Magnetism Flashcards
(108 cards)
1
Q
What is a permanent magnet? And what is an example
A
Permanently magnetic forever/Made from iron/cobalt/nickle
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
3
Q
What are the 3 magnet types
A
Permanent,temporary, electromagnetic
4
Q
What is electro magnet
A
Magnetic field provided by electrical current to a coil wire (copper)
5
Q
What cannot be magnetised
A
Copper, aluminium, brass, silver , gold, titanium
6
Q
What does velocity mean
A
Speed
7
Q
3 ways to demagnetize
A
Heat, AC Current, Vibration/tapping
8
Q
3 ways to magnetise
A
Tapping,stroking,DC current
9
Q
Magnetic flux is directly proportional to magnetic motive force, true or false
A
True
10
Q
What is EMF
A
Electro motive force
11
Q
Are turns on coil and voltage directly proportional? True or false
A
True
12
Q
What direction are flux lines
A
North Pole to South Pole/ south to north through magnet
13
Q
What direction are flux lines flowing inside magnet
A
South Pole to North Pole
14
Q
Explain angle of declination
A
Angular difference between true north and magnetic north
15
Q
Magnetic flux is strongest at poles. True or false
A
True
16
Q
Magnetic flux and magnetic field are same thing? True or false
A
True
17
Q
Explain reluctance
A
The opposition to the flux lines (similar to resistance)
18
Q
What has a high reluctance
A
Air
19
Q
What has a low reluctance
A
Soft iron
20
Q
What has a mid reluctance
A
Hard steel
21
Q
What type of reluctance is easiest for flux lines to set up
A
Low reluctance
22
Q
Name some ferromagnetic material
A
Iron, nickel, cobalt
23
Q
Ferrous metals are easily magnetised true or false
A
True
24
Q
Name some paramagnetic materials and can they be magnetised?
A
Platinum, titanium, chromium, yes but not as easily as ferrous metals
25
What are diamagnetic materials and can they be magnetised
Copper, brass,aluminium. No cannot be magnetised
26
What direction does current flow out of load and source
Negative to positive out of source/ positive to negative through load
27
Left hand rule- what direction does thumb point on coil current
North Pole
28
Left hand rule: what does thumb do in current flow (conductor)
Direction of current/ fingers direction of flux
29
What is magnetite
A rare mineral which is permanent magnet, also called lodestone
30
What is magnetomotive force (mmf)
‐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
31
What is electron theory
Electrons spin on their axis as they orbit nucleus. The spinning motion causes electrons to become tiny permanent magnets
32
Whenever an electric current flows through conductor a magnetic field forms around the conductor. True or false
True
33
What is domain theory
Small molecules of iron or other magnetic metal that arrange themselves in orderly pattern when metal is magnetised
34
What is a short circuit
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
35
What is a ground fault
When a hot comes into contact with the ground or ground boxes (metal box)
36
What insulation is on copper wire used for coils
Enamel coated
37
What is magnetic flux
Number of magnetic field lines passing through surface placed in magnetic field
38
What is flux density
The amount of flux passing through a unit area at the right angles to the magnetic field lines
39
What way does current travel in town parallel conductors when attracted
Current travels the same way
40
What way does current travel in conductors when repulsing
Travels Opposite ways through conductors (repulsion)
41
What is voltage, amp , resistance equivalent in magnetic circuit
Volts= MMF
Amp= Flux
Resistance= Reluctance
42
What is another word for conductor wound coil?
Helix
43
Does more turns = higher or lower MMF & EMF
Higher
44
What is a solenoid
A electromagnetic cylinder coil of wire which generates controller magnetic field
45
What is a solenoid used for
Used as a switch for power/ movable plunger turns switch on/off - used in cars
46
What is induction
a method to create or generate static electricity in a material by bringing an electrically charged object near it (transformers, generators)
47
How to convert cm to m and why
Multiply by 0.01 ex. 2000cm in m = 0.01 x 2000 = 20m
48
If current stops what happens to flux loops
They collapse until gone
49
Lines of force are inversely proportional to distance of the conductor. True or false
True
50
Explain magnetic force
The attraction or repulsion exerted between poles. Two parallel current carrying conductors will exert a force on each other
51
How to solenoids work
Plunger is attracted or repelled when energised. This movement opens or closes a switch or contact
52
What effects strength of electromagnet 4 things…
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
Loops closer together, more loops added, more current in the coil= more or less lines
More lines the more loops/closer/more current
54
What is a wiggy
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
What does hysteresis loop show
Shows the relationship between induced magnetic flux density (B) and magnetising force (H)
56
What’s another word for B-H loop
Hysteresis loop
57
Lower permeability = higher or lower coercivity,retentivity,magnetism,relictance
Lower perm= higher everything
58
Higher permeability= higher or lower coercively,retentivity,reluctance, magnetism ?
Higher perm= lower everything (remember they are opposite to each other - high perm= low and low perm = high)
59
What is retentivity?
Materials ability to retain lines of flux when magnetising force removed
60
Coercive force
The amount of force to remove residual magnetism from the material to demagnetise
61
Explain permeability
How easy a material is able to establish a magnetic flux
62
Explain residual magnetism/ residual flux
The magnetic flux density that remains in a material when magnetic force is 0
63
Why do we use iron core
To increase magnetic flux density as it’s low reluctance
64
Does the magnetic core need high or low reluctance
Low reluctance = more magnetic flux
65
What two losses are in magnetic circuit
Hysteresis (loss from heat)
Eddy current (swirls loss heat)
66
Electric motors are electrical to mechanical. T or f
True
67
Generators are mechanical to electrical.. T or F
True
68
What is a prime mover example
Wind turbine/waves/dam (something that spins)
69
Where is the biggest rate of change on sin wave
Beginning part. Just above the line
70
How many degrees per second on hertz
360
71
Hz=PXN / 120 what does this mean
Hertz = poles x speed divided by 120
Hz= Hertz in seconds
P= Poles
N= Speed in RPM
72
Advantage of ac current
Current can be stepped up or down more easily
Ac motors are smaller/cheaper/simpler
73
Advantage of dc current
Required to produce magnetic field for alternator
Electrochemical processes e.g electroplating
The use of dc traction motors
74
Permeability is not constant true/false
True- permeability of ferrous materials decrease as flux increases (BH graph)
75
What is the knee (saturation) point
When permeability curve levels out when material is full of flux lines
76
When saturated any increase in magnetising is a waste of energy t/f
True
77
What’s the two types of saturation
Theoretical- all domains are aligned
Practical- no longer practical to increase
78
Hysteresis graph- Tall and narrow curves make good temporary magnets true false
True. They have very little residual
79
Wide and flat curves indicate material will be good permanent magnet due to high retentivity true/false
True- hysteresis shape
80
What are air gaps used for
To adjust solenoids magnetic strength - Air has higher reluctance and lower permeability- fringing ( flux density will be lowest in the air gap)
81
How can solenoid current be varied
Using a rheostat (variable resistor)
82
What is a relay
Electromagnetic switch - can manage large amount of voltage ( normally open / normally closed contact)
83
Two types of relay circuits
The control circuit = lower current
The power/operating circuit= high volts
84
Three applications of a relay
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
What is rms/effective
Root mean square = average value dc current of ac waveform
86
What is alternation
One part of the sound wave
87
Time taken to complete one full cycle on soundwave
Period
88
What is hertz
How many cycles do you get in one second on signwave (frequency)
89
What is one full complete signwave called
A cycle
90
20 cycles = how many hertz?
20 1cycle= 1 hz
91
Signwave period =0.002 second. How many hz in one second
1 divide 0.002 = 500hz
92
What is PTC
Positive temperature coefficient = as temp increased resistance increases
93
What would a good core material have
High resistance and low retentivity
94
What is BH graph used for
Work out electromagnets permeability
95
What is frequency (Hz)
The number of compete cycles that occur in one second
96
Explain period, frequency & cycle
Period = time taken in seconds for one complete cycle
Cycle= one full wave cycle 360degrees
Frequency(Hertz) = how many cycles per 1 second
97
What is flemmings left hand rule
Is used for motors:
Thumb point to thrust
Forefinger points to flux
Middle to current.
98
What is one cycle
Is one full wave cycle on sine wave (360 degrees)
99
What is period in sine wave
Time taken in seconds for one complete cycle
100
How to convert flux lines to flux(Webber)
Divide by 10topower8
Ex. 8000 flux lines to Webber =
8000\10 to the power 8 = 0.00008
101
Hard steel or iron better for permanent magnet
Hard steel
102
Flux is inversely proportional to what
Conductor length
103
6 characteristic of flux lines
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
What are magnetic screens used for
To attract or deflect magnetic line from an area. Ferrous materials used reduce Eddie current
105
What Flemings left hand rule measure
Thumb- force/thrust
Forefinger= magnetic field
Middle= current
106
Narrow hysteresis loop
Small amount of energy lost Good for transformers and motor cores
107
Wide large hysteresis loop
Retains large amount of residual magnetism- Good for permanent magnets
108
How to control voltage 3 ways
Increase/decrease flux density by rheostat
Increase or decrease length of conductor
Increase/ decrease speed rotating
