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Unit 5 Physics > P2 > Flashcards

P2 Flashcards

(68 cards)

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1
Q

What is the main area of energy management related to temperature control?

A

Cooling or heating spaces to the desired temperature

Reverse cycle vapour compression is a cost-effective method for this.

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2
Q

What are the two main types of systems used for temperature control?

A

Refrigerators and heat pumps

Both use similar basic equipment.

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3
Q

What is the primary function of refrigerators?

A

Cool an enclosed space and reject heat outside.

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4
Q

How do heat pumps function differently from refrigerators?

A

Draw in heat from the ground or outside air and move it into a room or hot water tank.

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5
Q

What distinguishes refrigerators and heat pumps?

A

The positioning of the heat exchangers and which space is being temperature controlled.

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6
Q

What is the role of the expansion valve in refrigeration systems?

A

Provides good temperature control but does not recover the small amount of work done during expansion.

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7
Q

Define Coefficient of Performance (CoP).

A

The inverse of thermal efficiency with values greater than 1.

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8
Q

In terms of CoP, what does ‘heat moved, Q’ represent?

A

What is ‘useful’.

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9
Q

In terms of CoP, what does ‘work input, W’ signify?

A

The cost.

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10
Q

What is the maximum theoretical CoP value for refrigeration?

A

Qin/W = Td/(T - T).

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11
Q

What is the maximum theoretical CoP value for heat pumps?

A

Qout/W = TH/(TH - Td).

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12
Q

How can you improve the actual CoP value?

A

Minimise temperature gap (TH - Td)

This can be done by using oversize heat exchangers and tapping large steady heat sources.

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13
Q

List some common refrigerants.

A
  • Chlorofluorocarbons (CFCs)
  • Alkanes
  • Ammonia
  • Carbon dioxide.
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14
Q

Why are high latent heats of vaporisation/condensation beneficial?

A

They keep heat transfer processes isothermal and near reversible.

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15
Q

What is a good working fluid for heat engines?

A

Water/steam.

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16
Q

What should be considered when choosing refrigerants for refrigerators and heat pumps?

A

Boiling and freezing points must match with system temperatures.

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17
Q

Fill in the blank: The heat pump takes heat from a ground coil at ______.

A

8°C.

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18
Q

Describe one way to adapt supermarket cooling systems to utilize waste heat.

A

Position heat exchangers to capture waste heat for heating the shop.

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19
Q

What is elastic behaviour?

A

The ability of a material to spring back to its original shape and size after being stretched, squashed or otherwise distorted.

Elastic behaviour is a fundamental property of materials in physics.

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20
Q

What does Hooke’s law state?

A

Force, F, is proportional to extension, represented as F = kAx, where k is a constant.

Hooke’s law applies to stretching (tension) or squeezing (compression) of materials.

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21
Q

What are the key points of the tensile stress-strain curve for a non-ferrous metal?

A
  • Original state
  • Limit of proportionality
  • Elastic limit (yield point)
  • Increased yield strength after plastic deformation
  • Failure (UTS)

UTS stands for Ultimate Tensile Strength.

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22
Q

What does the term ‘strain’ represent?

A

Strain, ε = Δx/L

Strain is a dimensionless ratio representing deformation relative to original length.

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23
Q

What is the formula for tensile stress?

A

Stress = Force / Cross-sectional area

Tensile stress is a measure of the internal forces in a material.

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24
Q

What is Young’s modulus (elastic modulus)?

A

E = stress / strain

Units are N m-2 or Pa, and it measures a material’s stiffness.

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25
What is the elastic limit (yield strength)?
The highest tensile stress with full elastic recovery; beyond this point, the material yields to give a permanent plastic deformation. ## Footnote Hooke's law and Young's modulus fail to apply shortly before this point.
26
What is the work done (energy stored) in a spring represented by?
W = 1/2 * F * Δx = 1/2 * k * Δx² ## Footnote This formula is derived from the area under the F vs. Δx graph.
27
What is elastic hysteresis?
Elastic hysteresis occurs in materials like rubber, characterized by internal friction during stretching and recovery. ## Footnote Energy loss due to internal friction is represented by the area enclosed by the hysteresis loop.
28
What happens to work done during elastic hysteresis?
Work turns to heat due to internal friction. ## Footnote This property makes materials useful for cushioning shock and damping oscillations.
29
Calculate the maximum tensile stress for a structural steel rod with a yield strength of 250 MPa.
250 MPa ## Footnote This is the maximum stress the rod can carry without permanent distortion.
30
Fill in the blank: The formula for strain is _______.
ε = Δx/L
31
What does 'plastic' mean in materials science?
'Plastic' means 'able to be formed'. It describes materials experiencing a permanent change in shape without completely breaking up, after they pass the elastic limit. ## Footnote 'Plastic' deformation occurs when stress exceeds the yield strength.
32
What happens during plastic deformation?
When the stress exceeds the yield strength, the crystalline structure of metals allows layers of atoms to glide over one another, resulting in a permanent set. ## Footnote This deformation remains even after the stress is removed.
33
What factors affect the amount of plasticity in materials?
The amount of plasticity depends on the microcrystalline structure of the metal, which varies between materials. ## Footnote Most non-metals show little or no plasticity and are considered brittle.
34
What is ductility?
Ductility is the ability to be shaped by plastic flow under tension. ## Footnote Examples include drawing into rods, wires, or tubes and deep drawing of sheet material into bowls.
35
How does temperature affect ductility?
Ductility depends on temperature; if cooled sufficiently, even metals can become brittle.
36
What is malleability?
Malleability is the ability to be shaped by plastic flow under compression. ## Footnote Examples include rolling or hammering into sheets and cold forming by stamping or pressing.
37
Can a material be malleable but not ductile? Give an example.
Yes, some metals, like lead, are malleable but not ductile.
38
What is required for producing stainless steel tubes?
Stainless steel tubes are produced by drawing through a die, a tensile process that requires ductility.
39
How are cap heads of steel screws produced?
Cap heads of steel screws are produced by drop forging, a compressive process requiring malleability.
40
What processes are used to produce lead sheet?
Lead sheet is produced by hammering and rolling, which are compressive processes that depend on malleability.
41
What does Young's modulus measure?
Young's modulus measures the stiffness of a material.
42
What is ultimate tensile strength (UTS)?
Ultimate tensile strength (UTS) is the maximum stress that a material can withstand while being stretched or pulled before necking.
43
What is creep in materials?
Creep is the slow, permanent deformation of materials under constant stress over time, especially at high temperatures.
44
What does 'plastic' mean in materials science?
'Plastic' means 'able to be formed'. It describes materials experiencing a permanent change in shape without completely breaking up, after they pass the elastic limit. ## Footnote 'Plastic' deformation occurs when stress exceeds the yield strength.
45
What happens during plastic deformation?
When the stress exceeds the yield strength, the crystalline structure of metals allows layers of atoms to glide over one another, resulting in a permanent set. ## Footnote This deformation remains even after the stress is removed.
46
What factors affect the amount of plasticity in materials?
The amount of plasticity depends on the microcrystalline structure of the metal, which varies between materials. ## Footnote Most non-metals show little or no plasticity and are considered brittle.
47
What is ductility?
Ductility is the ability to be shaped by plastic flow under tension. ## Footnote Examples include drawing into rods, wires, or tubes and deep drawing of sheet material into bowls.
48
How does temperature affect ductility?
Ductility depends on temperature; if cooled sufficiently, even metals can become brittle.
49
What is malleability?
Malleability is the ability to be shaped by plastic flow under compression. ## Footnote Examples include rolling or hammering into sheets and cold forming by stamping or pressing.
50
Can a material be malleable but not ductile? Give an example.
Yes, some metals, like lead, are malleable but not ductile.
51
What is required for producing stainless steel tubes?
Stainless steel tubes are produced by drawing through a die, a tensile process that requires ductility.
52
How are cap heads of steel screws produced?
Cap heads of steel screws are produced by drop forging, a compressive process requiring malleability.
53
What processes are used to produce lead sheet?
Lead sheet is produced by hammering and rolling, which are compressive processes that depend on malleability.
54
What does Young's modulus measure?
Young's modulus measures the stiffness of a material.
55
What is ultimate tensile strength (UTS)?
Ultimate tensile strength (UTS) is the maximum stress that a material can withstand while being stretched or pulled before necking.
56
What is creep in materials?
Creep is the slow, permanent deformation of materials under constant stress over time, especially at high temperatures.
57
Why is understanding how and why materials fail important?
It is fundamental to securing safety and reliability ## Footnote Understanding material failure is crucial for engineering and safety applications.
58
What is ultimate tensile strength (UTS)?
The highest stress a material can sustain, just before it breaks ## Footnote UTS is a key measure of material performance.
59
What is yield strength in ductile materials?
The stress at which plastic deformation starts ## Footnote Below yield strength, a material can return to its original shape.
60
How do brittle materials differ from ductile materials regarding deformation?
Brittle materials only have a UTS and no yield point or plastic deformation ## Footnote Examples include concrete and glass.
61
What triggers brittle failure in materials?
Stress concentration at the tips of tiny imperfections and cracks ## Footnote Cracks grow rapidly, leading to catastrophic failure.
62
What happens to most ductile materials as they are worked?
They harden until stress levels become high enough to cause brittle failure ## Footnote This process can alter the material's properties significantly.
63
What is creep in materials?
A slow version of plastic deformation that increases with temperature ## Footnote Creep can lead to failure if components no longer fit or neck in.
64
What causes fatigue in materials?
Repeated cycles of loading and unloading ## Footnote Fatigue leads to gradual hardening and crack growth.
65
At what stress levels does fatigue failure occur?
At stress levels well below the normal UTS value ## Footnote This makes fatigue a critical consideration in material design.
66
What are the operating conditions for turbine blades?
High centrifugal forces and high temperatures ## Footnote These conditions contribute to material stress and potential failure.
67
What are common factors affecting the wings, tail, and fuselage of aircraft?
Repeated high stresses and vibrations ## Footnote These factors can lead to material fatigue and eventual failure.
68
What is the temperature condition for fan blades in turbines?
Low temperatures ## Footnote This is in contrast to the high temperatures experienced by other turbine components.

Unit 5 Physics (3 decks)

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