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Physics > 5-Choosing Materials > Flashcards

5-Choosing Materials Flashcards

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1
Q

What is the definition of stress?

A

The force per unit cross-sectional area when opposing forces act on a body.

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2
Q

What is the definition of strain?

A

Extension per unit length due to applied stress

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3
Q

What is the definition of the Young’s Modulus?

A

Stress / Strain

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4
Q

What is the Young’s Modulus a measure of?

A

The stiffness

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5
Q

What is the definition of a crystal?

A

A solid in which atoms are arranged in a regular array

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6
Q

What does Crystalline mean?

A

A solid consisting of crystals which are arranged randomly.

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7
Q

What does amorphous mean?

A

Where the atoms are arranged randomly

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8
Q

What is a polymeric solid?

A

A solid made up of chain-like molecules

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9
Q

What is a ductile material?

A

A material that can be drawn into wires.

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10
Q

What is elastic strain?

A

Strain that dissapears when the stress is removed, so the specimen returns to its original length.

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11
Q

What is plastic strain?

A

When the specimen does not return to its original length after the stress is removed.

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12
Q

What is a dislocation?

A

An incomplete plane of atoms

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13
Q

What is a grain boundary?

A

A boundary between crystals in a poly crystalline structure

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14
Q

What is the elastic limit?

A

Where the deformation becomes plastic, so it can no longer return to its original length

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15
Q

What is Ductile fracture?

A

Localised thinning which increases the stress as the area is smaller

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16
Q

What is a brittle material?

A

A material with no plastic flow, which fails by brittle fracture

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17
Q

What is brittle fracture?

A

Fracture under tension of brittle materials by crack propagation

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18
Q

What is the hardness of a material?

A

A material’s resistance to plastic deformation

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19
Q

What is the toughness of a material?

A

How much deformation a material can undergo before fracture.

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20
Q

If a material is stiff, what does it mean?

A

It is difficult to stretch or bend

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21
Q

If a material is malleable, what does it mean?

A

The material can be shaped easily

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22
Q

Describe Lord Rayleigh’s oil drop expreiment:

A
  • Measure the diameter and calculate the radius of the oil drop
  • Place the drop on still water which has a layer of powder
  • Measure the radius of the patch after it has spread about
  • Volume of the oil patch=volume of the oil drop
  • h= (4r^3) / (3r^2)
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23
Q

What is the number of atoms measure in?

A

m^-3

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24
Q

What is the equation for the number of atoms?

A

Density of element / mass of atom

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25
Q

What is the volume of a single atom?

A

1 / Number of atoms = mass of atom /density

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26
Q

What is the diameter of an atom?

A

10^-10 5

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27
Q

What is the order of magnitude size of an atom?

A

The cube root of the volume of a single atom

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28
Q

What does STM stand for?

A

Scanning Tunneling Microscope

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29
Q

What does AFM stand for?

A

Atomic Force Microscope

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30
Q

What can AFMs and STMs show?

A

Individual atoms on the surface of materials and allow atomic diameters to be measured.

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31
Q

Metals are crystalline, what does this mean?

A

Indivual particles are arranged in a regular pattern over distances many times the spacing between the particles

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32
Q

What are malleability and ductility a consequence of?

A

Plastic behaviour

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33
Q

What is plastic behaviour explained by?

A

The movement of dislocations within the crystalline structure.

34
Q

What does SEM stand for?

A

Scanning electron microscope

35
Q

What does Scanning electron microscopes show?

A

Large scale structures

36
Q

On what scale can AFMs detect changes?

A

On an atomic scale

37
Q

What do AFMs and STMs tell us about?

A

The arrangement of atoms on the surface of a material.

38
Q

What don’t AFMs and STMs tell us about?

A

The arrangement of atoms on the interior of a material.

39
Q

What technique can show that metals are crystalline?

A

X-ray crystallography

40
Q

What is the reasoning for some pure metals being malleable and ductile?

A

Dislocations

41
Q

What do dislocations allow layers of atoms to do?

A

Move one atom at a time

42
Q

What do dislocations do to the energy required to deform a material?

A

They greatly reduce the energy required to deform the material

43
Q

What are the dislocations like in ceramics?

A

They are not mobile so they cannot move through the material, which makes ceramics brittle

44
Q

Why are metal alloys less ductile than pure metals?

A

The different sized atoms pin down the dislocations, making slippages between the layers of atoms more difficult.

45
Q

What are metal alloys formed by?

A

The addition of metallic elements that usually have differently sized atoms

46
Q

When do solids form?

A

When liquids cool

47
Q

Is glass crystalline or amorphous?

A

Amourphous

48
Q

What does rapid cooling of a liquid result in?

A

Particles being trapped in an amorphous state, resembling the arrangement of a liquid

49
Q

What does slow, controlled cooling of a liquid lead to?

A

A single, pure crystal, which can be arranged in a perfect way

50
Q

What is a polycrystalline material?

A

A material that consists of a number of grains all orientated differently to one another, but with regular structure within each grain.

51
Q

What is the strength of materials affected by?

A

Tiny cracks and flaws in the structure of a material

52
Q

Describe what happens to a glass rod that has a tiny crack on its surface:

A
  • The glass becomes strained elastically
  • At the tip of the crack, atoms are pulled apart
  • The next 2 atoms are pulled apart
  • The crack moves through the material like a zip being undone
53
Q

What is toughness a measure of?

A

The energy required to extend cracks through a material

54
Q

What is tougher, a brittle substance or a tough metal?

A

A tough metal, as more energy is required to extend the cracks in it.

55
Q

When stress is applied to a crack in a metal, what happens?

A

The metal deforms plastically in the region of the crack which makes the crack broader, reducing the stress around the crack,

56
Q

What makes a material strong?

A

If it has a high breaking stress.

57
Q

What is tough the opposite of?

A

Brittle

58
Q

What are the 3 types of bonds between atoms?

A

Covalent, ionic and metallic.

59
Q

What makes the bonds in ionic compounds and ceramic unique?

A

The bonds are directional

60
Q

What do directional bonds mean?

A

The atoms are locked in place and cannot slip, making them hard and brittle

61
Q

What atoms can form covalent bonds?

A

Atoms that share electrons

62
Q

Are covalent bonds directional?

A

Yes

63
Q

What type of bonds do metals have?

A

Non-directional bonds

64
Q

Why are metals stiff?

A

Because the bonds are strong

65
Q

Atoms in metals are ionised, what does this mean?

A

The free electrons move between the ions

66
Q

Why are metals ductile?

A

Because the ions can slip

67
Q

What does stretching a metal do?

A

It pulls the bonds apart, causing the gaps to open up a little

68
Q

What are the array of positive ions bonded by in metals?

A

Electrons

69
Q

What is the elastic extensibility of metals?

A

0.1%

70
Q

What is the elastic extensibility of polymers?

A

1%

71
Q

Why is polyethene very ‘floppy’?

A

Because it is free to rotate about its bonds

72
Q

Why is polyethene strong and flexible?

A

The bonds are difficult to break and they are free to rotate.

73
Q

What do you do to a polymer to make it less flexible?

A

Add cross links

74
Q

What are cross links?

A

Where polymer chains are tied together at regular intervals along the chain.

75
Q

What happens when rubber is heated with sulfur?

A

The sulfur atoms form cross-links with the polymer chains

76
Q

What does the technique of vulcanisation mean for rubber?

A

It can be chemically adapted for many uses including tyres.

77
Q

In unstretched rubber, what do chains meander randomly between?

A

Sulfur cross-links

78
Q

In stretched rubber what happens to the chain bonds?

A

They rotate

79
Q

What happens when you let go of the stretched rubber?

A

The chains fold up again and the rubber contracts

80
Q

What is the plastic extensibility of polymers?

A

> 100%

Physics (22 decks)

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