D3 Flashcards
Define: magnetic field
region in which a magnetic material is acted upon by a magnetic force.
Which way will a compass point when in path of a flux line?
the north pole of the compass always points toward the south pole
Which way do magnetic lines of force flow?
out of the north pole and into the south pole.
Magnetic lines of force always form complete
loops
Magnetic lines of force represent tension along their length that tend to
make them as short as possible
Do magnetic lines of force cross?
no they do not intersect or cross they repel each other
Define: Reluctance
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
List three ways to produce a magnet
electric current
stroking with a magnet
aligning with a magnetic field and tapping
List three ways to demagnetize
Using heat
Using vibration
Using an AC current
Define: Curie temperature
the temperature at which a ferromagnetic material rearranges itself and the alignment of the domains is lost even after cooling.
Precautions to avoid damaging magnets
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
the dot in the diagram means current is flowing
out of the paper or toward you
the cross on the diagram means the current is
flowing into the paper
same direction currents
attract
opposite currents
repel
List the factors affecting the strength of an electromagnet
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
Define: leakage flux
lines of force leave and enter the coil before reaching the ends
Define: ferromagnetic material
permit the setting up of flux lines more easily than others
Define: Magnetic flux
the total number of flux lines in a magnetic curcuit
Symbol is Phi
Unit is the weber which equals 108 flux lines. Wb
Define: Magnetomotive force
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
Define: Flux density
Symbol is beta or B
The unit is tesla or T
tesla equals webers per square metre
Is the counterpart of current
Define: Magnetizing force
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
Define: Permeability
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
Define: Theoretical saturation
when all the domains in the magnetic material have aligned themselves with the magnetic field
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
Define: Hysteresis
the lagging of the flux density behind the magnetizing force.
Define: Residual magnetism
the amount of flux density remaining in the magnetic material after the magnetizing force has been removed.
Define: Coercive force
the amount of magnetizing force required to remove the residual magnetism
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
B-H curve graph is used to determine an electromagnet’s
Permeability
Define: hysteresis loss
is the area inside the curve
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
To produce or induce 1 volt the wire must cut
10^8 lines of flux per second.
Page 312 in DC Fundamentals
