chapter 3

Question 1

3 types of magnets

Answer 1

.natural lodestone
.permanent magnets magnets like these are are magnitised electrically by means of a solenoid.
.electromagnets when magnetic field passes through a coil a magnetic field similar to that of a bar magnet is established.

Question 2

lines of force

Answer 2

the space in which magnetic forces act , is termed magnetic field

Question 3

magnetic flux

Answer 3

entire quantity of magnetic lines surrounding a magnet , taken as a whole , is called a magnetic flux.

Question 4

flux density

Answer 4

number of lines per unit area is termed flux density.

Question 5

magnetic poles

Answer 5

all magnets have a north seeking pole and a south seeking pole. they are simply the dividing line between the magnet and air or other surrounding mediums.

Question 6

one of the phenomenea of an electric current is its ability to produce

Answer 6

a magnetic field around the current carriying conductor.

Question 7

magnetism associated with current carrying conductors can be intensified by

Answer 7

by forming the conductor into a coil.

Question 8

magnetic fmagnetic field strength is of an eleectromagnet is is dependtant on

Answer 8

mmf and lenghth of solenoid

Question 9

magnetic field strength of coil can be strengthened by

Answer 9

increase in current

placing an iron core inside solenoid.

Question 10

standard transformer

Answer 10

used to step a voltage up or down with corresponding increase or decrease in voltage.

Question 11

advantages of standard transformers

Answer 11

. eeasy construction
.efficiency at full load is high 97%
.they have no moving parts
.requires little maintainence.

Question 12

core of a transformer is built out of

Answer 12

aalternate layers of lightly insulated alloyed steel laminations.

Question 13

coils of transformers are made of

Answer 13

thinly insulated copper conductors

Question 14

cooling of transformers

Answer 14

the diispation of heat losses generated in a transformer is of great nb since the rating of the transformer is determined not only by these losses but also by the temperature rise of the transformer.

Question 15

open air cooling is applicable to

Answer 15

smaller transformers

Question 16

oil cooling of a transformer

Answer 16

oil that the the core of the transformer is immersed in has a duel perpose , improves the insulation qualities and conduxts heat away from the windings and core to the cooling surfaces of the tank.

Question 17

principle of operating a transformer

Answer 17

coils are electrically seperate by means op insulation

coils are magnetically coupled by means of a laminated iron core.

Question 18

when a ac is applied to the primary winding:

Answer 18

. ac current flow through it.
.sets up an alternating flux around the iron core.
.which links to the secondary winding
.inducing in it an emf of the same frequancy.

Question 19

define mutual inductance

Answer 19

if two coils are so arranged that the magnetic flux set up by the current in the primary coil links with the secondary coil , the two coils are said to have a mutual inductance. it is measured in henry

Question 20

define the term the henry

Answer 20

two circuits have a mutual inductance of one henry if the emf induced in the secondary circuit is 1 volt when the current in primary circuit changes at the rate of 1 ampere per second.

Question 21

ideal transformers

Answer 21

transformer assumed to havae no losses.

Question 22

what is an auto transformer

Answer 22

primary and secondary windings are connected electrically and magnetically

Question 23

uses for auto transformers

Answer 23

voltage regulation regulation on transmission lines , reduced voltage starting of three phase motors and high voltage starting of discharge lamps.

Question 24

name one danger of an auto transformer

Answer 24

with a step up step down transsformer , there is a distinct danger of hte full primary voltage appeaaring across the terminals of the secondary winding in the event of an open circuit occuring in the seccondary winding

Question 25

electromsgnetic induction

Answer 25

when a conductor moved through a magnetic field , an emf is induced in that conductor. if the conductor forms a closed circuit , an electricc current will flow. this is in accordance with faradays laws of electromagneetic induction