Magnetism D3 Review

Question 1

What is reluctance?

Answer 1

The opposition to the setting up of lines of flux.

Question 2

When selecting a core material for an AC electro-magnet, what two characteristics of the material should be considered when selecting the best material to be used?

Answer 2

A high electrical resistance ( the opposition to current flow in an AC or DC cct) and a low retentivity (the material’s ability to retain magnetism after the magnetism force has been removed).

Question 3

What is the reason for using laminated cores?

Answer 3

To reduce the eddy current loses

Question 4

What is a B-H graph used for?

Answer 4

To identify an electro-magnet core permeability (the measurement of a material’s ability to conduct magnetic lines of flux. The standard is air with a value of 1)

Question 5

In magnetism, what is the ‘Ohms Law’ equivalent for magnetic circuits?

Answer 5

Flux = magnetomotive force / reluctance

Amps = electromotive force / resistance

Question 6

What determines an electromagnets North and South polarity?

Answer 6

The polarity of the source

Question 7

Given the choice between hard steel, soft iron, copper or aluminium, which would be the best choice for a magnetic screen?

Answer 7

Soft iron because of its low reluctance (resistance to magnetism)

Question 8

When the air gap is increased in a magnetic circuit, what happens to the flux density (the number of magnetic lines contained in a certain area)?

Answer 8

Decreases

Question 9

Define coercive force

Answer 9

The magnetizing force to overcome residual magnetism (the amount of magnetism left in an object AFTER the magnetizing force has been removed).

Question 10

What if affected, if anything, if a hard iron core is replaced with an aluminium core in an electromagnet?

Answer 10

The flux (magnetic lines of force) weakens

Question 11

When two parallel conductors, in closed proximity, are carrying currents in the same direction, they would X each other.

Answer 11

Attract

Question 12

When related to an electric circuit, what term would permeability (measurement of a material’s ability to conduct magnetic lines of flux) be closest compared to?

Answer 12

Conductivity ( the measure of the ability of the material to conduct an electrical current).

Question 13

What is the term that relates to the fact that the flux density lags behind the magnetizing force?

Answer 13

Hysterisis

Hysteresis loss: Power loss in a conductive material caused by molecular friction. Hysteresis loss is proportional to frequency.

Question 14

Materials that exhibit the same features as iron in a magnetic field are referred to as X materials

Answer 14

Ferromagnetic materials. A material that will conduct magnetic lines of force easily such as iron. Ferromagnetic materials have a permeability much greater than air (1).

Question 15

Define reluctance

Answer 15

The opposition to the setting up lines of flux.

Question 16

When selecting a core material for an AC electro-magnet, what two characteristics of the material should be considered when selecting the best material to be used?

Answer 16

1- High electrical reluctance; the higher resistance, the lower the current will flow

2- Low retentivity; so that after removing the electric current to the substance doesn’t get magnetized and return to its original state.

Question 17

What is the reason for using laminated cores?

Answer 17

To reduce the eddy current loses; by making the core of a stack of plates electrically insulted from each others, rather than a solid block.

All transformers operating at low frequency use laminated or similar cores.

Question 18

What is used the B-H for?

Answer 18

To calculate an electromagnet core permeability by establishing the relationship between B ( flux density) and H( Magnetizing force) to determine the permeability ( ‘mu’).

Question 19

What determines an electromagnets north and South polarity?

Answer 19

• The polarity of the source
• The way the magnet wire is wound around the core

Question 20

Given the choice between hard steel, soft iron, copper or aluminium, which would be the best choice for a magnetic screen?

Answer 20

Soft iron; because of its low reluctance (easily magnetized materials)

Question 21

When the air gap is increased in a magnetic circuit, what happens to the flux density?

Flux density: : the electric flux passing through a unit area perpendicular to the direction of the flux.

Answer 21

Decreases

Question 22

What is affected, if anything, if a hard iron core is replaced with an aluminum core in an electromagnet

Answer 22

The flux weakens

Question 23

When two parallel conductors, in close proximity, are carrying in the same direction, they would X each other.

Answer 23

Attract each other

Question 24

When related to an electric circuit, what term would permeability be closest compared to?

Answer 24

Conductivity

Ie How much voltage is required to get an amount of electric current flow.

Question 25

What is the term that relates to the fact that the flux density lags behind the magnetizing force?

Answer 25

Hysteresis

It results in a power loss as heat in many electrical machines.

Question 26

List five characteristics of magnetic lines of force.

Answer 26

• Flux lines are directional; North to south ( via the air) and South to North ( inside the conductor)
• Flux lines do not cross each other
• Flux lines tend to be as short as possible
• Flux lines form a complete loop
• Flux lines in the same direction attract each other.

Question 27

Materials that exhibit the same features as iron in a magnetic field are referred to as X materials.

Answer 27

Ferromagnetic materials

Ie: those that exhibit strong magnetic effects.

Examples: iron, nickel, cobalt and permanent magnetic called neodymium magnets.