Self tests 1-3

Question 1

What is another name for magnetic lines of force?

Answer 1

magnetic flux

Question 2

why don’t magnetic lines of force collapse into the centre of a magnet?

Answer 2

the magnetic lines of force do not collapse due to mutual repulsion

Question 3

T/F? Magnetic lines of force never cross

Answer 3

True

Question 4

T/F? Magnetic lines of force start at the north pole and end at the south pole

Answer 4

False

Question 5

T/F? It is not possible to have a north pole without having an accompanying south pole

Answer 5

True

Question 6

T/F? The north pole of the earth is actually a south magnetic pole

Answer 6

True

Question 7

T/F? The arrows on the magnetic lines of force always point from north to south outside of the magnet, and south to north on the inside of the magnet

Answer 7

True

Question 8

T/F? Unlike poles of magnets repel each other

Answer 8

False

Question 9

T/F? Magnetic screens are made of materials with a low reluctance

Answer 9

True

Question 10

T/F? Magnetic screens block magnetic flux lines

Answer 10

False

Question 11

List three ways of creating magnets?

Answer 11

• using electrical current
• stroking with a magnet
• by linking with another magnetic field and tapping

Question 12

List three ways of demagnetizing a material?

Answer 12

• Using heat
• AC current
• tapping or vibrating excessively

Question 13

Which method of demagnetizing is commonly used by watchmakers?

Answer 13

AC current method

Question 14

What is meant by the term “curie” temperature?

Answer 14

temperature at which the molecular structure changes

Question 15

When a piece of ferromagnetic material is brought near a magnet, the ferromagnetic material becomes magnetized. This process is referred to as ?

Answer 15

Magnetic induction

Question 16

Is it true that any time a charge moves, there must be a magnetic field set up around that charge?

Answer 16

Yes

Question 17

why are there no magnetic poles in the field surroundings current-carrying conductor?

Answer 17

there is no magnetic material

Question 18

As the direction of current in a conductor is reversed, what happens to compasses surrounding the conductors?

Answer 18

the compass also reverses direction

Question 19

As the current in a conductor is increased, what happens to the magnetic lines of force surrounding the conductor?

Answer 19

More flux lines are made and forced outwards

Question 20

When current remains steady, does the field disappear?

Answer 20

No it remains steady

Question 21

Is the field around the conductor more or less dense closer to the conductor?

Answer 21

more dense

Question 22

As the current in a conductor is decreased, what happens to the magnetic lines of force surrounding the conductor?

Answer 22

They collapse inward

Question 23

I’m moving four times as far away from a current-carrying conductor, the density of the flux lines will have ____________ by ____________ times?

Answer 23

decreased by four

Question 24

Parallel conductor carrying currents in the same direction _________ each other and parallel conductors carrying currents in the opposite directions ___________ each other?

Answer 24

attract , repel

Question 25

Using the left hand rule for coils, the thumb points in the direction of?

Answer 25

North pole of coil

Question 26

four things that affect the strength of an electromagnetic coil?

Answer 26

- number of turns - core material - spacing between - vary current through the coil

Question 27

Materials that act in a similar way to iron in a magnetic field are referred to as?

Answer 27

ferromagnetic

Question 28

Two examples of non-magnetic material?

Answer 28

air, wood

Question 29

Most practical way of varying the strength of an electromagnet?

Answer 29

vary the current through the coil

Question 30

What device connected in series with the electromagnet is normally used to do this?

Answer 30

rheostat

Question 31

why must electrical bus bars be securely braced?

Answer 31

Because strong magnetic forces occur between bus bars if there is a short

Question 32

In the dot-and-cross convention used to indicate the direction of current flow a ___________ represents a current coming out of the page at you?

Answer 32

dot

Question 33

The magnetic flux in a circuit is directly proportional to the ___________ and inversely proportional to the ___________ of the magnetic circuit?

Answer 33

Magnemotive force, reluctance

Question 34

Opposition to the formation of flux lines in magnetic circuit is called?

Answer 34

Reluctance

Question 35

The point at which all of the domains in the magnetic material have aligned themselves with the magnetic field is referred to as?

Answer 35

theoretical saturation

Question 36

Residual magnetism is a measure of the amount of __________ remaining in the magnetic material after the magnetizing force has been removed?

Answer 36

flux density

Question 37

The amount of reverse magnetizing force required to remove the residual magnetism is called the?

Answer 37

coercive force

Question 38

The type of core material used for lifting electromagnets would have a ___________ retentivity.

Answer 38

low

Question 39

The spreading out of the flux lines in an air gap is referred to as _____

Answer 39

fringing

Question 40

Magnetic cores that are constructed of alternating layers of magnetic material and electrical insulator are said to be ____________

Answer 40

laminated

Question 41

As the reluctance of a magnetic circuit is increased, the flux density of the circuit is ___________?

Answer 41

decreased

Question 42

if the magnemotive force applied to a magnetic circuit is increased, the flux density of the circuit is __________?

Answer 42

increased

Question 43

The ease with which flux is set up in a unit length and cross-section of a material is referred to as the _________?

Answer 43

permeability

Question 44

The term for the lag of flux density behind magnetizing intensity is __________?

Answer 44

hysteresis

Question 45

Magnetic cores are usually constructed from materials with a high ___________ and a low ___________ ?

Answer 45

permeability, reluctance

Question 46

The amount of energy lost due to the constant reversals of the magnetic field in AC circuits is referred to as ____________ loss

Answer 46

hysteresis

Question 47

What form does this energy loss normally take?

Answer 47

Heat