D3

Question 1

Define: magnetic field

Answer 1

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

Question 2

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

Answer 2

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

Question 3

Which way do magnetic lines of force flow?

Answer 3

out of the north pole and into the south pole.

Question 4

Magnetic lines of force always form complete

Answer 4

loops

Question 5

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

Answer 5

make them as short as possible

Question 6

Do magnetic lines of force cross?

Answer 6

no they do not intersect or cross they repel each other

Question 7

Define: Reluctance

Answer 7

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

Question 8

List three ways to produce a magnet

Answer 8

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

Question 9

List three ways to demagnetize

Answer 9

Using heat
Using vibration
Using an AC current

Question 10

Define: Curie temperature

Answer 10

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

Question 11

Precautions to avoid damaging magnets

Answer 11

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

Question 12

the dot in the diagram means current is flowing

Answer 12

out of the paper or toward you

Question 13

the cross on the diagram means the current is

Answer 13

flowing into the paper

Question 14

same direction currents

Answer 14

attract

Question 15

opposite currents

Answer 15

repel

Question 16

List the factors affecting the strength of an electromagnet

Answer 16

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

Question 17

Define: leakage flux

Answer 17

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

Question 18

Define: ferromagnetic material

Answer 18

permit the setting up of flux lines more easily than others

Question 19

Define: Magnetic flux

Answer 19

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

Question 20

Define: Magnetomotive force

Answer 20

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

Question 21

Define: Flux density

Answer 21

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

Question 22

Define: Magnetizing force

Answer 22

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

Question 23

Define: Permeability

Answer 23

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

Question 24

Define: Theoretical saturation

Answer 24

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

Question 25

Define: Practical saturation

Answer 25

when it is no longer practical to try to increase the flux density.
occurs at a point on the curve called the knee

Question 26

Define: Hysteresis

Answer 26

the lagging of the flux density behind the magnetizing force.

Question 27

Define: Residual magnetism

Answer 27

the amount of flux density remaining in the magnetic material after the magnetizing force has been removed.

Question 28

Define: Coercive force

Answer 28

the amount of magnetizing force required to remove the residual magnetism

Question 29

Hysteresis loops that have curves that are tall and narrow are good for what

Answer 29

electromagnets because they magnetize easily and retain very little residual magnetism

Question 30

B-H curve graph is used to determine an electromagnet’s

Answer 30

Permeability

Question 30

Define: hysteresis loss

Answer 30

is the area inside the curve

Question 31

For generators the amount of emf that is induced is proportional to three factors

Answer 31

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

Question 32

To produce or induce 1 volt the wire must cut

Answer 32

10^8 lines of flux per second.

Page 312 in DC Fundamentals