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A Level Physics OCR B > 4: Testing Materials > Flashcards

4: Testing Materials Flashcards

1
Q

What is Hooke’s law?

A

Extension is proportional to force

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2
Q

Describe what happens to a wire when it is supported at the top and has a weight attached to the bottom. Talk about forces

A

It stretches

The weight pulls down with force F, producing an equal and opposite force at the support

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3
Q

What is k, in the equation F=kx?

A

Force or stiffness constant. Also for springs called the spring constant

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4
Q

Describe tension in a material (When a material is stretched)

A

Stretching a material creates tension across it. Forces of tension act along the same line as the forces stretching the material but in the opposite direction at each end of the material - they ‘pull’ on the object at either end of the material

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5
Q

What is need for a spring to change length?

A

The application of a pair of opposite forces

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6
Q

Describe the extension in the spring. When is it negative?

A

The extension of a spring is proportional to the force applied. If the forces are compressive, the spring is squashed and the extension is negative

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7
Q

In springs, is k the same if the forces are compressive or tensile?

A

The same value

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8
Q

Do all materials obey Hooke’s law?

A

No, but they all obey it up to a point

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9
Q

When does Hooke’s law stop working?

A

When the load is great enough

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10
Q

What does a material obey Hooke’s law look like on a force against extension graph?

A

Straight line through (0,0)

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11
Q

When does the graph of force against extension start to curve?

A

When the force becomes great enough. Starts to curve after the limit of proportionality

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12
Q

What happens if a material exceeds the elastic limit?

A

The material will be permanently stretched

When all force is removed, the material will be longer than at the start

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13
Q

What happens to the relationship between force and extension after the elastic limit?

A

The material will stretch further for a given force

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14
Q

A stretch can be elastic or ….

A

Plastic

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15
Q

When does a material show elastic and plastic deformation?

A

Elastic up to the elastic limit then plastic after the limit

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16
Q

What does elastic deformation mean?

A

The material returns to its original shape once the forces are removed

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17
Q

Describe elastic deformation microscopically

A

When a material is out under tension, the atoms of the material are pulled apart from one another
Atoms can move slightly relative to their equilibrium positions, without changing position in the material
Once the load is removed, the atoms return to their equilibrium, distance apart

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18
Q

If a deformation is plastic, the material is ….

A

Permanently stretched

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19
Q

Describe plastic deformation microscopically

A

Some atoms in the material move position relative to one another
When the load is removed, the atoms don’t return to their original positions

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20
Q

Describe the investigation of extension by stretching an object. How can you find k?

A

Support the object being tested at the top and measure its original length
Add masses one at a time to the bottom of the object
After each weight is added, measure the new length of the object, then calculate extension
Plot a graph of force (weight), against extension. Where the line of best fit is straight, the object obeys Hooke’s law, so gradient = k. If you’ve loaded the object beyond its limit of proportionality, the graph will start to curve

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21
Q

Describe the set up for the experiment investigating extension

A

Object, with weights attached, attached to a clamp, which is attached to a clamp stand. A ruler is put next to the clamp stand to measure the length of the object

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22
Q

If the force on the material stretches the material is it compressive or tensile?

A

Tensile

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23
Q

If a force squashes the material is it compressive or tensile?

A

Compressive

24
Q

What causes a strain?

A

A stress

25
Q

Define stress

A

The tension (the force applied) divided by the cross-sectional area

26
Q

What are the units of stress?

A

Pa or N/m²

27
Q

Define strain

What are the units?

A

The extension, i.e. the change in length, divided by the original length of the material
There are no units, it’s given as a number or percentage

28
Q

What is the fracture stress?

A

A stress big enough to break a material

29
Q

Describe the fracture stress microscopically

A

The effect of the stress is to start to pull the atoms apart from one another
Eventually the stress becomes so great that atoms separate completely, and the material fractures (breaks). Distress at which this occurs is called the fracture stress

30
Q

What does the fracture stress look like on a graph of stress against strain?

A

It is the point where the line or graph stops

31
Q

What is the ultimate tensile strength?

A

This is the maximum stress that the material can withstand before breaking

32
Q

What is elastic strain energy?

A

The energy stored in a stretched material

33
Q

When is work done in deforming a material?

A

When a material is stretched or compressed

34
Q

On a graph of force against extension what represents the work done?

A

The area under the graph

35
Q

Before the elastic limit where is all the work done, by stretching or compressing the material, stored?

A

Before the elastic limit, all the work done in stretching or compressing the material is stored as energy in the material

36
Q

What is the work done on an elastic material equal to?

A

The work done on an elastic material, in stretching it, is equal to the energy stored in the material as elastic strain energy

37
Q

Explain why the work done = Fx/2

A

Work done equals force * displacement
However, the force on the material isn’t constant. It rises from zero up to force F. To calculate the work done, use the average force between zero and F, F/2

38
Q

Describe what happens with work done once a material is stretched be on the elastic limit

A

If the material is stretched beyond the elastic limit, some work is done separating atoms. This energy will not be stored as elastic strain energy, and so is it released when the force is removed

39
Q

Up to the limit of proportionality which two values are proportional to each other?

A

Stress and strain

40
Q

Explain, when testing Young’s modulus, why the wire should be as thin and as long as possible

A

The longer and thinner the wire, the more extends for the same force. This reduces the uncertainty in your measurements

41
Q

Describe the experiment for the investigation of Young’s modulus of a very long wire

A

Calculate the cross-sectional area and the unstrechted length of the wire.
If you increase the weight, the wire stretches and the marker moves.
Increase the weight in steps, recording the marker reading each time. Use a digital scales to accurately find the weight added at each step
Use your results to calculate the stress and strain on the wire and plot a stress-strain graph

42
Q

From a stress strain graph how can you find the Young’s modulus?

A

The gradient of the graph gives you the Young’s modulus.

43
Q

What does the area under a stress strain graph give you?

A

The elastic strain energy per unit volume

44
Q

Explain the term brittle

A

Brittle materials break suddenly without deforming plastically.
If you apply a force to brittle material, it won’t deform plastically, but will suddenly snap when the force gets to a certain size.
Brittle materials can also be quite weak if they have cracks in them.

45
Q

Explain the term ductile

A

Ductile materials can be drawn into wires without losing their strength.
You can change the shape of ductile materials by drawing them into wires or other shapes. Important thing is that they keep their strength when they’re deformed like this

46
Q

Why is copper perfect for making electric wires?

A

Copper is ductile, and with its high electrical conductivity this means that it’s at the ideal for electric wires

47
Q

Describe the term strong

A

Strong materials can withstand high stresses without the forming or breaking.
Strength is a measure of how much a material can resist being deformed (bent, stretched, fractured etc.) by a force without breaking. This can be resisting a pulling force (tensile strength) or a squeezing force (compressive strength)

48
Q

Describe the term stiff

A

Changing the shape of stiff materials is really difficult as they are resistant to both bending and stretching. Stiffness is measured by the Young’s modulus – the higher the value of the stiffer the material

49
Q

Describe the term tough

A

Toughness is a measure of the energy and material can absorb before it breaks. Really tough materials can absorb a lot of energy so are very difficult to break.

50
Q

Stress strain graphs for ductile materials…

A

Curve

51
Q

On the stress strain graft for a ductile material, what is the elastic limit?

A

At this point the material starts to behave plastically.

From this point onward, the material would no longer return to its original shape once the stress was removed

52
Q

On a stress strain graph for a ductile material, what is the limit of proportionality?

A

After this, the graph is no longer a straight line but starts to bend.
At this point, the material stops obeying Hooke’s law, but would still return to its original shape if the stress was removed

53
Q

On a graph of stress against strain for a tactile material, what does the graph look like before the limit of proportionality?

A

The graph is a straight line through the origin. This shows that the material is obeying Hooke’s law

54
Q

On a stress strain graph for ductile material, what is the yield stress?

A

Here the material suddenly starts to stretch without any extra load.
The yield stress is the stress at which a large amount of plastic deformation takes place with a constant or reduced load

55
Q

Describe briefly the stress and strain graph for a brittle material

A

Brittle materials don’t tend to behave plastically. They fracture before they reach the elastic limit, therefore most of the graph is a straight line

56
Q

Investigation of Young’s modulus:

Describe how to accurately find the length of the unstretched wire

A

Clamp the wire to the bench. Start with the smallest weight necessary to straighten the wire (don’t include this weight in your final calculations).
Measure the distance between the fixed end of the wire and the marker – this is your unstretched length

57
Q

Is silicon a conductor, semi-conductor or an insulator?

A

Semi-conductor

A Level Physics OCR B (61 decks)

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