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Physics AS > Section 5: Materials > Flashcards

Section 5: Materials Flashcards

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1
Q

What is density?

A

A measure of compactness of a substance. It relates the mass of a substance to how much space it takes up.

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2
Q

What is the equation for density?

A

P = m/v

Density (kgm^-3) = mass (kg) / volume (m^3)

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3
Q

What does the density of an object depend on?

A

What it’s made of. A materials density isn’t affected by shape or size.

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4
Q

What does the average density of an object determine?

A

Whether it floats or sinks

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5
Q

What is the density of water?

A
  1. 00 gcm^3

1. 00cm^3 of water has a mass of 1.00g

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6
Q

What is Hooke’s Law?

A

If a light metal wire with original length L is supported at the top and then had a weight attached to the bottom, it stretches. The weight pulls down with force F. Once the wire has stopped stretching, the forces will be equilibrium and there will be an equal and opposite reaction force at the support.

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7
Q

What did Robert Hooke discover in 1676?

A

That the extension of a stretched wire is proportional to the load force F. This relationship is called Hooke’s Law

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8
Q

What is the equation for Hooke’s Law?

A

F = k deltaL

Force (N) = stiffness constant (Nm^-1) x extension (m)

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9
Q

What does k depend on in Hooke’s Law?

A

The object being stretched

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10
Q

What does Hooke’s Law also apply to?

A

Springs

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11
Q

What is k usually described as?

A

The spring stiffness or the spring constant. It has the same value whether the forces are tensile or compressive

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12
Q

What does the force extension graph for Hooke’s Law show?

A

A straight line through the origin with the gradient representing k and the it curves off

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13
Q

Why does the graph curve off?

A

When the force becomes great enough, the graph starts to curve because it has reached the elastic limit

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14
Q

What is the elastic limit?

A

This is the point that if you carry on stretching the material it will be permanently stretched.

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15
Q

Where do metals generally obey Hooke’s Law to?

A

The limit of proportionality. It is very close to the elastic limit. It is the point at which the force is no longer proportional to the extension. It is also known as the Hooke’s Law limit

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16
Q

What experiment can you use to test how the extension of an object varies with the force used to extend it?

A

Attach a spring to a clamp stand with a rule from the bottom of the spring. Measure the length of the spring (original length) weights should then be added one at a time and the new length measured. After each weight is added the extension of the object can be calculated.

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17
Q

How can you calculate extension?

A

New length - original length

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18
Q

What does it mean if the object is elastic?

A

The material will return to its original length once the forces are removed. So it has no permanent extension

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19
Q

What does loading and unloading mean?

A

Loading means increasing the force on the material and unloading means reducing the force on the material

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20
Q

What happens when a material is put under tension?

A

The atoms of the material are pulled apart from one another

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21
Q

What happens if you stretch a plastic material?

A

The material will be permanently stretched after the force has been removed

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22
Q

How do plastic materials deform?

A

When the material is stretched, the atoms in the material move position relative to each other. When the load is removed, the atoms don’t return to their original positions. A Metal is stretched past its elastic limit shows plastic deformation

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23
Q

What’s the difference between tensile and compressive forces?

A

If the forces stretch the material they are tensile and if the forces squash the material they’re compressive forces

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24
Q

What is the definition of tensile stress?

A

The force applied divided by the cross sectional area

25
Q

What’s the equation for working out tensile stress?

A

Stress = F/A

The unit of stress is Nm^-2 or pascals

26
Q

What does stress cause?

A

Strain

27
Q

What is the definition of tensile strain?

A

The change in length (extension) divided by the original length of he material

28
Q

What is the equation for working out strain?

A

Strain = delta L / L

Strain doesn’t have any units because it’s a ratio. It doesn’t matter if the forces applied are tensile or compressive the same equation applies

29
Q

What’s the difference between tensile forces and compressive forces when working out strain?

A

Tensile forces = positive

Compressive forces = negative

30
Q

When does stress increase?

A

When a greater and greater tensile force is applied

31
Q

What is the effects of stress?

A

It starts to pull all the atoms apart from one another. Eventually the stress becomes too great and the material breaks. This is called the breaking stress.

32
Q

What is the ultimate tensile stress?

A

The maximum stress that the material can withstand

33
Q

Where is the elastic strain energy shown on a force extension graph?

A

It is given by the area under the graph - the potential energy

34
Q

What is the energy stored equal to?

A

The work done on the material by stretching it

35
Q

How can you calculate work done?

A

Work done = force x displacement

36
Q

How do you calculate work done on a force extension graph?

A

The force acting on the material isn’t constant so you have to work out the average

Work done = 1/2F x delta L

37
Q

What is the equation for working out elastic strain on a force extension graph?

A

Since work done = energy it is…

E = 1/2F x delta L

38
Q

Because Hooke’s Law is being obeyed, F = K delta L a new equation can be formed

A

E = 1/2K(delta L)^2

39
Q

What is always conserved when stretching?

A

Energy

40
Q

If a material is elastic and work is done what happens?

A

All the work done is stored as elastic strain energy

41
Q

What is the conservation of energy like in springs?

A

Stretched - elastic strain energy is stored
When the end of the spring is released, the stored elastic strain energy is transferred to kinetic energy and gravitational potential energy

42
Q

Overall what can the energy changes in an oscillating spring be summed up as?

A

Change in kinetic energy = change in potential energy

43
Q

What can be done for the energy conservation in transport designs?

A

Crumple zones that deform plastically in a crash

44
Q

What is the young modulus?

A

It’s a measure of how stiff an object is. It’s really useful for comparing the stiffness of different materials for example if you’re trying to find the best material to use for making a particular product

45
Q

What is the young modulus in terms of a graph?

A

The straight constant bit of the stress strain graph

46
Q

How can you calculate the young modulus?

A

E = tensile stress / tensile strain

(F / A) / (delta L / L)

(F x L) / (A x delta L)

47
Q

What are the units of the young modulus the same as?

A

Stress because strain has no units

48
Q

What experiment could you use to determine the young modulus of a material?

A
  1. The wire should be as thin and long as possible to reduce uncertainty
  2. Find cross sectional area (use a micrometer)
  3. Clamp wire to the bench so you can hang weights. Start with a small weight to straighten the wire but don’t use this in your experiment calculations
  4. Measure the distance between the fixed end of the wire and the marker
  5. If you increase weight the wire stretches and the marker moves
  6. Increase the weight in steps recording the marker each time
  7. Measure the extension
  8. You can use this to find stress and strain and plot a graph
49
Q

How can you find the young modulus using a graph?

A

The gradient of the stress strain graph gives you the young modulus

E = stress / strain = gradient

50
Q

What does the area under a stress strain graph give?

A

Energy stored per unit volume = the energy stored per 1m^3 of wire

51
Q

How can you calculate the energy per unit volume?

A

1/2 x stress x strain

52
Q

What are the three most important points of the stress strain graph?

A
  • P - the limit of proportionality - material stops obeying Hooke’s Law but would still return to original size and shape if stress was removed
  • E - The elastic limit at this point the material starts to behave plastically -from point E onwards the material would no longer return to its original size and shape once the stress was removed
  • Y - the yield point - here the material suddenly starts to stretch without any extra load. The yield point or yield stress is the stress at which a large amount of plastic deformation takes place with a constant or reduce load
53
Q

What is the area between the two lines of the force extension graph?

A

The work done to permanently deform the wire

54
Q

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

A

The graph doesn’t curve. It starts with a straight line through the origin. Brittle materials also obey Hooke’s Law, however when the stress reaches a certain point the material snaps so it doesn’t deform plastically

55
Q

Name some brittle materials.

A

Ceramics are brittle because they tend to shatter. Also chocolate bars are brittle you can break off one square of chocolate without changing the shape of the whole thing

56
Q

What is the structure like of ceramics?

A

Ceramics are made by melting certain materials, and then letting them cool. The arrangement of atoms in a ceramic can be crystalline or polycrystalline - where there are many regions (or grains) of crystalline structure. The atoms in each grain line up in a different direction. They are bonded in a giant rigid structure which makes them stiff and very brittle.

57
Q

What happens when stress is applied to a brittle material?

A

Tiny cracks in the materials surface get bigger and bigger until the material brakes completely. This is called brittle fracture. The cracks in brittle materials are able to grow because these materials have a rigid structure.

58
Q

When are structures not brittle?

A

When the atoms are able to move and prevent any cracks getting bigger.

Physics AS (9 decks)

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