to the flux lines (similar to resistance)

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18
Q

What has a high reluctance

A

Air

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19
Q

What has a low reluctance

A

Soft iron

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20
Q

What has a mid reluctance

A

Hard steel

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21
Q

What type of reluctance is easiest for flux lines to set up

A

Low reluctance

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22
Q

Name some ferromagnetic material

A

Iron, nickel, cobalt

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23
Q

Ferrous metals are easily magnetised true or false

A

True

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24
Q

Name some paramagnetic materials and can they be magnetised?

A

Platinum, titanium, chromium, yes but not as easily as ferrous metals

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25
Q

What are diamagnetic materials and can they be magnetised

A

Copper, brass,aluminium. No cannot be magnetised

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26
Q

What direction does current flow out of load and source

A

Negative to positive out of source/ positive to negative through load

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27
Q

Left hand rule- what direction does thumb point on coil current

A

North Pole

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28
Q

Left hand rule: what does thumb do in current flow (conductor)

A

Direction of current/ fingers direction of flux

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29
Q

What is magnetite

A

A rare mineral which is permanent magnet, also called lodestone

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30
Q

What is magnetomotive force (mmf)

A

‐the force that causes the setting up of flux lines in a magnetic circuit
‐magnetomotive force in a magnetic circuit = # of coil turns x the current through the coil

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31
Q

What is electron theory

A

Electrons spin on their axis as they orbit nucleus. The spinning motion causes electrons to become tiny permanent magnets

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32
Q

Whenever an electric current flows through conductor a magnetic field forms around the conductor. True or false

A

True

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33
Q

What is domain theory

A

Small molecules of iron or other magnetic metal that arrange themselves in orderly pattern when metal is magnetised

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34
Q

What is a short circuit

A

Short circuit is occurs when the current finds a way to bypass the appliance on a path that has little or no resistance. Hot and neutral touch

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35
Q

What is a ground fault

A

When a hot comes into contact with the ground or ground boxes (metal box)

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36
Q

What insulation is on copper wire used for coils

A

Enamel coated

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37
Q

What is magnetic flux

A

Number of magnetic field lines passing through surface placed in magnetic field

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38
Q

What is flux density

A

The amount of flux passing through a unit area at the right angles to the magnetic field lines

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39
Q

What way does current travel in town parallel conductors when attracted

A

Current travels the same way

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40
Q

What way does current travel in conductors when repulsing

A

Travels Opposite ways through conductors (repulsion)

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41
Q

What is voltage, amp , resistance equivalent in magnetic circuit

A

Volts= MMF
Amp= Flux
Resistance= Reluctance

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42
Q

What is another word for conductor wound coil?

A

Helix

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43
Q

Does more turns = higher or lower MMF & EMF

A

Higher

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44
Q

What is a solenoid

A

A electromagnetic cylinder coil of wire which generates controller magnetic field

45
Q

What is a solenoid used for

A

Used as a switch for power/ movable plunger turns switch on/off - used in cars

46
Q

What is induction

A

a method to create or generate static electricity in a material by bringing an electrically charged object near it (transformers, generators)

47
Q

How to convert cm to m and why

A

Multiply by 0.01 ex. 2000cm in m = 0.01 x 2000 = 20m

48
Q

If current stops what happens to flux loops

A

They collapse until gone

49
Q

Lines of force are inversely proportional to distance of the conductor. True or false

A

True

50
Q

Explain magnetic force

A

The attraction or repulsion exerted between poles. Two parallel current carrying conductors will exert a force on each other

51
Q

How to solenoids work

A

Plunger is attracted or repelled when energised. This movement opens or closes a switch or contact

52
Q

What effects strength of electromagnet 4 things…

A

1: the spacing between coils
2: the number of turns in coil
3: the magnitude of current flowing through the coil
4: the type of material from which the core is made

53
Q

Loops closer together, more loops added, more current in the coil= more or less lines

A

More lines the more loops/closer/more current

54
Q

What is a wiggy

A

Solenoid tester. Tests the voltage on a solenoid coil. Spring presses down on the coil and when energised moves the spring to display voltage

55
Q

What does hysteresis loop show

A

Shows the relationship between induced magnetic flux density (B) and magnetising force (H)

56
Q

What’s another word for B-H loop

A

Hysteresis loop

57
Q

Lower permeability = higher or lower coercivity,retentivity,magnetism,relictance

A

Lower perm= higher everything

58
Q

Higher permeability= higher or lower coercively,retentivity,reluctance, magnetism ?

A

Higher perm= lower everything (remember they are opposite to each other - high perm= low and low perm = high)

59
Q

What is retentivity?

A

Materials ability to retain lines of flux when magnetising force removed

60
Q

Coercive force

A

The amount of force to remove residual magnetism from the material to demagnetise

61
Q

Explain permeability

A

How easy a material is able to establish a magnetic flux

62
Q

Explain residual magnetism/ residual flux

A

The magnetic flux density that remains in a material when magnetic force is 0

63
Q

Why do we use iron core

A

To increase magnetic flux density as it’s low reluctance

64
Q

Does the magnetic core need high or low reluctance

A

Low reluctance = more magnetic flux

65
Q

What two losses are in magnetic circuit

A

Hysteresis (loss from heat)
Eddy current (swirls loss heat)

66
Q

Electric motors are electrical to mechanical. T or f

A

True

67
Q

Generators are mechanical to electrical.. T or F

A

True

68
Q

What is a prime mover example

A

Wind turbine/waves/dam (something that spins)

69
Q

Where is the biggest rate of change on sin wave

A

Beginning part. Just above the line

70
Q

How many degrees per second on hertz

A

360

71
Q

Hz=PXN / 120 what does this mean

A

Hertz = poles x speed divided by 120

Hz= Hertz in seconds
P= Poles
N= Speed in RPM

72
Q

Advantage of ac current

A

Current can be stepped up or down more easily

Ac motors are smaller/cheaper/simpler

73
Q

Advantage of dc current

A

Required to produce magnetic field for alternator

Electrochemical processes e.g electroplating

The use of dc traction motors

74
Q

Permeability is not constant true/false

A

True- permeability of ferrous materials decrease as flux increases (BH graph)

75
Q

What is the knee (saturation) point

A

When permeability curve levels out when material is full of flux lines

76
Q

When saturated any increase in magnetising is a waste of energy t/f

A

True

77
Q

What’s the two types of saturation

A

Theoretical- all domains are aligned
Practical- no longer practical to increase

78
Q

Hysteresis graph- Tall and narrow curves make good temporary magnets true false

A

True. They have very little residual

79
Q

Wide and flat curves indicate material will be good permanent magnet due to high retentivity true/false

A

True- hysteresis shape

80
Q

What are air gaps used for

A

To adjust solenoids magnetic strength - Air has higher reluctance and lower permeability- fringing ( flux density will be lowest in the air gap)

81
Q

How can solenoid current be varied

A

Using a rheostat (variable resistor)

82
Q

What is a relay

A

Electromagnetic switch - can manage large amount of voltage ( normally open / normally closed contact)

83
Q

Two types of relay circuits

A

The control circuit = lower current
The power/operating circuit= high volts

84
Q

Three applications of a relay

A

1: remote control ( for inaccessible locations
2: automation (no human necessary)
3: used for safety (high voltage can be switched on using lower voltage)

85
Q

What is rms/effective

A

Root mean square = average value dc current of ac waveform

86
Q

What is alternation

A

One part of the sound wave

87
Q

Time taken to complete one full cycle on soundwave

A

Period

88
Q

What is hertz

A

How many cycles do you get in one second on signwave (frequency)

89
Q

What is one full complete signwave called

A

A cycle

90
Q

20 cycles = how many hertz?

A

20 1cycle= 1 hz

91
Q

Signwave period =0.002 second. How many hz in one second

A

1 divide 0.002 = 500hz

92
Q

What is PTC

A

Positive temperature coefficient = as temp increased resistance increases

93
Q

What would a good core material have

A

High resistance and low retentivity

94
Q

What is BH graph used for

A

Work out electromagnets permeability

95
Q

What is frequency (Hz)

A

The number of compete cycles that occur in one second

96
Q

Explain period, frequency & cycle

A

Period = time taken in seconds for one complete cycle

Cycle= one full wave cycle 360degrees

Frequency(Hertz) = how many cycles per 1 second

97
Q

What is flemmings left hand rule

A

Is used for motors:

Thumb point to thrust
Forefinger points to flux
Middle to current.

98
Q

What is one cycle

A

Is one full wave cycle on sine wave (360 degrees)

99
Q

What is period in sine wave

A

Time taken in seconds for one complete cycle

100
Q

How to convert flux lines to flux(Webber)

A

Divide by 10topower8

Ex. 8000 flux lines to Webber =
8000\10 to the power 8 = 0.00008

101
Q

Hard steel or iron better for permanent magnet

A

Hard steel

102
Q

Flux is inversely proportional to what

A

Conductor length

103
Q

6 characteristic of flux lines

A

1: they do not flow and once lines set up are fixed/stationary

2: always form complete loops

3: lines of force repel each other

4:line of force like to be as short as poss

5: line of force never cross

6: line arrange themselves so maximum lines set up

104
Q

What are magnetic screens used for

A

To attract or deflect magnetic line from an area. Ferrous materials used reduce Eddie current

105
Q

What Flemings left hand rule measure

A

Thumb- force/thrust
Forefinger= magnetic field
Middle= current

106
Q

Narrow hysteresis loop

A

Small amount of energy lost Good for transformers and motor cores

107
Q

Wide large hysteresis loop

A

Retains large amount of residual magnetism- Good for permanent magnets

108
Q

How to control voltage 3 ways

A

Increase/decrease flux density by rheostat
Increase or decrease length of conductor
Increase/ decrease speed rotating

Magnetism (1 decks)

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