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M3 > 3.10 Magnetism > Flashcards

3.10 Magnetism Flashcards

1
Q
A
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2
Q

What has been the historical significance of magnetism?

A

Utilised since the invention of the magnetic compass during the Chinese Han dynasty, around 2000 years ago.

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3
Q

List some applications of permanent magnets.

A
  • Motors
  • Generators
  • Microphones
  • Loudspeakers
  • Hard drives
  • Sensors
  • Switches
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4
Q

Who began the discovery of the connection between electricity and magnetism?

A

Michael Faraday in 1831.

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5
Q

What is the major discipline that emerged from the study of electricity and magnetism?

A

Electromagnetism.

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6
Q

What is magnetism caused by?

A

The motion of electric charges.

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7
Q

What is the region in which magnetic effects are observed called?

A

Magnetic field.

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8
Q

Name three elements that are described as being magnetic.

A
  • Iron
  • Nickel
  • Cobalt
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9
Q

What happens to items containing magnetic elements when near permanent magnets?

A

They are attracted and can be magnetised.

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10
Q

Why do the magnetic fields of most elements cancel out?

A

Due to the many electrons present and their arrangements.

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11
Q

True or False: Each atom of a magnetic element acts like a small, permanent magnet.

A

True.

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12
Q

Fill in the blank: The understanding of electromagnetism is fundamental to the design and maintenance of _______.

A

[electrical components and systems].

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13
Q

What is the effect of the particular arrangements of electrons in magnetic elements?

A

They create a small, residual magnetic effect.

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14
Q

What is the Earth’s magnetic field also known as?

A

Geo-magnetic Field

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15
Q

How is the Earth’s magnetic field generated?

A

By a self-exciting dynamo process in the planet’s interior

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16
Q

What generates the magnetic field in the Earth’s outer core?

A

Electrical currents flowing in slowly moving molten iron

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17
Q

What is the process in the Earth’s outer core likened to?

A

A naturally occurring electrical generator

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18
Q

What causes the deviation of a compass from true north?

A

Pockets of magnets (Lodestones) in the crust or rocks

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19
Q

What is the angle called that represents the deviation of the compass from true north?

A

Declination or magnetic declination

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20
Q

What is a significant problem for aircraft relying on magnetic compasses?

A

The variation of magnetic declination with geographic location and time

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21
Q

If the Earth is pictured as a permanent magnet, which pole attracts the north pole of a compass?

A

The south pole of the Earth

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22
Q

What is the north magnetic pole?

A

The point at which the Earth’s magnetic field lines point vertically downwards

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23
Q

What happens when a permanent magnet is freely suspended?

A

It comes to rest with one end pointing towards the north pole of the Earth

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24
Q

What is the north-seeking pole of a permanent magnet?

A

The end that points towards the north pole of the Earth

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25
Q

What is the primary use of a magnetic compass?

A

To find the direction of magnetic north and therefore all other directions

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26
Q

What causes the compass needle to deviate from magnetic north?

A

Pockets of magnets and shifting magnetic fields

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27
Q

What does a magnetic compass contain that allows it to function?

A

A small, light magnet called a needle

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28
Q

What happens to the compass needle in the presence of a stronger magnetic field?

A

It aligns with the field around the stronger magnet

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29
Q

What are the three typical representations of the Earth’s magnetic fields?

A

A: Theoretical magnetic field with smooth lines, B: Kinks causing deviation, C: Snapshot of shifting magnetic fields

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30
Q

Fill in the blank: The deviation of the compass from true north is an angle called _______.

A

declination

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31
Q

What materials does a permanent magnet attract?

A

Magnetic materials containing one or more magnetic elements.

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32
Q

What is the force of attraction of a magnet strongest?

A

At each end (pole) of the magnet.

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33
Q

What happens when like poles of magnets interact?

A

They repel each other.

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34
Q

What happens when unlike poles of magnets interact?

A

They attract each other.

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35
Q

What is a plotting compass used for?

A

To map the direction of magnetic force around a magnet.

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36
Q

What do the lines produced by a plotting compass represent?

A

Lines of magnetic flux.

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37
Q

What direction does magnetic flux flow according to the model?

A

From north to south around the magnet.

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38
Q

What happens to the strength of the magnetic field where lines of magnetic flux are closer together?

A

The magnetic field is stronger.

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39
Q

What is the shape of the lines of magnetic flux?

A

Closed loops.

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40
Q

Where do lines of magnetic flux exit and enter a magnet?

A

Exit at the north pole and enter at the south pole.

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41
Q

What direction do lines of magnetic flux point externally?

A

Away from the north pole and towards the south pole.

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42
Q

Fill in the blank: Magnetic lines of flux always form _______.

A

closed loops.

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43
Q

True or False: Magnetic lines of flux can cross each other.

A

False.

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44
Q

What is the effect of magnetic lines of flux on each other?

A

They repel each other sideways.

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45
Q

What is the behavior of magnetic lines of flux in terms of their length?

A

They try to shorten themselves.

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46
Q

What path do magnetic lines of flux always take?

A

The path of least reluctance.

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47
Q

Is there a known insulator for magnetic lines of flux?

A

No known insulator exists.

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48
Q

What are Weiss domains?

A

Small magnets within materials where the magnetic fields of individual atoms are aligned in the same direction

Weiss domains are crucial in understanding how materials can become magnetised.

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49
Q

What happens to an iron bar when subjected to a strong magnetic field?

A

The magnetic domains align in the same direction, reinforcing each other’s magnetic fields

This results in the iron bar becoming magnetised and exhibiting a magnetic field.

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50
Q

What is a permanent magnet?

A

An iron bar that has been magnetised and exhibits a magnetic field

The alignment of Weiss domains contributes to its permanent magnetism.

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51
Q

List methods for magnetising materials.

A
  • Stroking with another magnet
  • Hammering in the direction of a magnetic field
  • Heating in a magnetic field
  • Passing an electric current through a coil (electro-magnet)

These methods utilize various physical principles to align the magnetic domains.

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52
Q

What occurs when a permanent magnet is broken into pieces?

A

Each piece acts as a smaller permanent magnet with its own north and south pole

This phenomenon illustrates that magnetism is a property of the material at a fundamental level.

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53
Q

What is demagnetisation?

A

The process of removing magnetism from a workpiece or tool

Demagnetisation may be necessary for various applications, especially in electronics.

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54
Q

How can demagnetisation be achieved?

A
  • Turning off the current of an electro-magnet
  • Slowly moving away from a strong alternating magnetic field (degaussing)
  • Using mechanical force (striking or dropping)
  • Heating beyond the Curie temperature

Each method affects the alignment of magnetic domains in different ways.

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55
Q

What is degaussing?

A

The process of slowly moving an item away from a strong alternating magnetic field to demagnetise it

This is often necessary for components after exposure to strong electromagnetic events like lightning strikes.

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56
Q

What is the Curie temperature for iron?

A

769 °C

Heating beyond this temperature may demagnetise iron, but magnetism can recur if the temperature falls below it.

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57
Q

What happens if you heat a magnetic material beyond its Curie temperature?

A

The magnetism may be lost, but it can recur if the temperature drops below the Curie temperature

Regular exposure to high temperatures can lead to permanent loss of magnetism.

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58
Q

Fill in the blank: The Curie temperature for cobalt is _______.

A

1075 °C

This temperature is significant for understanding the thermal stability of cobalt’s magnetic properties.

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59
Q

True or False: Nickel has a Curie temperature of 356 °C.

A

True

This temperature is essential for applications involving nickel in magnetic fields.

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60
Q

What range of Curie temperatures do some ferrites have?

A

50 – 600 °C

Ferrites exhibit varying thermal properties which can affect their magnetic behavior.

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61
Q

What is magnetic shielding?

A

Magnetic shielding is used to create a shield for a device or component that requires protection from magnetic interference.

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62
Q

What does a steel ring do in magnetic shielding?

A

The steel ring attracts the lines of magnetic flux to itself because it has high permeability.

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63
Q

What happens to components placed inside a steel ring in the context of magnetic shielding?

A

Components placed inside the ring should not be subjected to a magnetic field applied in the direction shown.

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64
Q

True or False: Nothing can stop magnetic flux from flowing from north to south.

A

True.

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65
Q

What is the principle behind magnetic shielding?

A

It redirects lines of magnetic flux using good conductors.

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66
Q

What are some examples of devices that may require magnetic shielding?

A

Electro-mechanical instruments such as ammeter and voltmeters.

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67
Q

What can affect electro-mechanical instruments in legacy aircraft?

A

Stray magnetic fields, such as the earth’s and fields created in cables when a current is passing through them.

68
Q

How can stray magnetic fields be mitigated in sensitive instruments?

A

By encasing them in a metal case.

69
Q

What two materials are compared in terms of reluctance in magnetic screening?

A

Air and soft iron.

70
Q

Fill in the blank: Air has a high _______ while soft iron has a low _______.

A

reluctance.

71
Q

What happens to magnetic flux when equipment is surrounded by soft iron?

A

Most of the flux will pass through the soft iron, rather than the air inside it.

72
Q

What is reluctance analogous to?

A

Resistance.

73
Q

What property does soft iron exhibit when under the influence of a magnetizing force?

A

Soft iron readily magnetizes and demagnetizes.

74
Q

What are the three types of magnetic materials?

A

Ferromagnets, Paramagnets, Diamagnets

75
Q

What is a characteristic of ferromagnetic materials?

A

Readily magnetises and easily orient their electron spins to an external magnetic field

76
Q

What distinguishes soft ferromagnetic materials from hard ferromagnetic materials?

A

Soft materials become demagnetised spontaneously; hard materials retain their magnetism

77
Q

List some applications of ferromagnetic materials.

A
  • Electric motors
  • Generators
  • Transformers
  • Telephones
  • Loudspeakers
  • Magnetic stripes on credit cards
78
Q

Name common examples of ferromagnetic materials.

A
  • Iron
  • Cobalt
  • Nickel
  • Metallic alloys
  • Rare earth magnets
79
Q

What is magnetite?

A

A ferromagnetic material formed by the oxidation of iron into an oxide

80
Q

What is the Curie temperature?

A

The maximum temperature where the ferromagnetic property disappears due to thermal agitation

81
Q

What is the Curie temperature of iron?

A

About 1043 K

82
Q

What is hysteresis in ferromagnetic materials?

A

The tendency to remember their magnetic history after an external magnetic field is removed

83
Q

What is remanence?

A

The fraction of saturation magnetisation retained when the driving field is removed

84
Q

What is magnetostriction?

A

The mechanical response of ferromagnetic materials to an impressed magnetic field

85
Q

What are paramagnetic materials?

A

Materials with a small and positive susceptibility to magnetic fields

86
Q

List some examples of paramagnetic materials.

A
  • Aluminium
  • Titanium
  • Oxygen
  • Iron oxide
87
Q

What occurs to paramagnetic materials when the external magnetic field is removed?

A

They do not retain their magnetic properties

88
Q

What is Curie’s law in relation to paramagnetic materials?

A

Magnetic properties decrease as temperature increases at a constant external magnetic field

89
Q

What are diamagnetic materials?

A

Materials that are repelled by a magnetic field

90
Q

Give examples of diamagnetic materials.

A
  • Water
  • Plastics
  • Gold
  • Copper
  • Mercury
91
Q

How can you determine if an object is diamagnetic or paramagnetic?

A

By observing how it aligns itself in a magnetic field

92
Q

What causes the repulsion in diamagnetic materials?

A

Induced magnetic field occurs in the opposite direction of the external magnetic field

93
Q

What distinguishes diamagnetic materials from paramagnetic and ferromagnetic materials?

A

Diamagnetic materials have all paired electrons, while others have unpaired electrons

94
Q

What is an electromagnet?

A

A type of magnet where the magnetic field is produced by an electric current.

95
Q

What happens to the magnetic field of an electromagnet when the current is turned off?

A

The magnetic field is not present.

96
Q

What are the main components of an electromagnet?

A
  • Insulated wire wound into a coil
  • A magnetic core made from ferromagnetic or ferrimagnetic material
97
Q

What is a ‘turn’ in the context of an electromagnet?

A

Each loop of the coil.

98
Q

What is the advantage of an electromagnet over a permanent magnet?

A

The magnetic field can be quickly changed by controlling the amount of electric current.

99
Q

Does an electromagnet require a power supply?

A

Yes, it requires a continuous supply of electric current.

100
Q

List some uses of electromagnets in aircraft systems.

A
  • Motors
  • Generators
  • Relays
  • Loudspeakers
  • Hard-disc drives
101
Q

Why is it difficult to obtain very large strength magnetic fields with permanent magnets?

A

The material becomes magnetically saturated.

102
Q

What is the principle of electromagnetism?

A

A magnetic field is created by passing an electric current through a wire.

103
Q

Describe the shape of the magnetic field produced by a straight conductor.

A

Circular and concentric to the conductor.

104
Q

What happens to iron filings sprinkled around a conductor when current passes through it?

A

The filings take up a shape indicating the magnetic field.

105
Q

What indicates the strength of a magnetic field?

A

The concentration of the lines of flux; tighter lines indicate a stronger field.

106
Q

What three components are required to create an electromagnet?

A
  • A conductor (usually a helical coil)
  • An electrical supply and associated circuitry
  • A core to increase strength and efficiency
107
Q

What occurs when the switch of a solenoid is closed?

A

The current flows through the coil, producing a magnetic field around the conductor.

108
Q

Fill in the blank: An electromagnet is sometimes known as a _______.

A

[solenoid]

109
Q

True or False: Electromagnets do not require any external power supply.

A

False.

110
Q

What is generated around a conductor when an electric current flows through it?

A

A magnetic field

Every electric current produces a magnetic field.

111
Q

What happens to a compass needle placed near a wire carrying a current?

A

It is deflected due to the magnetic field

If the current is sufficiently large, the compass needle comes to rest in a direction perpendicular to the wire.

112
Q

What is the Corkscrew Rule also known as?

A
  • Maxwell’s Right Hand Thumb Rule
  • Right Hand Grasp Rule
  • Right Hand Grip Rule
  • Corkscrew Rule

It predicts the direction of the magnetic field around a current-carrying conductor.

113
Q

How does the Corkscrew Rule predict the direction of the magnetic field?

A

By visualizing the corkscrew being turned in a clockwise direction

This clockwise direction indicates the magnetic field direction when viewed from above.

114
Q

In the Corkscrew Rule, what does the direction of the corkscrew’s movement represent?

A

The direction of current flow in the conductor

The corkscrew moves away from the observer as the current flows.

115
Q

How is the direction of current flow in a conductor typically represented?

A

As an arrow

A cross represents current flowing into the page, while a dot represents current flowing out of the page.

116
Q

What is the Hand Clasp Rule used for?

A

To determine the direction of the magnetic field lines and current around a straight current-carrying conductor, solenoid, or coil inductor

It utilizes the right hand to show the relationship between current and magnetic field direction.

117
Q

In the Hand Clasp Rule, what does the thumb represent?

A

The direction of current

The curled fingers show the direction of the magnetic field lines of force.

118
Q

What does the Right Hand Rule for Determining Polarity of a Solenoid indicate?

A

The direction of current flow determines the polarity of the coil

Clockwise current flow results in a South Pole at one end and an North Pole at the other end.

119
Q

What happens to the polarity of a coil if the current flows anti-clockwise?

A

The end of the coil becomes the North Pole and the other end becomes the South Pole

This is determined when facing the coil’s end.

120
Q

What is magnetomotive force (mmf)?

A

A measure that causes a magnetic flux in a circuit, analogous to electric current

mmf is produced by current flowing through a coil and is often expressed in ampere-turns (At)

121
Q

How is magnetomotive force calculated?

A

mmf = Current x Number of Turns (At)

F = I x N, where I is the current in amps and N is the number of turns

122
Q

What is the SI unit of magnetomotive force?

A

Ampere (A)

Although often expressed as ampere-turns (At), this does not conform to SI standards

123
Q

What is magnetic field strength (H)?

A

A measure of the intensity of magnetic effects at any point in the magnetic field

Defined as mmf per unit length, measured in ampere-turns per metre (At/m)

124
Q

How is magnetic field strength calculated?

A

H = IN/l

Where I is the current, N is the number of turns, and l is the length of the magnetic circuit in metres

125
Q

What is the relationship between current flow and magnetic field strength?

A

Magnetic field strength is proportional to current flow

If current increases, magnetic field strength increases and vice versa

126
Q

What is magnetic flux (Φ)?

A

The total magnetic flux in a magnetic circuit, comparable to current flow in an electric circuit

Once established, maintaining flux does not require energy dissipation

127
Q

How is magnetic flux calculated?

A

Φ = B x A

Where B is the flux density and A is the cross-sectional area of the magnetic circuit

128
Q

What is the SI unit of magnetic flux?

A

Weber (Wb)

129
Q

What is magnetic flux density (B)?

A

The amount of flux per unit cross-sectional area in a magnetic field

It is analogous to current density in an electric circuit

130
Q

How is magnetic flux density calculated?

A

B = Φ / A (T)

Where Φ is magnetic flux and A is the area in m²

131
Q

What is permeability (μ)?

A

The ratio of flux density (B) to magnetic field strength (H)

In free space, represented by μo and valued at 4π x 10^-7 (1.257 x 10^-6) in SI units

132
Q

What effect does inserting an iron core into a coil have on flux density?

A

Increases the flux density

This is due to the higher permeability of the core material

133
Q

What is relative permeability (μr)?

A

The ratio of flux density of the core to flux density of air

The relative permeability of air is considered to be one

134
Q

How is absolute permeability (μ) defined?

A

μ = μoμr

Where μo is the permeability of free space and μr is the relative permeability of the medium

135
Q

True or False: Magnetic flux density weakens with increasing distance from a straight current-carrying wire.

A

True

136
Q

What is the relationship between magnetic flux density and current in the vicinity of a current-carrying wire?

A

Magnetic flux density is directly proportional to the current in amperes

137
Q

What does the hysteresis loop show?

A

The relationship between the applied magnetising force H and the resulting flux density B over one complete cycle of alternating current.

138
Q

What happens to flux density B when the core is saturated?

A

It cannot conduct any more flux, resulting in a plateau in the graph.

139
Q

What is residual magnetism?

A

The magnetism that remains in a material after the applied field drops to zero.

140
Q

Define magnetic remanence.

A

The residual magnetism represented by the value Br, which indicates the internal flux density after the magnetising force is removed.

141
Q

What is coercive field strength Hc?

A

The amount of applied field needed to reduce the flux density B to zero after achieving magnetic remanence.

142
Q

What is the significance of the saturation point?

A

It is the point at which increasing the magnetic field no longer increases magnetisation.

143
Q

What is permeability?

A

The ratio of Flux Density (B) to the magnetising force (H).

144
Q

What is the standard value of permeability of free space?

A

4π x 10^-7 H/m.

145
Q

What does retentivity measure?

A

The ability of a material to retain a certain amount of residual magnetic field after the magnetising force is removed.

146
Q

What is coercive force?

A

The amount of reverse magnetic field needed to return a material’s magnetic flux to zero.

147
Q

Define reluctance.

A

The opposition that a ferromagnetic material shows to the establishment of a magnetic field.

148
Q

What is the unit of reluctance?

A

At/Weber.

149
Q

What are eddy currents?

A

Loops of electrical current induced within conductors by a changing magnetic field.

150
Q

Fill in the blank: Eddy currents flow in closed loops, along planes ______ to the magnetic field.

A

perpendicular

151
Q

What effect do eddy currents have according to Lenz’s law?

A

They create a magnetic field that opposes the magnetic field that created them.

152
Q

True or False: Eddy currents can only be induced in moving conductors.

A

False

153
Q

What is a key application of eddy currents?

A

Induction heating furnaces.

154
Q

How can eddy currents be minimized in machinery?

A

By using laminated magnetic cores.

155
Q

What does the area inside the hysteresis curve represent?

A

The energy lost from the circuit per unit volume of the core, mainly as heat.

156
Q

What characterizes magnetically hard materials?

A

They are difficult to magnetise and demagnetise.

157
Q

What characterizes magnetically soft materials?

A

They are easy to magnetise and demagnetise.

158
Q

What type of materials are strong permanent magnets made from?

A

Magnetically hard materials

159
Q

What happens to the magnetic field strength of permanent magnets if they are struck or dropped?

A

It is reduced

160
Q

What should be avoided to prevent damage to permanent magnets?

A

Intense vibration

161
Q

Why should permanent magnets be kept away from external magnetic fields?

A

To prevent demagnetisation

162
Q

What is the maximum temperature that can be tolerated by magnets before reduction in magnetic flow occurs?

A

250°C

163
Q

What happens to magnets at temperatures higher than 250°C?

A

Permanent demagnetisation occurs

164
Q

What can be done to revert the magnetic properties of a magnet after reducing its temperature?

A

Decreasing the temperature will revert it to its previous state

165
Q

What must not be changed when storing magnets long-term?

A

The magnetic circuit

166
Q

What are ‘keepers’ in relation to magnet storage?

A

Ferromagnetic components connected between the poles of the magnet

167
Q

What is the purpose of using keepers when storing magnets?

A

To allow lines of flux to flow through them

M3 (19 decks)

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