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Physics > Unit 2: Materials > Flashcards

Unit 2: Materials Flashcards

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1
Q

What is the formula for density?

A

Density = mass / volume

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2
Q

What are the SI units for density?

A

kg/m^3

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3
Q

What is the density of water?

A

1000kg/m^3

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4
Q

What happens when an object is subjected to opposing forces?

A

It may be stretched or compressed.

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5
Q

What are stretching forces?

A

Tensile forces

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6
Q

What are squashing forces?

A

Compressive forces

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7
Q

What do tensile forces do to an object?

A

They tend to stretch the object, putting the material under tensile stress and causing an extension in the object.

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8
Q

How can you investigate the behaviour of a spring under tension?

A

The spring can be suspended vertically, masses are then hung from the bottom of the spring and the extension is measured.

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9
Q

What is Hooke’s law?

A

The extension of a string is proportional to the force applied to it up until the limit of proportionality.

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10
Q

What can Hooke’s law be written as?

A

F = kΔl where l is the change in length of the spring (the extension) and k is the spring constant.

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11
Q

What is the spring constant a measure of?

A

The stiffness of the spring i.e. how much force it takes to stretch it by a given distance.

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12
Q

How is the spring constant represented on a force-extension graph?

A

It is the gradient of the graph.

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13
Q

What are the units of the spring constant?

A

N/m

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14
Q

What are springs and wires that obey Hooke’s law said to show?

A

Elastic behaviour.

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15
Q

What can Hooke’s law be written as?

A

F = kΔl where l is the change in length of the spring (the extension) and k is the spring constant.

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16
Q

What are springs and wires that obey Hooke’s law said to show?

A

Elastic behaviour.

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17
Q

What does elastic behaviour consist of?

A

The objects return to their original length when the tensile force is removed.

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18
Q

When is a spring said to have been stretched past its elastic limit?

A

When a force is large enough that it causes a permanent extension so that the spring does not return to its original size even when the force is removed.

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19
Q

What does plastic behaviour consist of?

A

When an object shows permanent deformation even when the force is removed.

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20
Q

What is the formula for stress?

A

Stress (σ) = force (F) / cross-sectional area (A)

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21
Q

What is stress measured in?

A

N/m^2 which is known as the pascal (Pa)

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22
Q

What is the formula for strain?

A

Strain (ɛ) = extension (Δl) / original length (l)

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23
Q

Why does stress have no unit?

A

Because it is a ratio of two lengths.

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24
Q

Will a thicker wire stretch more or less for the same tensile force applied to it as a narrower wire?

A

It will stretch less for the same tensile force.

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25
Q

Will a longer wire stretch more or less for the same tensile force applied to it as a shorter wire?

A

It will stretch more for the same tensile force.

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26
Q

What is the elastic limit?

A

This is the maximum stress that can be applied to a material without causing a permanent extension.

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27
Q

What is the elastic limit?

A

This is the maximum stress that can be applied to a material without causing a permanent extension.

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28
Q

What is the yield point?

A

Beyond the elastic limit, a point is reached at which there is a noticeably larger permanent change in length. This results in plastic behaviour.

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29
Q

What is strength?

A

Some materials can withstand large stresses before they fracture (break). These are strong or high-strength materials.

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30
Q

What is the breaking stress?

A

This is the maximum stress that can be applied to a material without it breaking.

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31
Q

What is stiffness?

A

This is a measure of how difficult it is to change the size or shape of a material.

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32
Q

What kind of wires will have a high stiffness?

A

Thick wires, short wires and steel wires.

33
Q

Why do steel wires have a higher stiffness than copper wires?

A

The copper extends more per unit force.

34
Q

What is ductility?

A

A ductile material can be easily and permanently stretched.

35
Q

What is brittleness?

A

A brittle material cannot be permanently stretched.

36
Q

What is a good example of a ductile material and why?

A

Copper as it can be easily drawn out into a thin wire.

37
Q

What happens to a brittle material when a force is applied to it?

A

The material soon breaks after the elastic limit is reached.

38
Q

What is a good quality of some brittle materials?

A

They are strong in compression.

39
Q

What are two examples of brittle materials?

A

Concrete and cast iron.

40
Q

How can you tell if a material obeys Hooke’s law by looking at a force-extension graph?

A

The graph will have a straight line (i.e. a constant gradient) that goes through the origin.

41
Q

What does a steeper gradient on a force-extension graph mean for a material that obeys Hooke’s law?

A

It has a higher stiffness.

42
Q

If a material does not undergo plastic deformation before breaking, then what property does it possess?

A

Brittleness

43
Q

If a material does not undergo plastic deformation before breaking, then what property does it possess?

A

Brittleness

44
Q

If a material obeys Hooke’s law up to the elastic limit, after which it undergoes plastic defloration, then what property is it likely to possess?

A

Ductility

45
Q

When are force-extension graphs used and when are stress-strain graphs used?

A

Force-extension graphs apply only to the specimen under test whereas stress-strain graphs apply to the material under test (regardless of the dimensions of the specimen).

46
Q

What happens to a ductile material when it is subjected to a high tensile strength and it undergoes considerable plastic deformation?

A

During the plastic stage and when the material is just about to fail, it will ‘neck’.

47
Q

What does the stress-strain graph look like for a ductile material?

A

It has a positive constant gradient, it then dips and the gradient increases again but it is curved outwards.

48
Q

Why does necking occur?

A

The cross-sectional area of the wire gets narrower and because stress = force / area, the neck experiences an increase in tensile stress.

49
Q

What happens at the neck?

A

This is where the material fails so it provides an early warning of when the material is about to fail.

50
Q

What is brittle fracture due to?

A

The rapid extension of surface or internal cracks in the material.

51
Q

How quickly does brittle fracture occur?

A

It is sudden and catastrophic - there is little or no plastic deformation as a warning.

52
Q

Why does brittle fracture occur when there are internal cracks?

A

There is concentration of stress around the cracks - the sharper the crack, the greater the stress.

53
Q

Why does brittle fracture occur when there are internal cracks?

A

There is concentration of stress around the cracks - the sharper the crack, the greater the stress.

54
Q

When is crack movement easier and when is it more difficult?

A

It is easier under tension and more difficult under compression.

55
Q

What can pre-stressing do to brittle materials?

A

It can increase their strength.

56
Q

What does the stress-strain graph look like for a brittle material?

A

It is a straight line that has a positive gradient and it stops at the breaking stress.

57
Q

What is elastic strain energy?

A

When a wire is stretched by a force, provided the elastic limit is not exceeded, then the work done (energy change) is stored as elastic strain energy.

58
Q

What does the area under a stress-strain graph represent?

A

It represents the total work done in stretching an object and is therefore equal to the elastic strain energy stored.

59
Q

How can you derive the formula for the elastic strain energy stored?

A

Work done by force F in extending the wire by a small extension δl is δW = Fδl, the total work done in fully extending the wire is therefore the sum of the small areas Fδl: W = ΣδW = ΣFδl (which equals the area under the graph). As the area of a triangle is 1/2 x base x height, elastic strain energy is 1/2FΔl.

60
Q

What is the formula for the elastic strain energy stored in a spring?

A

1/2k(Δl)^2

61
Q

What is the definition used to get the formula for the elastic strain energy stored in a spring?

A

F = kΔl

62
Q

What is the definition used to get the formula for the elastic strain energy stored in a spring?

A

F = kΔl

63
Q

What is the Young modulus a measure of?

A

The stiffness of a material.

64
Q

What is the formula for the Young modulus?

A

E = tensile stress / tensile strain

65
Q

What are the units of the Young modulus?

A

N/m^2 or pascal (Pa)

66
Q

Up until what point does the Young modulus apply?

A

The limit of proportionality.

67
Q

Does it matter if the samples under test for the Young modulus have different dimensions?

A

No

68
Q

What does that apparatus for the experimental determination of the Young modulus of a material look like?

A

There is a support beam where two wires are hung - one is the control wire and the other is the long wire under test. There is a spirit level bubble between the two wires and masses are attached to the wires - there is also a micrometer attached to where the masses hang off of on the long wire under test.

69
Q

How long should the long wire under test be in Searle’s apparatus?

A

At least one metre.

70
Q

What are the key points of the experimental method when determining the cross-sectional area and the length of the wire?

A

Two identical wires are fixed in parallel, both wires are initially loaded to straighten them, the micrometer attached to the test wire is adjusted in order to bring the spirit level horizontal, the micrometer reading is then taken, a metre rule is used to measure the original length of the test wire, a second micrometer is used to measure the diameter of the wire in several places to account for any unevenness in the wire diameter and the average reading is used to calculate the cross-sectional area.

71
Q

What are the key points of the experimental method when determining the extension of the wire?

A

The test wire is then loaded and the micrometer attached to the test wire is adjusted to bring the spirit level horizontal once more, the extension is then calculated, further loads are added and the procedure is repeated until a range of readings has been obtained, a second set of results is obtained during unloading and a graph of force against extension is plotted.

72
Q

What is the formula for the elastic strain energy stored in a spring?

A

1/2k(Δl)^2

73
Q

What are the key points of the experimental method when determining the extension of the wire?

A

The test wire is then loaded and the micrometer attached to the test wire is adjusted to bring the spirit level horizontal once more, the extension is then calculated, further loads are added and the procedure is repeated until a range of readings has been obtained, a second set of results is obtained during unloading and a graph of force against extension is plotted.

74
Q

What is the equation of the line of the graph plotted from Searle’s apparatus?

A

F = (EA/l) Δl

75
Q

How can the Young modulus be calculated from the force-extension graph?

A

E = gradient x l/A

76
Q

What does the gradient of a stress-strain graph represent?

A

The Young modulus

77
Q

What measures improve the accuracy of Seattle’s experiment?

A

A long thin wire is used to produce as large an extension as possible for every load added, a control wire is used so that changes in length due to temperature changes (or to a sagging support) do not affect the results, a metre rule is accurate enough as it provides small percentage uncertainties and a micrometer of 0.01mm precision should be used in order to provide small percentage uncertainties.

78
Q

When a stress-strain graph has a postive gradient and then it bows outwards and returns to back to the x axis yet not at the value of 0, what can be said?

A

It obeys Hooke’s law up until the curved part, then the elastic limit is reached so the material undergoes plastic deformation. When the stress is removed the material has undergone a permanent change in length as it does not return though the origin.

Physics (18 decks)

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