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Physics > Section 5 Materials > Flashcards

Section 5 Materials Flashcards

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1
Q

What is density?

A

It is a measure of the ‘compactness’ of a substance.
The material’s mass per unit volume

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2
Q

What is the equation of density?

A

p = m/V

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3
Q

What does the density of an object depend on?

A

The material it is made of

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4
Q

What is Hooke’s Law?

A

The applied force on the spring is directly proportional to the extension of the spring, provided the spring has not been extended beyond its limit of proportionality.

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5
Q

What is the equation for the pressure on solids?

A

P = F/A

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6
Q

What is the equation for pressure in fluids?

A

P = pgh

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7
Q

What is the equation for Hooke’s Law?

A

F = kx

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8
Q

What is the spring constant?

A

The constant of proportionality for the extension of a spring under a force. The higher the spring constant, the greater the force needed to achieve a given extension.

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9
Q

The extension or compression of a spring is proportional to what?

A

the force applied

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10
Q

How do you know if a spring is obeying Hooke’s Law from a force-extension graph?

A

There should be a straight line through the origin

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11
Q

On a force-extension graph, what is the gradient?

A

the spring constant , k

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12
Q

What is the elastic limit of a spring?

A

When the force becomes great enough, the force-extension graph starts to curve. This point on the graph is the elastic limit.

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13
Q

What happens if you keep applying force past the elastic limit?

A

The material will be permanently deformed. When all the force is removed, the material will no longer than at the start.

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14
Q

What is another name for the limit of proportionality?

A

Hooke’s law limit

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15
Q

Where is the limit of proportionality on a force-extension graph?

A

It is the point beyond which the force is no longer proportional to extension.
It is just before the elastic limit.
It is where the straight line ends and starts to curve.

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16
Q

What is the equation for extension?

A

new length - original length

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17
Q

How can you investigate the extension of springs?

A

The object under test should be supported at the top, using a clamp, and a measurement of its original length taken using a ruler. Weights should then be added one at a time to the other end of the object.
The weights used will depend on the object being tested - you should do a trial investigation if you can to work out the range and size of weights needed. You want to be able to add the same size weights each time and add a large number of weights before the object breaks, to get a good picture of how the extension of the object varies with the force applied to it.
After each weight is added, the extension of the object can be calculated.
Finally, a graph of load against extension should be plotted to show the results.

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18
Q

What does it mean if a deformation is elastic?

A

The material returns to its original shape once the forces are removed - so it has no permanent extension.

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19
Q

How can you tell if a material is elastic from a force-extension graph?

A

It has 2 different curves. The top one being loading and the bottom one being unloading. It also returns to its original length after the extension.

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20
Q

What happens to the atoms in a material when this material is put under tension and when this load is then removed?

A

When a material is put under tension, the atoms of the material are pulled apart from one another. Atoms can move small distances relative to their equilibrium positions without actually changing positions in the material. Once the load is removed, the atoms return to their equilibrium distances apart.

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21
Q

What are tensile forces?

A

When the forces stretch the material

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22
Q

What are compressive forces?

A

When the forces squash the material

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23
Q

What is the equation for stress?

A

Stress = force/area

24
Q

What is the equation for strain?

A

strain = extension/original length

25
Q

What are the units for stress?

A

pascals or Nm-2

26
Q

What are the units for strain?

A

There are no units as it is a ratio

27
Q

As a greater and greater tensile force is applied to a material, what happens to the stress on it ?

A

The stress on it increases

28
Q

What is meant by breaking stress?

A

The effect of 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 breaks.

29
Q

What is meant by the ultimate tensile stress?

A

The maximum stress that the material can withstand.

30
Q

What two values do engineers have to consider when designing structures?

A

ultimate tensile stress
breaking stress

31
Q

What is the elastic strain energy?

A

Before the elastic limit, all the work done in stretching is stored as potential energy in the material, which is the elastic strain energy.

32
Q

How do you find the elastic strain energy on a force-extension graph?

A

area under the graph

33
Q

How do you find the work done of a spring?

A

Wd = 1/2 x F x extension

34
Q

How do you find the energy in a spring?

A

E = 1/2 x k x extension squared

35
Q

How is energy conserved in an elastic deformation?

A

If the deformation is elastic, all the work done is stored as elastic strain energy in the material. When the stretching force is removed, this stored energy is transferred to other forms - e.g. an elastic band is stretched and then fired across the room.

36
Q

How is energy conserved in a plastic deformation?

A

If the deformation is plastic, work is done to separate atoms. The energy is not stored as strain energy, and is most dissipated as heat.

37
Q

When a vertical spring with a mass suspended vertically below is stretched, how is energy conserved?

A

When a vertical spring with a mass suspended vertically below is stretched, elastic strain energy is stored in the spring. When the end of spring with the mass is released, the stored elastic energy is transferred to kinetic energy (as the spring contracts) and gravitational potential energy (as the mass gains height). The spring then begins to compress and the kinetic energy is transferred back to stored elastic strain energy.

38
Q

What can the energy changes in an oscillating spring be said as?

A

change in kinetic energy = change in potential energy

39
Q

What types of energy does potential energy include?

A

gravitational energy
elastic strain energy

40
Q

When you apply a load to stretch a material, what does it experience?

A

tensile stress and tensile strain

41
Q

What is the equation of Young’s Modulus?

A

stress/strain

42
Q

What is the method of the Young Modulus Experiment required practical?

A
  1. The test wire should be thin, and as long as possible. The longer and thinner the wire, the more it extends for the same force - this reduces the uncertainty in the measurements.
  2. First you need to find the cross-sectional area of the wire. Use a micrometer to measure the diameter of the wire in several places and take an average of your measurements. By assuming that the cross-section is circular, you can use the formula for the area of a circle.
  3. Clamp the wire to the bench so you can hang weights off one end of it. Start with the smallest weight necessary to straighten the wire (don’t exclude this weight in your final calculations).
  4. Measure the distance between the fixed end of the wire and the marker - this is your unstretched length.
  5. Then if you increase the weight, the wire stretched and the marker moves.
  6. Increase the weights in steps, recording the marker reading each time - the extension is the difference between this reading and the unstretched length.
  7. You can use your results from this experiment to calculate the stress and strain of the wire and plot a stress-strain curve. The gradient of this line is the Young’s Modulus.
43
Q

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

A

Young’s Modulus of that material

44
Q

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

A

strain energy per unit volume

45
Q

What are the 3 important points on a stress-strain graph?

A

limit of proportionality
elastic limit
yield point

46
Q

What does it mean by limit of proportionality on a stress-strain graph?

A

After this point, the graph starts to bend.
At this point, the material stops obeying Hooke’s Law, but would still return to its original size and shape if the stress was removed.

47
Q

What does it mean by the elastic limit on a stress-strain graph?

A

At this point, the material starts to behave plastically. From this point onwards, the material would no longer return to its original size and shape once the stress was removed.

48
Q

What is meant by the yield point on a stress-strain graph?

A

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

49
Q

On a force-extension graph, why does the unloading line sometimes not line up with the loading line?

A

If the material has deformed plastically, the extension will have changed permanently so the unloading line will be lower than the loading line. This is because the length with no load applied will now be longer than the original length.

50
Q

After a metal wire has been stretched past its limit of proportionality, what happens to the curve?

A

It starts to curve downwards because force is no longer directly proportional to the extension.

51
Q

On a force-extension graph, once the wire has been stretched past its limit of proportionality, why are the loading and unloading lines still parallel?

A

When the load is removed, the extension decreases. The unloading line is parallel to the loading line because the stiffness (k) is still the same - the forces between the atoms are still the same as they were during the loading.

52
Q

On a force-extension graph, what does the area between the loading and unloading lines represent?

A

the work done to permanently deform the wire

53
Q

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

A

The graph starts with a straight line through the origin as brittle materials obey Hooke’s Law. However, when the stress reaches a certain point, the material snaps - it doesn’t deform plastically.

54
Q

What is the structure of brittle materials?

A

The atoms in each grain line up in a different direction. However, they are arranged, the atoms are bonded in a giant rigid structure. The strong bonds between the atoms make them stiff, while the rigid structure means that they are brittle materials.
When a stress is applied to a brittle material, any tiny cracks at the material’s surface get bigger and bigger until the material breaks completely. This is called brittle fracture. The cracks in brittle materials are able to grow because these materials have a rigid structure.

55
Q

What is meant by brittle fracture?

A

When a stress is applied to a brittle material, any tiny cracks at the material’s surface get bigger and bigger until the material breaks completely. This is called brittle fracture. The cracks in brittle materials are able to grow because these materials have a rigid structure.

Physics (15 decks)

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