D3 Flashcards

1
Q

Define: magnetic field

A

region in which a magnetic material is acted upon by a magnetic force.

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2
Q

Which way will a compass point when in path of a flux line?

A

the north pole of the compass always points toward the south pole

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3
Q

Which way do magnetic lines of force flow?

A

out of the north pole and into the south pole.

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4
Q

Magnetic lines of force always form complete

A

loops

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5
Q

Magnetic lines of force represent tension along their length that tend to

A

make them as short as possible

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6
Q

Do magnetic lines of force cross?

A

no they do not intersect or cross they repel each other

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7
Q

Define: Reluctance

A

the property that opposes the setting up of flux lines
Symbol is Rm
Unit is ampere-turn per weber A.t/Wb
counterpart is resistance

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8
Q

List three ways to produce a magnet

A

electric current
stroking with a magnet
aligning with a magnetic field and tapping

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9
Q

List three ways to demagnetize

A

Using heat
Using vibration
Using an AC current

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10
Q

Define: Curie temperature

A

the temperature at which a ferromagnetic material rearranges itself and the alignment of the domains is lost even after cooling.

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11
Q

Precautions to avoid damaging magnets

A

do not expose to heat
do not expose to AC current
do not drop or expose to excessive vibration
do not store near other magnets

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12
Q

the dot in the diagram means current is flowing

A

out of the paper or toward you

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13
Q

the cross on the diagram means the current is

A

flowing into the paper

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14
Q

same direction currents

A

attract

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15
Q

opposite currents

A

repel

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16
Q

List the factors affecting the strength of an electromagnet

A

spacing between coils for an air-core electromagnet
number of turns in the coil
magnitude of the current flowing through the coil
type of material of which the core is made

17
Q

Define: leakage flux

A

lines of force leave and enter the coil before reaching the ends

18
Q

Define: ferromagnetic material

A

permit the setting up of flux lines more easily than others

19
Q

Define: Magnetic flux

A

the total number of flux lines in a magnetic curcuit
Symbol is Phi
Unit is the weber which equals 108 flux lines. Wb

20
Q

Define: Magnetomotive force

A

the force that causes the setting up of flux lines in a magnetic curcuit
(mmf) counterpart is (emf)
Symbol is Fm
Unit is ampere-turn A.t

21
Q

Define: Flux density

A

Symbol is beta or B
The unit is tesla or T
tesla equals webers per square metre
Is the counterpart of current

22
Q

Define: Magnetizing force

A

is the measure of the mmf per unit leng in a magnetic circuit.
(magnetizing intensity or magnetizing gradient)
Symbol is H
Unit is ampere-turn per metre A.t/m

23
Q

Define: Permeability

A

the measure of the ease with which flux may be set up in a unit length and cross section of a material.
symbol mu
Unit is Wb/A.t.m or H/m

24
Q

Define: Theoretical saturation

A

when all the domains in the magnetic material have aligned themselves with the magnetic field

25
Define: Practical saturation
when it is no longer practical to try to increase the flux density. occurs at a point on the curve called the knee
26
Define: Hysteresis
the lagging of the flux density behind the magnetizing force.
27
Define: Residual magnetism
the amount of flux density remaining in the magnetic material after the magnetizing force has been removed.
28
Define: Coercive force
the amount of magnetizing force required to remove the residual magnetism
29
Hysteresis loops that have curves that are tall and narrow are good for what
electromagnets because they magnetize easily and retain very little residual magnetism
30
B-H curve graph is used to determine an electromagnet's
Permeability
30
Define: hysteresis loss
is the area inside the curve
31
For generators the amount of emf that is induced is proportional to three factors
The number of lines of force (flux) The number of turns of wire The speed of the motion of wires cutting the lines of flux
32
To produce or induce 1 volt the wire must cut
10^8 lines of flux per second. Page 312 in DC Fundamentals