Magnetism and Electromagnetism

Question 1

What is magnetism?

Answer 1

A measurable characteristic of a moving electrical charge.

Question 2

There are three classifications of magnetic materials; what are these in order of magnetic ability?

Answer 2

1. Ferromagnetic materials (most magnetic - Iron)
2. Paramagnetic materials (sometimes magnetic - Aluminum)
3. Diamagnetic materials (never magnetic - Carbon)

Question 3

What is the difference between a permanent and temporary magnet?

Answer 3

A temporary magnet loses it’s magnetism quickly after it’s magnetising force is lost.

A permanent magnet will lose it’s magnetism after a much longer time.

Question 4

What is magnetic flux?

Answer 4

The lines of force of a magnetic field are collectively known as flux.

Symbol is Φ.

Question 5

What is flux density?

Answer 5

Flux density is the quantity of flux lines through a cross sectional area.

Flux density determines the strength of the magnetic field.

Symbol is ß.

Question 6

What is permeability?

Answer 6

The ease at which flux magnetizes a material.

Symbol is µ.

Question 7

What are the seven characteristics of magnetic flux?

Answer 7

Lines:

• Form closed loops
• Are directional
• Do not intersect
• Try to shorten themselves
• Repel each other
• Seek the path of least reluctance
• Can enter any material.

Question 8

What is reluctance?

Answer 8

The opposition of a material to accepting a magnetic path through itself.

Symbol is R_m.

Question 9

What is residual magnetism?

Answer 9

Leftover aligned magnetic domains after a magnetising force is removed.

Question 10

What is retentivity?

Answer 10

A material’s ability to retain magnetism.

Question 11

What is saturation?

Answer 11

When all magnetic domains in a material are aligned.

Any further application of a magnetising force will only produce waste.

Question 12

Which material is more reluctant, air or soft iron?

Answer 12

Air is.

Question 13

Magnetic flux flows from “_____”, outside of a magnet.

Answer 13

North pole to south pole.

Question 14

How would one use a left hand rule for coils?

Answer 14

• Thumb points to north pole.
• Fingers point in direction of electron flow.

Question 15

How would one use a left hand rule for conductors?

Answer 15

• Thumb points in the direction of electron flow.
• Fingers curl in the same way as the magnetic field produced (north to south).

Question 16

What are three factors affecting the strength of an electromagnet?

Answer 16

• The amount of current in the coil.
• The number of turns in the coil.
• The type of core used.

Question 17

What are the two most common types of Iron losses?

How are they reduced?

Answer 17

• Hysteresis losses: The flip flopping of magnetic domains in the core due to ac current, lost as friction heat.
  
  • Reduced with a silicon steel alloy core.
• Eddy current losses: Heat losses due to induced circulating current in the core.
  
  • Reduced by lamination of the core.

Question 18

What is Lenz’s law of induction?

Answer 18

Induced voltage will cause a current to flow and it’s magnetic effect will oppose the change which produced it.

Question 19

What is inductance symbolized by, and what is it measured in?

Answer 19

Symbolized by L.

Measured in Henrys (H).

Question 20

What is Flemming’s left hand rule?

Answer 20

• Thumb points in the direction of conductor motion.
• Firefinger points in the direction of magnetic field from north to south.
• Middle finger points in the direction of electron flow. (always perpendicular to forefinger)

Question 21

What is Faraday’s law of induction?

Answer 21

A voltage will be produced across a conductor which cuts across magnetic flux.

This voltage is proportional to the rate of cutting flux.

1 Volt is produced by cutting across 100 million lines of force in 1 second.

Question 22

What is the most noticeable effect of inductance?

Answer 22

Current takes longer to establish and dissipate.

Question 23

What is mutual induction?

Answer 23

When current through one coil magnetically induces a voltage on a coil mounted on the same core but not electrically connected.

Question 24

When both electrons in a pair of an atom are spinning in opposite directions, the net value of magnetism is…?

Answer 24

0.

Question 25

Which materials are added to high carbon steel to make permanent magnets?

Answer 25

• Cobalt
• Molybdenum
• Chrome-tungsten
• Alnico-5 (most common).