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Electrical Foundation > Magnetism and Magnetic Induction > Flashcards

Magnetism and Magnetic Induction Flashcards

1
Q

The angular difference between true north and magnetic north is known as the ________

A

Angle of declination

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2
Q

________ are magnets that do not require any power or force to maintain their field

A

Permanent magnets

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3
Q

________ are metals that are easily magnetized, such as iron, nickel, and cobalt

A

Ferromagnetic materials

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4
Q

________ are metals that can be magnetized, but not as easily as ferromagnetic materials

A

Paramagnetic materials

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5
Q

True or false: diamagnetic materials are either metal or nonmetal materials that cannot be magnetized

A

True

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6
Q

Name 3 examples of paramagnetic materials

A

Platinum, titanium, and chromium

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7
Q

Name 4 examples of diamagnetic materials

A

Copper, brass, wood, and antimony

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8
Q

Name 4 characteristics of magnetic lines of force

A
  1. They form closed loops
  2. They posses direction
  3. They repel each other
  4. They cannot intersect or cross
  5. Magnetic lines of force represent tension along their length which tends to make them as short as possible
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9
Q

Flux lines are ________ near the poles of a magnet and become ________ as they move further away

A

Stronger; weaker

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10
Q

Complete the following: A basic law of physics states that whenever an electric current flows through a conductor, ________

A

A magnetic field is formed around the conductor

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11
Q

________ depend on an electric current flow to produce a magnetic field

A

Electromagnets

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12
Q

________ is a measure of a material’s ability to become magnetized

A

Permeability

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13
Q

________ is the property that opposes magnetic flux

A

Reluctance

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14
Q

________ occurs when all the molecules of the magnetic material are lined up

A

Saturation

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15
Q

The amount of magnetism left in a material after the magnetizing force has stopped is called _________

A

Residual magnetism

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16
Q

________ is the amount of magnetizing force necessary to remove residual magnetism from a material

A

Coercive force

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17
Q

________ is the lagging of the flux density behind the magnetizing force

A

Hysteresis

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18
Q

When looking at a material’s magnetization curve (hysteresis loop), curves that are tall and narrow are best suited for ________

A

Electromagnets. A tall and narrow magnetization loop indicates that the material has a high permeability and low retentivity value

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19
Q

A wide and relatively flat curve on a B-H graph indicates that the material would be well suited for use as a ________

A

Permanent magnet. A wide and flat curve indicates that the material has a high level of residual magnetism (retentivity) and would require a strong coercive force to remove this magnetism

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20
Q

________ refers to the amount of energy lost due to the constant reversal of the magnetic field in an AC circuit

A

Hysteresis loss

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21
Q

Early natural magnets were known as ________

A

Lodestones

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22
Q

True or false: The direction of current flow through the wire determines the polarity of the magnetic field

A

True

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23
Q

Name 3 methods used to demagnetize an object

A

Placing the object in an AC magnetic field and pulling it away, striking it, and heating it (Curie temperature)

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24
Q

In order for a material to lose it’s magnetism by heat, it must reach a certain temperature known as ________

A

Curie temperature

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25
Q

True or false: a piece of magnetized material has its molecules in a state of disarray

A

False. This describes a non-magnetized material. Magnetized materials have their molecules aligned in an orderly pattern

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26
Q

A helically wound coil that produces a strong magnetic field is known as a ________

A

Solenoid

27
Q

What is the easiest and most practical method for varying the strength of an electromagnet

A

Vary the current. Increasing the current will strengthen the electromagnet, decreasing the current will weaken it

28
Q

A device known as a ________ is used when storing magnets to help prevent their magnetic field from weakening over time

A

Keeper

29
Q

________ is a term that refers to the path along which flux lines follow as they form their complete loops

A

Magnetic circuit

30
Q

________ refers to the total number of flux lines in a magnetic circuit

A

Magnetic flux (Q)

31
Q

Current in an electrical circuit is the equivalent of ________ in a magnetic circuit

A

Magnetic flux (Q)

32
Q

The ________ is the unit of magnetic flux and is equal to ________ flux lines

A

Weber, abbreviated as Wb; 10 to the power of 8 (100,000,000)

33
Q

________ is the force that causes the setting up of flux lines in a magnetic circuit

A

Magnetomotive force (Fm)

34
Q

The magnetomotive force in a magnetic circuit is equal to ________

A

The number of coil turns in the circuit times the current through the coils (Fm = N x I)

35
Q

The ________ is the unit of magnetomotive force

A

Ampere-turn, abbreviated as A.t.

36
Q

________ is the opposition to the setting up of flux lines in a magnetic circuit

A

Reluctance (Rm)

37
Q

Resistance in an electrical circuit is the equivalent of ________ in a magnetic circuit

A

Reluctance (Rm)

38
Q

The ________ is the unit of reluctance

A

Ampere-turn per weber, abbreviated as A.t./Wb

39
Q

True or false: reluctance in a magnetic circuit increases as the length of the circuit increases, and decreases as the cross-sectional area of the circuit increases

A

True

40
Q

________ is the measure of the amount of flux per unit cross section of a magnetic circuit

A

Flux density (B)

41
Q

The ________ is the unit of flux density

A

Tesla, abbreviated as T

42
Q

________ is the measure of the magnetomotive force per unit length in a magnetic circuit

A

Magnetizing force (H)

43
Q

The ________ is the unit of magnetizing force

A

Ampere-turn per metre, abbreviated as A.t./m

44
Q

________ is the measure of the ease with which flux may be set up in a unit length and cross section of a material

A

Permeability (U)

45
Q

The ________ is the unit of permeability

A

Weber per ampere-turn metre, or Henries per metre, abbreviated as Wb/A.t.m. or H/m

46
Q

The permeability of free space (air) is ________

A

4 pi x 10 to the power of -7

47
Q

When a coil is wound around a nonmagnetic material such as wood or plastic, it is know as a(n) ________. When a coil is wound around a magnetic material such as iron or soft steel, it is known as a(n) ________

A

Air-core magnet; iron-core magnet

48
Q

Like residual magnetism, another term that is used to describe a material’s ability to retain magnetism is ________

A

Retentivity

49
Q

Complete the following statement: whenever a conductor cuts through magnetic lines of flux, ________

A

A voltage is induced in the conductor

50
Q

Complete the following statement: the polarity of an induced voltage is determined by the ________

A

Polarity of the magnetic field in relation to the direction of movement

51
Q

The left-hand rule for current-carrying conductors states that the thumb points in the direction of ________ and the fingers indicate the direction of the ________

A

Electron flow (current); magnetic field

52
Q

The left-hand rule for current-carrying coils states that the thumb points to the ________ and the fingers indicate the __________

A

North pole; direction of electron flow (current)

53
Q

Name the 3 factors that determine the amount of voltage induced in a conductor

A
  1. The number of turns of wire
  2. The strength of the magnetic field (flux density)
  3. The speed of the cutting action
54
Q

In order to induce 1 volt in a conductor, the conductor must cut ________ lines of magnetic flux in ________

A

100,000,000; 1 second (1 Wb per second)

55
Q

Lenz’s law states that an induced voltage or current ________

A

Opposes the motion that causes it

56
Q

True or false: inductors always oppose a change of current

A

True

57
Q

One volt is induced in a conductor when magnetic lines of flux are cut at a rate of ________

A

1 weber per second

58
Q

True or false: induced voltage is always opposite in polarity to the applied voltage

A

True

59
Q

Two conductors each carrying a current traveling in the same direction will create magnetic fields that ________ one another

A

Attract

60
Q

An exponential curve is divided into ________ time constants, with each being equal to ________ of some value

A

Five; 63.2%

61
Q

Inductance is measured in units called ________

A

Henrys (H)

62
Q

The amount of inductance an inductor will have is determined by the ________ and the ________

A

Number of turns of wire; physical construction of the coil

63
Q

According to Faraday’s law of induction, ________ are loops of electrical current induced within conductors by a changing magnetic field in the conductor

A

Eddy currents

Electrical Foundation (12 decks)

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