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