Ants, atoms and chips! - Wikispaces5.pdf · Ants, atoms and chips! Question 50S: Short Answer ... Rewrite the width of a transistor in metres, ... Question 80S: Short Answer

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Ants, atoms and chips!Question 50S: Short Answer

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Thinking about atoms means thinking about objects smaller than any you are used to. But thatdoesn’t mean you can’t think about how big they are. These simple questions are to get you used toways of thinking about very small things.

Ants1. Estimate the length of an ordinary ant.

2. If ants of this type were marching single file, leaving no gaps, lengthways along a meter rule, howmany would be on the rule at any one time?

3. A particular type of ant of length x is marching down a piece of wood of length L. Write anexpression for the number N of ants that would fit lengthways on the piece of wood. (Hint: this isexactly the same as you have just done; only using algebra instead of numbers.)

Atoms and chipsA modern microchip contains transistors each of the order of 0.1m wide.

4. Rewrite the width of a transistor in metres, using standard form.

An atom is typically 110-10 m in diameter.

5. How many atoms are there typically across the width of a microchip?

You have seenThat simple calculations that you can do in your head can be written down as algebraic equations

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and used to solve more complex problems.

How to calculate how many atoms are in a given object.

Further questions on metalsQuestion 80S: Short Answer

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1. A student finds the following short passage. Explain each of the points made, giving greaterdetail. Pay particular attention to the bold terms.

Cracks occur in all substances. When a material is stretched energy is stored in it.If the energyneeded to open and deepen a crack is less than that stored, the crack can propagate. Such amaterial undergoes brittle fracture. A crack is a macroscopic phenomenon, influenced bymicroscopic structure.

The propagation of a crack can be stopped by the presence of a few dislocations which makethe material ductile. Dislocations are a microscopic phenomenon. On the other hand, thepresence of many dislocations or foreign atoms can harden a material and such hard materialsare likely to fail through brittle fracture.

The diagram shows a dislocation that has formed inside a specimen. The arrows show the directionof shear stress being applied to the material.

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2. Draw a series of diagrams showing how the dislocation will move under the influence of thisstress.

Grain size is of particular importance to metallurgists.

3. How do grain boundaries interact with dislocations?

4. How will the stress required to cause plastic flow vary with grain size?

5. How can grain size be controlled and what effect does this control have on the microstructure?Give an account of one method.

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Questions on polymersQuestion 100S: Short Answer


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What to doAnswer these questions on the paper. Some will require access to File 10D 'Materials database', andsome will require further research.

Try these1. What is the difference between a thermosetting and a thermoplastic polymer? Give an example of

each type and one of its uses.

2. Look at the stress–strain curve for polythene and explain why polymers are often called plastics.

strain %

0 100 200

20

300 400

3. A specimen of rubber of cross sectional area 2 mm2 is extended in length from 0.1 m to 0.15 mby a force of 0.4 N. Use these results to predict the force needed to extend a piece of the same

material with 4 mm2 cross section from a length of 0.50 m to 0.75 m.

4. Use the materials database from chapter 4 (File 10D 'Materials database') to find the Youngmodulus of polystyrene, high-density polythene, mild steel and soda glass, and arrange them inorder of increasing stiffness.

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5. Describe, in molecular terms, what happens when rubber is stretched and then released. Usediagrams to illustrate your answer.

6. Hair can be considered as a thermoset polymer, where long protein (keratin) chains are heldtogether by cross-links composed of two sulphur atoms (so-called disulphide bridges). At whattemperature does hair melt?

7. Match the polymer to its use.

Polymers: polypropylene, low-density polythene, polystyrene, polyvinylchloride, high-densitypolytheneUses: margarine tubs, plastic cutlery, window frames, freezer bag, strong carrier bag.

8. Describe the differences in properties between ordinary and expanded polystyrene and accountfor them in terms of their structure.

9. A bucket to hold water could be made from mild steel, wood or polypropylene. Say whether thepolymer is a good choice or not, in comparison with the other materials, backing up yourargument with relevant information from the database.

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BoneQuestion 110S: Short Answer


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One of nature's compositesHere you look at how the structure of bone, a composite, influences its mechanical properties. Youwill need to consult the File 10D 'Materials database' from chapter 4.

Bone is a complex composite. It consists of bone cells embedded in a matrix containing collagenfibres and so-called ground substance which, in turn, contains a high concentration of disorderedhydroxyapatite (HPA) – calcium phosphate crystals. The protein collagen is the main structuralmaterial of the body.

1. Where else is collagen found?

2. As with other composites, the separate components of bone bring desirable properties to thematerial – HPA contributes hardness, collagen strength and flexibility. Consult the database andcomment on how bone compares to iron and wood in its tensile strength and its density.

3. Again consulting the database, write a paragraph to 'sell' bone as a new material to a companylooking for a structural material.

The image shows the way bone tissue is arranged in a hip bone. Note the two layers: compact boneon the outside and spongy bone on the inside and the tiny spikes, known as trabeculae, which growalong the direction of greatest stress.

Source

Open the JPEG file

Open the JPEG file

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Source

Open the JPEG file

4. Why is bone so light?

Springs connected to the Young modulusQuestion 150S: Short Answer

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A school laboratory has a demonstration model, which is meant to illustrate the stretching of bondsbetween atoms as a piece of material is stretched. The model has three horizontal planes of smallballs linked horizontally by rods and vertically by springs. There are nine balls in each horizontallayer. All the springs are identical and may be taken to be 50 mm long. The dimensions are shown, allin millimetres. When a vertical stretching force of 18 N is applied, the height of 100 mm increases to110 mm.

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100 mm

forces

springs

balls

rods

1. What is the restoring force for the 10% strain for a single spring?

2. What is the force constant (restoring force per unit extension) for a single spring?

Now you are ready to make a link between the Young modulus of a metal (stress / strain) and thespringiness of bonds between individual atoms.

Imagine layers of atoms in a square array, each atom at a distance r 0 from its nearest neighbour in

each direction.

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r r0 + r

r0

r0

force

force

Suppose a wire with many such layers is stretched a little so that each layer is now r 0 + r from

those above or below it.

3. If there are n gaps between layers in the length of the wire, how long is the wire before it isstretched?

4. By how much has the wire extended?

5. What is the strain in terms of r 0 and r ?

Now think of the bond holding each atom as being like a spring, so that there is a force of kr pullinga pair of atoms together.

6. k is the stiffness of the spring. What are the units of k?

7. There are m atoms in each layer. What is the force pulling adjacent layers together?

8. The stress is the force per unit area. In terms of the spacing r 0 , how many atoms are there per

square metre of a layer?

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9. What is the stress in terms of k, r 0 and r ?

10. What is the Young modulus in terms of k , r 0 ?

11. For steel, the Young modulus is 2 1011 Pa. The atomic spacing is about 3 10–10 m. What is

the stiffness of the interatomic bond?

Scaling exercisesQuestion 20E: Estimate


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IntroductionNo matter how complex, everything that exists is made up of the small scale; for example, large citiesare made up of buildings made of small individual bricks. Similarly an oak tree is a complicatedstructure that makes acorns – but the acorn includes all the information needed for making the oaktree. Outer space (stars and galaxies, etc) is made up of inner space (protons and electrons, etc). Ifwe want to study both inner and outer space, it is important to develop some way for comparing thesizes of numbers involved.

Getting a grip on numbersAssume the diameter of an atom is 10–10 metres.

1. Estimate the length of a fingernail.

2. How many atoms are there along the length of a fingernail?

3. This number is too big to appreciate. Scale it up to get a better understanding; i.e. if each atomwere 1 cm across, how long would the finger be on the same scale?


This gives a better idea of the relative size of the atom to an everyday object.

The emptiness of spaceIn the last 20 years, with the popularity of science fiction as shown in films and on TV, with humanand alien spacecraft zapping around planets, stars and galaxies in a matter of minutes, it's very easyto forget just how vast and empty most of the universe is. This exercise may help you get some ideaof the scale of distances in interplanetary space.

Planet Diameter/ km

Distance from theSun / millions of km

Mercury 4878 57.91

Venus 12 104 108.21

Earth 12 756 149.60

Mars 6794 227.94

Jupiter 142 800 778.34

Saturn 120 000 1427.01

Uranus 51 800 2869.60

Neptune 49 500 4496.70

Pluto 2400 5900.00

The Sun itself has a diameter of 1.4 million kilometres, more than 100 times that of the Earth.

Using these data, plan a model of the planets. Use a scale of 1cm : 100 000 kilometres. On this scale,10 cm distance in your model corresponds to one million km in real life. So to start with, we couldrepresent the Sun as being a 14 cm ball (an orange, say) at the centre of your model. In this model,how big are the planets, and how far away from the Sun are they? You may well be surprised at thedistances involved, so check your calculations carefully if your answers don't seem to make sense!

4. To take this further, what is the fastest speed that you could fly in a plane? Or in a military plane?

If you could travel at 1700 m s–1 (five times the speed of sound) how long would it take to reachthe Sun?


Estimating with materialsQuestion 180E: Estimate

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Making estimatesEach of these estimates questions requires that you deal intelligently with the numbers. You shouldbe working to two significant figures and quoting the answer to one. If you need to invent a number totake the argument forward then try to provide some justification for the size of this number.

For more advice on making rough estimates, look at Making rough estimates in Data andMeasurement Skills.

Try these1. Your school / college is concerned about the cost of breakages in the canteen. There is a stock of

100 cups used twice-daily and, on average, there are five breakages a week. The cups cost 50peach. The catering committee proposes to replace these with cheaper, disposable, expandedpolystyrene cups which cost only 89p for a pack of 50. Work out the yearly cost for both types ofcup. Are the polystyrene cups really cheaper? And are there any hidden costs to thechangeover?

2. The cost of the raw material for making polystyrene cups is £600 per tonne (1000 kg) – althoughthis price is variable. A single cup sells for 1.78p. Ignoring any mark-up for profit, estimate thecost of manufacturing a polystyrene cup.

3. Estimate the number of atoms of graphite rubbed onto paper in writing the letter 'A'.

4. In work hardening, the generation of lengths of new dislocation is critical. In a soft metal the total

length of dislocations is about 1011 metres per cubic metre (m m–3) of material; in cold worked

metal the length is about 1015 m m–3. Estimate the length of dislocation line in a sugar-cube sized

piece of metal, in km. Do the same calculation for a several tonnes coil of cold-rolled steel.


The relative atomic mass of silicon is 28 and its density is 2.3 103 kg m–3.

5. How many silicon atoms are there in a volume of 1 m3?

6. A dopant is introduced into the silicon at a concentration level of about 1022 atoms / m3.

(a) How many atoms of silicon will there be for every atom of dopant introduced into the material?

(b) What volume does this amount of silicon occupy?

7. Explain why this calculation suggests that there is a lower limit on the dimensions of a slab of n-or p-type silicon used in a microchip.

Visible structuresQuestion 10C: Comprehension


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What to doAnswer the three sets of questions following the three short passages and images.

Wood grain


Read the following passage. It is based on an extract from Patterns in Nature by Peter S Stevens,published by Penguin Books (1976).

Source

Open the JPEG file

Look at the photographs of wood showing the patterns formed by the grain. They look like thepatterns of streamlines in a flowing liquid. The pattern seems to break into swirls and eddies, makingthe wood look turbulent.

Does wood flow like a liquid? Is it produced by streams and currents? The answer is no. The woodsimply grows along lines of stress. Material is added in response to stress. Where the lines are closetogether the forces are high, where they are far apart they are weak. It is stress rather than woodwhich 'flows'.

1. When a young tree is planted it is often supported by a stake. Why do gardeners remove thestake after a few years and leave the tree to be blown by the wind?

2. In wood, material is laid down naturally along the lines of stress. Do engineers and architectsmake complex structures safe and stable in the same way? Explain.


fibre direction

fibre direction

3. Here are two versions of a chair. In which version is the wood used correctly? Explain how theunsatisfactory chair is likely to fail.

Mica

Open the JPEG file

Open the JPEG file

Look at the photograph of mica. It is a mineral, an aluminosilicate, in which silicate tetrahedra arelinked together to form sheets. The material can be cleaved to give thin transparent sheets. They

have a high tensile strength (3100 MN m–2) and can withstand very high temperatures. Mica can beused for the windows of furnaces.

4. What does this behaviour suggest to you about the structure of mica?


Wire ropeAnd if one prevail against him, two shall withstand him; and a threefold cord is not quickly broken.

(Ecclesiastes 4. 12)

Because the steel is subdivided into many strands it is safer. If one strand breaks in tension thefracture will not spread to neighbouring strands. Wire rope can safely be made from brittle,high-tensile steel.

Open the JPEG file

5. Think of another example of a material which has a fibrous structure. Explain the advantages (ordisadvantages) this type of structure gives.

Photoelastic stress imagesQuestion 40C: Comprehension


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What to doLook carefully at the images. Then answer the questions.

Polarised light view of stresses in a model of the walls of Beauvais Cathedral

Source

Open the JPEG file

A plastic model of the building in cross section is loaded with weights to simulate the effect of wind


forces (called 'wind loading' and an important design factor for tall buildings). The model is heated to150 C and allowed to cool.

The colours seen in polarised light indicate the stress in the material. When the colour bands areclosely spaced, the stress gradient is steeper. Black areas indicate no stress.

1. Sketch the building in outline and label it to show one region of very low stress and one region ofvery high stress.

Photoelastic stress patterns showing the interaction of fibres and matrix in a composite

Source

Open the JPEG file

This image shows a sample of composite held under tension and photographed through crossedpolarisers. The epoxy matrix is reinforced with short rigid bundles of graphite fibres.

2. Where is the stress concentrated?

Tendon elasticityQuestion 50C: Comprehension


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Developing an argumentThis interesting correspondence occurred in the pages of New Scientist, November 1994. Beingphysics-trained we can go further with some calculations, to make these same points quantitatively.Read the passage and then answer the questions.

Q: Do runners waste any energy output by raising their bodies vertically every step? If so, whatpercentage is wasted, and why do they do it?

A: Running is a series of leaps, in which we rise and then fall by about 6 centimetres (at a goodmarathon speed), gaining and then losing potential energy. Also, we accelerate as we take off andslow down as we land, gaining and then losing kinetic energy. Potential energy fluctuationspredominate at low speeds, and kinetic energy fluctuations at high speeds. Most of the work requiredof our muscles serves to supply these components of energy.

The work referred to is wasted, in that there would be no need to do it if we ran smoothly on wheels.Measurements of oxygen consumption show that cycling needs less than half as much energy asrunning at the same speed. But we don't run on wheels – and would be poor at crossing roughground if we did.

We avoid part of the cost of running on legs instead of wheels by bouncing along like a rubber ball.Each time a foot lands on the ground, springs are stretched, and they recoil elastically as the footleaves the ground. About half of the (kinetic plus potential) energy lost and regained is stored up aselastic strain energy and returned, halving the work that the muscles have to do.

The springs involved are the Achilles tendon and the ligaments of the arch of the foot. The tendonstretches and recoils by about 5 per cent of its length. Stretching and recoil of the ligaments allow thearch of the foot to flatten, and then arch again.

R McNeill AlexanderUniversity of Leeds

1. Outline an experiment to determine the rise and fall distance for a person walking (or running).

A 70 kg runner moves at 10 mph (4.5 m s–1), with a rise / fall distance of 6.0 cm.

2. What is the change in gravitational potential energy with each stride?

3. His forward velocity fluctuates between a minimum of 4.4 m s–1 and a maximum of 4.6 m s–1.What is the change in kinetic energy?


4. What is the total energy lost and regained at each footfall?

5. If there were no springs in the legs/feet, how would this energy be lost?

6. Suggest two reasons why this might be a problem.

7. What are the main 'springs' our body uses in running?

This apparatus is used to gather force-extension data and the results are plotted as a graph.

load cell

clamp

tendon extensometer

clamp

actuator

a tendon being stretchedand allowed to recoil in atesting machine

muscle


0 3

0.5

1.0

area A

area B

extension / mm

one cycle of a typical test

8. What does the area under the graph represent? What is area B? What is area A?

For a typical 70 kg man running at marathon speed, the peak ground force is about 1900 N.

9. How could this be measured?

10. How many times body weight is this?

11. Why is it so large?

12. By drawing a triangle of forces, estimate the maximum force in the Achilles tendon.


6.4 kN

1.9 kN

ligaments

Achilles tendon

The cross-sectional area of the tendon is about 90 mm2.

13. Calculate the stress in the tendon.

14. Write this stress as a percentage of the breaking stress, 100 N mm–2.

The Achilles tendon is about 30 cm long.

15. What is its extension when it is stretched by 5%?

16. Using the equation E = ½ Fmaximum xmaximum for energy stored in a Hooke's law spring, estimatethe elastic energy stored.

17. What proportion does this stored energy represent compared with the energy the runner losesand regains with each stride?


Area A in the graph represents energy lost with each footfall. It is about 7% of the total energy lost(A+B), and it heats the tendon.

18. Estimate the volume of the tendon assuming the cross-sectional area of the tendon is 0.9 cm2.

19. Tendon tissue is just a little more dense than water. Estimate its mass.

20. The specific heat capacity of tendon is probably about 3.5 J g–1 K–1. How much would the

temperature rise after 100 strides?

21. The tendon cells will be killed by mild cooking if its temperature rises above 45 C. Is there a limiton how far it is safe to run? Explain your answer.

The foot ligaments can similarly store elastic energy, about 17 J per footfall. Running shoes may savea little too, but their chief function is to reduce the impact forces experienced by running on hardground.

22. What do each of the models shown below represent? Label each part.


force

time

force

time

force

time

Simple models of masses and coiling springs with the floor

low stiffness

high stiffness

23. What does the area under these F–t graphs represent?

24. Which graph shows the best protection for the runner?


Concrete:A material for all seasonsQuestion 120C: Comprehension


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A comprehension passageRead this article and then answer the following questions.

Open the Adobe Acrobat PDF file

Source

Concrete and fibreglass are both given as examples of composite materials in the passage.

1. Name one other example of a composite material you have studied and give an example of itsuse.

2. State three physical properties of the composite material that make it ideal for the use you havesuggested.

3. With reference to the passage, state two advantages of concrete compared with stone when usedas a building material.

Table 1 gives data of strength and density for some common engineering materials.

4. Which of the materials given in table 1 could be described as homogeneous (as described in lines37–38)?

5. Describe, in general terms, the relationship between the strength and density of a material.


6. How does the information in table 1 support the use of steel reinforcing bars to compensate forthe weakness of concrete in tension?

7. Teak wood and Douglas fir are weaker in compression than in tension. Considering the structureof the wood, suggest a reason for this.

8. Cast iron is a brittle material. Assuming that cast iron has the same stiffness in compression andtension, sketch graphs of stress against strain for both compression and tension and indicate theessential similarities and differences.

The author states, in line 60, that steel is ductile, making it an ideal reinforcing material for concrete.In line 72, the author suggests that metals become very brittle at very low temperatures.

9. Explain, in terms of atoms, why steel can become permanently stretched on impact.

10. Suggest, in terms of atoms, why metals become very brittle at very low temperatures.

Figure 1 shows a graph of the percentage of 28 day strength against age of concrete. A logarithmicscale has been used for the age of concrete.

11. Use the graph to obtain values of the percentage of 28 day strength at 1 month, 1 year and 5years and draw a sketch graph of the percentage of 28 day strength against the age of theconcrete using a linear scale for the age.

12. Suggest why a logarithmic scale was used to represent age in figure 1.

13. Explain, in your own words, why concrete might crack when it sets.


14. Use the formula, change in length = change in temperature original length , to show that a 2m long concrete block will contract by approximately 1 mm if it is cooled by 45 C after beingtaken out of its mould. The value of for concrete is given in the passage.

Wire ropes and suspension bridgesQuestion 130C: Comprehension


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Open the JPEG file

InstructionsRead the following passage and then answer the questions.

John Roebling, bridge builder and manufacturer of wire ropeJohn Roebling studied bridge building in Germany, from where he set sail to America in 1831 whenhe was 25 years old. He started a factory to manufacture wire rope. The business thrived and playeda major role in the history of bridge building in North America. Roebling designed suspension bridges.His masterpiece was the Brooklyn Bridge.

A wire rope is an example of a composite structure. Because the metal fibres are not embedded in amatrix it is not always recognised as such. If one fibre breaks, the gap prevents the released strainenergy from being transmitted to a neighbouring fibre. The tensile strength of a multistranded wirerope will therefore be greater than that of a single wire using the same amount of material.

Roebling was an articulate and prolific writer. To see the million-dollar projects that he conceivedactually become fully operating structures, he had to interact with politicians and businessmen and


convince them that what was proposed was not only structurally sound but also politically andfinancially sound.

In the 1840s the dependability of suspension bridges was seriously questioned by engineers andlay-persons alike. Telford's bridge across the Menai Strait in Northwest Wales had run into trouble inhigh winds. The chain pier at Brighton had been very severely damaged in storms. As a resultsuspension bridges were out of favour in Britain but Roebling did not accept this line of thinking.

He successfully designed a series of suspension bridges. He did not simply copy his own safedesigns. He concentrated his design judgement on how his bridge might fail. He achieved success byidentifying and confronting failure modes.

In a report submitted on 1 September 1867, to the president and directors of the New York BridgeCompany, Roebling begins his discussion of the practicability and strength of the bridge he proposedto link New York and Brooklyn by describing the breaking strength of 'a bar of good wrought iron'.After establishing that wire drawn from such a bar could support, at its breaking strain, 32 400 feet ofits own length hung vertically, he applies a factor of three and argues that one-third of that length, or10 800 feet, will, if left undisturbed and kept from oxidation, support its own weight any length of time.

Roebling went on to take into account the increase in tension when the rope was stretched 'across awide chasm'. He concluded

From the simple facts and considerations, it is plain that the central span of the East River Bridge,which is only 1,600 feet from centre to centre of the tower, is far within the safety limits of goodwire.

Roebling's Brooklyn Bridge was completed in 1883. The safety margin in the main cables of thebridge was six times. It spanned over a quarter of a mile between towers. Within another century themile span was being approached in structures like the Humber Bridge in England. At more than 4600feet the Humber was the longest span in the world when it was completed in 1981.

Questions1. What is meant by the 'safety margin' in a design?

In the passage above Roebling expresses the 'breaking strength' of a wire rope made from his 'goodwrought iron' in an unfamiliar way.

2. Use the information he gives to calculate the stress corresponding to Roebling's 'breakingstrength'.

3. Compare this value with values given in the literature for the tensile strength of wrought iron.(Note: 1 ft = 0.305 m. Take the density of wrought iron as 7.9 Mg m–3.)

4. Explain why your value is higher.

High-temperature superconductivityQuestion 160C: Comprehension


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Read the following passage and answer the questions.

Electrons on the moveIn 1911, the Dutch physicist Kamerlingh Onnes made a remarkable discovery. Three years earlier, hehad managed to liquefy helium, and this allowed him to reach temperatures close to absolute zero(–273 C) in his laboratory. He wanted to investigate the electrical properties of metals at lowtemperatures, because he knew that the resistance of a metal decreases as it is cooled. When hecooled a thread of pure mercury, he found that its resistance suddenly dropped towards zero at 4.2 K(4.2 degrees above absolute zero). What Onnes had observed was superconductivity. Many metalshave this property – their resistance becomes zero at very low temperatures.

The diagram below shows Onnes' results. He couldn't be sure that the mercury's resistance was

exactly zero; he could only say it was 0.11 just above 4.2 K, and that it fell to at least 10–5 at thiscritical temperature.

4.00 4.10 4.20 4.30 4.40

0.00

0.025

0.05

0.075

0.10

0.125

0.150

Hg

10–5

Temperature / K

Nowadays, we know that superconducting materials really do have zero resistance. A current couldflow through a superconductor for ever, without losing any energy. To understand better how this canhappen, we need to look more closely at the way in which electric current flows through metals.

Two factors can affect the resistivity of a metal: the temperature and the purity of the metal. If a wire isheated, its resistance increases. If it is cooled, its resistance decreases. The following graph showsthat the resistivity of a pure metal approaches zero as the temperature approaches zero. An impuremetal, however, retains some resistance, even at 0 K. To understand how these factors affectresistivity, we need to think about how a current flows through a metal. In a metal, an electric currentis a flow of 'free' electrons. These are electrons which are not bound to the atoms of which the metalis made. Typically, there will be one free electron for each atom, and so the concentration, n, of freeelectrons is similar to the number of atoms per unit volume in the metal.

They really are nearly 'free'. These electrons can move without hindrance through the regularlyspaced ions in the metal lattice. So it seems as if no potential difference is needed to maintain anelectric current. A potential difference is needed, in fact, because the free electrons are scattered bythermal vibrations, by impurities and by defects in the lattice. The electrons move in all directions, but


the potential difference maintains a small average drift speed along the conductor.

It took more than 40 years to find the explanation of superconductivity. The idea is that the electronscan all be in exactly the same state of motion, going along carrying a current. This cannot normallyhappen, because electrons are forbidden to have exactly the same state (for example, they occupydifferent states in the shells of atomic structure). However, it can happen if they pair off. One pair canbe in exactly the same state as another pair, as can photons in a laser. These pairs can then travelfreely, all moving together and carrying a permanent electric current.

The pairs ('Cooper pairs') form like this. One electron pulls the lattice ions a little nearer to itself, sothat there is a slight excess of positive charge near it. A second electron is slightly attracted by thispositive charge. Meanwhile the second electron is doing the same to the first one.

0 100 200 300

pure metalimpure metal

temperature / K

Superconductivity is a fascinating phenomenon, but it has yet to find many everyday applications.This is because of the need to maintain low temperatures using liquid helium, which is bothinconvenient and expensive. However, there are many specialist applications where convenience andexpense are less important. One area where superconducting wire has been used extensively is inelectromagnets (solenoids) which have found uses in laboratories where strong magnetic fields areneeded (for example, in particle accelerators), in magnetically levitated high-speed trains, and inmagnetic resonance imaging (MRI) body scanners, like the one shown in below.

Source

Open the JPEG file

Onnes himself quickly saw how a superconducting solenoid could be used to show that asupercurrent could flow forever. He set up a current in such a solenoid, and placed a compassnearby. The solenoid's field deflected the compass needle, and Onnes was fascinated to see how it


remained deflected for several days. This convinced him that a superconductor truly has zeroresistance.

How can we set up an 'eternal current' in a superconducting solenoid? The diagram below shows oneway to do this. The circuit includes a clever device, a 'superconducting switch'. This is a length ofsuperconducting wire with a magnetising coil wrapped round it. When current is supplied to themagnetising coil, the magnetic field destroys the superconductivity, and the switch is 'off'. With themagnetic field off, the wire becomes superconducting, its resistance is zero, and the switch is 'on'. Toestablish the desired current in the solenoid, the superconducting switch is first kept off. Current flowsfrom the power supply, around the coil. When the desired current is achieved, the superconductingswitch is turned on (magnetising current turned off). The current in the coil now flows through theswitch; it has a continuous superconducting path, and could flow forever without showing any signs ofdecreasing.

rER 0-10 k

power supply E = 10 Vr = 0.1

I

superconducting solenoid

x

liquid helium

superconductingswitch

A

B

magnetisingcoil

In the mid-1980s, a new type of superconducting material was found. These are not metals; they areoxides, ceramic materials. The exciting thing about these superconductors is that they becomesuperconducting at much higher temperatures than do metals. This is important because they can becooled using liquid nitrogen at 77 K. This is much cheaper and more convenient than using liquid


helium. Ceramic materials are brittle, which is a problem. Once these new materials are formed intothe desired shape, many different applications are possible. For example, superconducting motorscan be made which consume very little power. Such motors could drive pumps and fans in powerstations, saving up to 5% of the power generated. These 'high-temperature superconductors' pose atheoretical problem too. How do they conduct? It seems certain that Cooper pairs of electrons areinvolved, but what is the mechanism that binds them together?

1. Kamerlingh Onnes used an ammeter and a voltmeter to measure the resistance of his sample ofmercury. He could not be sure that its resistance was exactly zero below 4.2 K. Why not?

2. Onnes' experiment with a solenoid and a compass showed that the compass needle remaineddeflected by the magnetic field of the supercurrent for several days. Did this show that thesolenoid's resistance was zero?

3. Use the information below to calculate the number of atoms per cubic metre in copper:

density of copper = 8930 kg m–3

molar mass of copper = 63.55 g mol–1

Avogadro constant = 6.022 1023 atoms / mol

4. The concentration, n, of free electrons in copper is about 8.5 1028 m–3. Compare this figure withyour answer to the previous question. Estimate the number of free electrons contributed by each

atom in copper.

5. Free electrons in copper have a typical randomly directed speed of 1.5 106 m s–1. At room

temperature, they experience about 1014 collisions per second. Estimate the average distance an

electron travels between collisions.

6. At 4 K, the resistivity of pure copper is about 105 times smaller than at room temperature.

Estimate the average distance an electron travels between collisions at 4 K.


7. In the circuit shown, the current supplied to the superconducting solenoid can be altered byadjusting the variable resistor. What is the largest current which can be supplied. What is thesmallest current?

8. The superconducting switch has a resistance of 10 when it is magnetised and 0 when it isnot magnetised. In which position should switch B be set, if the superconducting switch is to beopen?

9. A current I = 5 A reaches point X in the circuit. How will it then flow, if the switch is closed? And ifswitch B is open?

How resistivity changes with temperatureQuestion 140D: Data Handling


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BackgroundMetals and semiconductors both conduct electricity. As the temperature changes, their resistivitieschange, but in very different ways.

This question includes two graphs for you to interpret, to help you to understand and explain thedifference between metals and semiconductors.

Resistivity–temperature graphs


4002000 600 800 10000

5

10

15

20

25

30

35

Temperature / K

PtAlAuCuAg

This graph shows how the resistivities of five different metals depend on temperature.

For platinum, many measurements were made, and the results are shown as a continuous curve. Forthe other four metals, measurements were made only at a few temperatures and the lines are drawnto connect data points.

1. Which of the metals shown has the lowest resistivity?

2. From the graph, find the resistivity of platinum at a temperature of 800 K.

3. Describe in words how the resistivity of platinum changes as the temperature increases.

4. How do the resistivities of the other metals change as the temperature changes?

5. Lines have been drawn to connect the data points for four of the metals. Do you think that thisgives a reasonable picture of how their resistivities change as the temperature increases?

6. Some resistance thermometers are made using a coil of platinum wire. As the temperature of thewire increases, its resistance increases. Measuring the resistance can then give a measure of thetemperature. Use the graph to explain how such a resistance thermometer can be used over awide range of temperatures, and why platinum is a better metal for this purpose than gold orsilver.


106

104

102

100

10–2

10–4

10–6

10–8

0 200 400 600 800 1000

Temperature / K

Silicon (calculated)

Platinum

This graph shows how the resistivity of silicon, a semiconductor, changes as the temperatureincreases. The values have been calculated using information about the properties of silicon.

Also included are the data for platinum; these are the same data used in the first graph.

7. Look at the resistivity scale on this graph. How does it differ from the scale used in the firstgraph? Why is this type of scale used here?

8. At 300 K, the resistivity of platinum is about 10–7 m. What is the resistivity of silicon at this

temperature? By how many orders of magnitude do the two resistivities differ?

9. Describe in words what the graph tells you about how the resistivity of silicon changes as thetemperature increases. How does this compare with the behaviour of platinum?

10. Use the idea of free (conduction) electrons to explain why these two materials behave in suchdifferent ways.


Questions on metalsQuestion 70X: Explanation–Exposition


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Getting startedThese questions are intended to help you think about the relationships between the microstructure ofmaterials and their macroscopic properties. To deal with real-life problems, you need a good grasp ofbasic concepts. So the questions begin with some simple reminders of terms and go on to help yourevise ideas of microstructure. It is important to understand how metallurgists can modify thesestructures; some questions deal with this too.

Questions1. Describe the behaviour of a material which is

stiff but not strong, and another which is

hard but not tough.

2. Look at the picture of a ball-bearing analogue. Identify each of the defects that are labelled on thepicture.

Open the JPEG file


3. A student bends a straight bar of copper into a u-shape. He is then unable to bend it straightagain. Explain why this happens (the answer is not that he has tired himself out!)

4. What would you do to a sample of metal in order to (i) anneal it, (ii) quench it? What change tothe properties of the material would you expect as a result of each treatment? How does themicrostructure of the material change as a result of each treatment?

Use the image above of a model of a grain boundary, a dislocation and a vacancy. It also appears inDisplay Material 150S 'Making models of metals'.

5. For each defect, estimate how much it distorts (stresses) the atomic structure around it. Ameasure of the distortion might be how far (in atomic diameters) you have to move from thedefect before its effect on the position of the nearby atoms becomes negligible.

Cadmium can be grown as a single crystal in the shape of a thin cylinder. When pulled in tension,steps appear on the surface.

6. Explain in terms of dislocation movement how these steps might have formed. Estimate howmany dislocations will need to move to form a step 0.1 mm high.

7. How does the addition of carbon to pure iron lead to a harder material? Explain the microscopiceffects that occur to harden the material.


Ductile materials 'neck' down when they are deformed in tension. You may well have seen thishappen in the laboratory (the figure shows the sequence of failure).

voids (inside)

concave surface

convex surface

Eventually the material fails in a 'cup-and-cone' fracture. Scanning electron micrographs show thatsmall cavities (usually referred to as 'voids') are forming in the region of the fracture.

8. Suggest some reasons why these voids might form.

9. Why does the specimen only neck down in one place?

The graph shows the force-extension curves for two steel wires with identical shapes; one steel has ahigh carbon content, the other is a mild steel with small amounts of carbon.


extension

A

B

10. Which wire is stronger?

11. Which wire is tougher?

12. Which is the mild steel specimen?

13. Account for the difference in mechanical properties of the steels, paying particular attention to theamount of carbon and to the way in which this will modify the behaviour of the steel at themicroscopic level.

ConductivityQuestion 170X: Explanation–Exposition


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1. Pure copper has a high conductivity. Copper is alloyed with tin to form bronze. How do youexpect the conductivity of bronze will compare with that of copper? Explain.


2. Gold is often used for the electrical connections on circuit boards. Name two properties make itsuitable for this purpose.

3. Sketch diagrams to show how a series of electrons hopping into vacancies in a semiconductingmaterial can give the appearance of a single vacancy or 'hole' moving in the opposite direction.

Thermistors and light-dependent resistors (LDRs) are both semiconductor devices whose conductivitydepends on their physical conditions.

4. Describe how a thermistor behaves, and explain what is happening inside the material.

5. Do the same as question 4, but for an LDR.

Biological tissuesReading 10T: Text to Read

Teaching Notes | Key Terms

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All the materials making up your body are made up of cells. There are many different types of cell.They group together to form the tissues and complex organs which make up the body. It is thedifferent properties of these cell types, and the way in which they are arranged, which give the tissuesand organs their unique properties: for example, the strength of bone, the strength and elasticity ofmuscles, and the elasticity of skin.

A tissue is the name given to a group of cells (which are usually all similar or at least of only a few


types) and the substance which surrounds them. Together, the cells and the intercellular substanceperform a particular function. Bone, tendon and skin are examples of tissues. The study of tissues iscalled histology.

Tissues can be identified by looking at sections under the light microscope.

Bone

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Cartilage and bone are the main supportive tissues in vertebrates. They are types of 'connective'tissues which are distinguished by the presence of a small number of different cells in a large quantityof intercellular material. Bone contains calcium which gives it its great strength.

This is a section through a developing long bone. Can you see the nuclei in some of the individualcells?

Tendon

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Tendons are cords of tough, fibrous connective tissues which attach muscles to bones and sotransmit the muscles' pull to the bones. They need to be very strong and also elastic.

This is a section through a tendon. You can see that it consists of a group of parallel fibres. Whatfeatures of this tissue do you think might contribute to its strength and elasticity?

Artery

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A blood vessel that carries blood from the heart to the tissues is called an artery. Arteries need tohave thick, elastic, muscular walls because they carry blood at high pressure.

This image shows the cross section through an artery. You should note the thick walls of muscletissue and the blood cells inside the vessel.


The pressure of the blood in veins is much lower than that in arteries. What differences do you thinkyou would see between this cross section and the section through a typical vein?

Skin

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In mammals, most heat exchange occurs through the skin, and it plays an important role intemperature control.

The skin consists of separate layers of different tissues. The top layer is called the 'epidermis', andthe middle layer the 'dermis'. The lowest layer consists of subcutaneous fat (adipose tissue). The toplayers of the epidermis consist of dead cells.

This image shows a cross section through thin human skin. Try to identify the three layers. Note thatthe adipose tissue layer is the only one where individual cells will be clearly visible.

Physics in Use: Presentation on materials – briefing for studentsReading 20T: Text to Read

Teaching Notes

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The taskIn this unit, we hope you will take a deeper interest in some of the wide range of materials peoplehave either discovered or invented, and used. It is no exaggeration to say the material worldprofoundly influences our civilisation and so our lives.

We have designed the course so that you gain the background knowledge to:

1. Research one material of your choice.

2. Make a presentation about it.

There are many novel materials, but you could also look at a traditional or historical material. Workingintensively, the total time you spend on this task should be in the range of three to five hours.

You are expected to:

1. Show the relationships between the bulk properties of the material and its use.

2. Set your material in a context.


This forms one of two coursework tasks for the AS course.

The criteria for assessment are given in the OCR Specification.

First you need to select a topic and use a number of resources to do your research (magazines,textbooks, websites, papers etc). The 'starters' provided may give you an idea or you may alreadyhave your own. Research takes time, particularly if you have to write off for some information. Youmust begin this early. It is important you keep a record of where you found a piece of information.

Next you need actually to digest and interpret and understand the information. It is not enough simplyto collect it. The skill of research is to accumulate information and then organise it to tell the story youwant to tell. In doing this you need to bear in mind your audience, which is your fellow students. If youcan't understand it, it is most unlikely your 'explanation' will help them.

That leads you on to think of the third aspect, the presentation itself. You will need to choose how topresent your research. Thinking about your presentation will force you to order your material andunderstand what is really important.

Finally, for this presentation you are asked to set the material you have chosen in a wider context. Itwould be easy to report only on the technical details of a material. Physics needs to connect with thereal world, so you are asked to inform your audience about the connection between your material andthe real world. This may be an example of its aesthetic, social, historical, political or economic impact,either locally, nationally, globally or on a much more human scale, closer to home.

BiomimeticsReading 30T: Text to Read


Quick Help

This information sheet will get you started on a case study of biomimetics. It contains a briefintroduction, which explains why this is a worthwhile topic to study, followed by a list of resourceswhich will point you in the direction of the information you need.

Before you start work, study the questions in the final section; these could help you focus yourthinking on this topic.

Why study nature?Long before humans had mastered even the most elementary materials (such as stone) or thesimplest of engineering principles (such as the wheel), nature had evolved thousands of elegant andintelligent solutions to the problems of everyday life. Think of traps like a spider's web, the economicpackaging of leaves and petals in a bud, the strength of wood and the toughness of mother-of-pearlwhich lines the shells of molluscs. Rather late in the day, humans have realised that they might beable to turn nature's material science to their own advantage. Modern analytical methods arerevealing the inner structures of biological materials, while a study of natural design principles may beapplied to human engineering problems. This new science is called biomimetics and it is 21st-centuryscience, ready to take its place alongside polymer science and metallurgy in offering us advancedmaterials and design solutions to a wide range of problems.


Resources to useThe following references will be useful:

PapersThese background papers explain the principles of biomimetics and give some examples of thescience in practice:

Vincent J 1996 Tricks of nature New Scientist (17 August) 38–40 (written by one of the world'sleading experts in biomimetics)

Reading 20T 'Materials from Nature'There are several short features on biological materials in New Scientist, but they will not be indexedunder biomimetics; look for the material itself, for example wood, bone.

BooksAmato I 1997 Stuff: the Materials the World is Made Of (Avon Books). Pages 157–70 This contains a discussion on biomimetic materials covering their history, some of the pros and cons,and a detailed description of the structures of several biological materials such as nut and molluscshells, along with an indication of their present and future applications.

Ball P 1997 Made to Measure: New Materials for the 21st Century (Princeton University Press). Chapter 4 contains a lengthy and detailed discussion on biological materials, including a section onspider silk, wood, mother-of-pearl and bone. There is a short section on biomimetics too, but thisdeals mainly with design principles rather than materials. Check the bibliography which containsmany extra useful references.

Working with Materials: Wood, Metal, Plastic 1996 Collins Real World Technology (CollinsEducational). All you need to know about wood, its properties and applications!

Lewington A 1990 Plants for People (Natural History Museum Publications). Lots of information about the origins and applications of plant materials such as cotton, silk and wood.

Vincent J F V 1990 Structural Biomaterials (Princeton University Press). Technical, but good for looking up facts and figures.

Getting started:Questions to think about1. What are the main differences in structure between biological materials and metals?

2. Is biomimetics all about copying nature?

3. Could biomimetics ever become as big as the plastics industry?

4. Wood is a very abundant material on the planet. Can we find new uses for it? What could itreplace?

5. What other biological materials have untapped potential? Could they find new applications in theirpresent form – or would it be better just to use the concept and create a new material from it?Can you find examples of each?


Introduction to proteinsReading 40T: Text to Read


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Proteins are essential components of all living things. They have a wide variety of biological functions,including transport (e.g. haemoglobin), nutrition (e.g. digestive enzymes such as trypsin) and support(e.g. the muscle protein myosin and collagen).

These substances are all very different, but they are all built up in the same way. Proteins are largemolecules which are built up as long chains of 'building blocks' called amino acids. Twenty differenttypes of amino acids are found naturally in proteins. The sequence of amino acids in a particularprotein chain determines the shape and function of that protein. An enormous number of proteins canbe built up from these 20 units. This is like building up thousands of words from the 26 letters of thealphabet – but some proteins are many thousands of units long!

Protein crystal

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Proteins are very small – far too small for their detailed structure to be visible even using the mostpowerful electron microscopes. Scientists use the technique of x-ray diffraction to calculate thestructures of proteins from the way in which x-rays are scattered by the arrangement of atoms in acrystal. We now know the structures of thousands of different proteins.

This is a picture of a protein crystal, as seen down an ordinary light microscope. Although proteinsare complicated molecules, their crystals usually have simple shapes, and look rather like crystalswhich can be grown from common substances like salt.

Diffraction patternWhen a beam of x-rays is shone at a protein crystal, different regular layers of atoms 'reflect' thex-rays. This produces beams of x-rays which 'scattered' in different directions. The scattered x-raysare more intense in some directions than others, so they form patterns which look like this:


Source

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The pattern of atoms making up the protein – the protein structure – can be reconstructed from thispattern using powerful computers.

FerritinThe structures of small proteins can be easiest to understand. Ferritin is a small protein which is builtup from about 180 amino acids. It is found in mammals, including humans; it is involved in storing ironin a soluble, non-toxic form so that it can be transported easily and safely around the body.

The structure of ferritin, or of any protein, can be represented in several different ways. Each of thesedisplay styles shows something different about the protein's shape or function.

'Wireframe' model

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In this picture the bonds between atoms are shown as 'sticks'. You can't see the atoms at all, but youcan tell where they are as there is an atom at each end of each bond. The bonds are colour-coded bythe element type: Carbon atoms are grey, oxygen atoms are red and nitrogen atoms are blue.Hydrogen atoms are not shown in this structure – this is quite common. There is therefore only oneother element shown here – sulphur. Can you guess, without looking at the structure, what colour isused to represent sulphur? Were you right?

What do you think that this style of displaying molecules is most useful for? (Hint: think about scalingup the molecule so that you can see it in more detail.)

'Ball-and-stick' model

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This is very similar to the 'stick' model but the position of each atom is also shown as a 'ball'. Theradius of each colour of ball is proportional to the size of an atom of each element.

This style is commonly used in plastic models of atoms and molecules.

'Spacefilling' model

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If you could shrink yourself down to atomic size, you would not see the bonds between atoms at all.Instead, you might see atoms as very roughly spherical 'clouds' of electrons surrounding the tinynuclei. The 'spacefilling' model shown here is therefore the most 'realistic'. Again, each atom is shownas a sphere, but the spheres are larger and the bonds are not visible.

Is it easy to see the structure of the molecule?

'Cartoon' model

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None of the styles you have seen already is very good at showing the way that a protein chain folds.This protein consists of four long coils called 'helices', and one shorter coil, all joined by loops. Goback to the other representations. Can you see the chain, even knowing roughly what it should looklike?

People have developed 'cartoon' representations of proteins to illustrate the path of the protein chain.This is still ferritin; it's now very easy to see the helices and loops. Each helix is coloured separately.

Schematic diagram of fold

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Sometimes it is useful to look at the protein as a two-dimensional 'schematic' illustration. This is a


two-dimensional schematic picture of ferritin, with the helices shown in the same colours. The chainstarts at the 'bottom' of the blue helix (blue arrow) and ends at the 'top' of the red helix. Can you seethe long loop between the green and yellow helices in the three-dimensional cartoon?

You should always remember that although this is a very useful way of looking at protein structure, itis totally unrealistic. Real proteins don't look at all like these cartoons!

Hip replacementsReading 50T: Text to Read


Quick Help

This information sheet will get you started on a case study of hip replacements. It contains a briefintroduction, which explains why this is a worthwhile topic to study, followed by a list of resourceswhich will point you in the direction of the information you need.

Before you start work, study the questions in the final section; these should help you focus yourthinking on this topic.

Bone repair:Materials science to the rescueBone serves us superbly as a scaffolding material which supports and protects our bodies. However,like other biological materials, it changes as we age (think of skin) because of biological andmechanical wear and tear. The joints, the places where bones meet, are especially vulnerable in thisrespect and damaged joints greatly impair mobility as well as being extremely painful. Fortunately adamaged joint can be replaced by a prosthesis (the general term for an artificial body part). The hipjoint is actually the most common prosthesis, with around 40 000 hip replacement operations beingperformed each year in the UK (and half a million world-wide). Replacements of the knee, shoulderand finger joints are also fairly common operations.

Joint replacements have a high success rate and it used to be that the prosthesis would last a personfor the rest of his or her life. However, as the population ages, and as more prostheses are given toyounger people, more prostheses are failing (their average lifetime is only 10–15 years). Up to 20% ofall joint replacement operations (costing around £5000) are repeats. It is not the material of theprosthesis itself which fails – more that the joint works loose because of the way the body responds tothe material (in a nutshell, the prosthesis weakens the surrounding bone). Here is a marvellousopportunity for materials scientists to save the NHS money and to give people a better quality of life –by searching for better materials for hip replacements. The researchers are already making goodprogress. Bone replacement materials (which could one day be used in joint replacements) arealready being tested in people, to repair bone damage in the skull, jaw, ear and spine.

Resources to useThe following references should be useful:

PapersBonfield W and Tanner E 1997 Biomaterials – a new generation Materials World (January) 18–20Professor Bonfield is one the leading experts in bone replacement materials – his description of his


work on Hapex, a composite bone mimic, is well worth reading. He also discusses materials currentlyused in hip replacement.

Pengelly A 1998 Implanting wisdom Materials World (December) 758–60A materials scientist describes, from his own experience, what it is like to have a hip replacement,discusses the problems and suggests some solutions.

Peppas N A and Langer R 1994 New challenges in biomaterials Science 263A more technical report which gives an overview of the whole field of biomaterials. You may like toinclude some of these ideas and examples in your case study.

BooksBall P 1997 Made to Measure: New Materials for the 21st Century (Princeton University Press)(chapter 5 Spare parts, structural repairs) pp 221–6A good introduction to the topic, and an overview of some of the new materials which are beingdeveloped for bone repair.

Callister W D Jr 1997 Materials Science and Engineering: an Introduction (Wiley) (chapter 23 section6 Artificial total hip replacement) pp 732–8Although this is a university textbook, don't be put off, because this section contains all the facts youneed about the materials currently used in hip replacements, as well as describing the requirementsneeded in materials to be used in the body.

Revise/learn some not too complex biology about bone and joints (any textbook of biology, anatomyor physiology).

VisitsFind out what your local hospital is doing in the area of joint replacement (check with your teacherfirst to see how best to approach them). You may be able to chat to an orthopaedic surgeon, find outhow many operations are done, see the prostheses and find out more about costs and generally findout how this aspect of materials science is benefiting your own community.

Getting started:Questions to think about1. What are some of the reasons for people needing hip and other bone replacements?

2. What are the properties of bone that bone replacement materials need to mimic? How far canthey do so?

3. What properties would you expect of a material that is to remain in the human body for severalyears (or even for life)?

4. Describe the properties (give figures, if you can) of the materials used in hip replacements (note:these include metals, ceramics and plastics).

5. Describe some of the new materials being developed for bone replacement, giving their expectedadvantage over current materials.

6. What are the benefits for individuals and for society as a whole in developing new bonereplacement materials?


Contact lensesReading 60T: Text to Read


Quick Help

This information sheet will get you started on a case study of contact lenses. It contains a briefintroduction, which explains why this is a worthwhile topic to study, followed by a list of resourceswhich will point you in the direction of the information you need.


Materials to see through:A century of contact lensesContact lenses are an excellent example of how materials to suit a particular purpose have evolvedover time. Having spectacles made of glass is one thing (and now even these are made of plastic) butimagine putting a piece of glass onto the surface of your eye in order to see better. Yet the firstcontact lenses, made in 1887, were actually made of glass, because there were no other suitablematerials available. It was only with the discovery of plastics, from the 1930s, that the use of contactlenses became widespread.

Today, they are made from a variety of transparent polymers, and there are several different types oflenses – from soft lenses which last for many months and lenses you can sleep in, to daily disposableones and even tinted lenses that change the colour of your eyes. In short, the contact lens wearerhas never had so much choice, thanks to advances in materials science.

The main classes of contact lens materials are listed below:

1. Hard contact lenses (not much worn now) are made from polymethylmethacrylate (PMMA)commonly known as Perspex. This is a transparent, rigid plastic which does not allow oxygen topass through it.

2. Soft contact lenses (long-term and disposable) are made from a class of polymers known ashydrogels. Most of them are based upon a polymer called hydroxyethylmethacrylate (HEMA)which contains hydroxyl (OH) side chains. These attract water molecules, forming a hydrogelmaterial which is part way between a solid and a liquid. Hydrogels are very flexible, and mouldeasily to the contours of the eye. They also allow some oxygen through, which is essential to thehealth of the eye. They contain between 35 and 80% water.

3. Several types of polymer containing silicon have been tried for rigid gas-permeable lenses that,because they allow oxygen through, are suitable for extended wear. Silicone rubber lenses, forinstance, offer no barrier to oxygen. However, most of the silicon-containing polymers are veryhydrophobic (in contrast to the hydrogels which are hydrophilic), which makes themfundamentally incompatible with the eye. There has been more success with gas-permeablelenses made of polymers containing fluorine side chains.

We require a great deal from a material to be used in contact lenses, and none of the examplesabove quite fits the bill – which is why the search for new and improved materials continues.


Resources to useRevise, or read up on, the optics of the eye and disorders of refraction such as short-sightedness(myopia) in a biology textbook (or ask a friend doing biology to explain this to you) so you can relatethis to the job contact lenses do.

If you wear glasses or contact lenses yourself, you could interview your optician (or write with a list ofquestions) to find out more about who wears which kinds of lenses and the factors involved inprescribing them.

Getting started:Questions to think about1. What properties must a contact lens material possess (go beyond material properties for this

one).

2. List the advantages and disadvantages of contact lenses compared with spectacles.

3. Which is the better type of material for contact lenses – hydrophobic or hydrophilic? Why? If youdon't understand these words, look them up.

4. What types of visual defect can contact lenses correct? (illustrate with graphics).

5. List the advantages and disadvantages of (a) hard contact lenses, (b) daily disposables and (c)extended-wear lenses.

Metal alloys:Then and nowReading 70T: Text to Read


Quick Help

This information sheet is intended to help you get started on a case study that looks at ourmanipulation of metals to produce predictable and desired properties. There is a brief introductionthat explains why this is a worthwhile topic to study and there are resources that will point you in thedirection of the information you need.

A history of metallurgyThe extraordinary rise of Homo sapiens over the past few thousand years goes hand in hand with themastery the species has attained over materials. From the first stone knives and axes to today'scomplex and tailor-made artefacts, our species has manipulated the raw materials of the planet andtransformed them to match our own needs.

In this case study you might choose to look at the broad history of metallurgy, at a specific material ofimportance in past times, or one that is important now.

The history of western European metallurgy goes something like:

Stone Age (up to 4500 BP) (BP means 'years before the present')


Bronze Age (about 4500 to 2500 BP)

Iron Age (from about 2500 BP)

Steel Age (200 BP)

Plastics Age (50–100 BP)

Superalloys Age (the one we are in!)

You could take a broad historical look at all the developments represented in this list.

Ask yourself questions about the materials and their fabrication and about the way that they came intouse. Why did we begin to use one material more than another? What made us change from onematerial to another? Was it some change in the needs of the people of the time? Or did somedevelopment enable a difficult material to be worked easily? Are there any parallels between the useof materials and the arts and sciences of the time? What about the architecture, what were bridgesand buildings constructed from? How did the materials affect the lives of the wealthy or of ordinarypeople? Did nomads use different materials from town dwellers?

Alternatively, you could consider just one material in more detail. Here are three lists of questions foryou relating to three of the materials: bronze, steel and the superalloys being developed today.

BronzeWhat is bronze? How was it made? To what uses was it put? Is bronze used today? Was it, duringthe Bronze Age, a common material or was it confined to the rich people? What are the tensileproperties of bronze? How do these reflect the uses to which it was put? In what ways did it changepeople's lives?

SteelSteel is another material that has been of great importance in our historical development. What is itshistory? Has it always been used the way it is today? What are pig iron, wrought iron, steel? Whichimportant scientists and engineers contributed to its history? Have all cultures used steel in the waywe do in the West?

SuperalloysIf you have access to Ivan Amato's book (see Resources to use) read chapter 7. He gives threedetailed examples of superalloys: the steel alloy used for fuel pump bearings in the Space Shuttle(this pump operates in probably one of the most hostile environments in the world), the developmentof a lighter but stronger metal for gears, and self-sealing steels that 'repair' themselves.

Choose one of these examples and try to amplify what Amato covers. What are the requirements fora bearing steel or a gear? What are the metals with a memory that he describes? What other usesmight these metals have?

Consider the Olson diagrams he describes. Try to understand how modern materials scientists aremore predictive about their new materials than people were able to be 50 or 100 years ago. What arethe reasons for this? (And if you want some light relief, look at the Olson diagram for ice cream andtry to find out something about how it is made in the kitchen and commercially!)

Resources to useThere are some large questions in this case study. You can find many books on the history of scienceand engineering that give accounts of the history of materials, including detailed descriptions of


processes.

Alexander W and Street A Metals in the Service of Man (Penguin)

Amato I 1997 Stuff: the Materials the World is Made Of (Avon Books)

Bronowski J 1973 The Ascent of Man (BBC Books)

Derry T K and Williams T I 1960 A Short History of Technology (Oxford University Press)

Gordon J E 1968 The New Science of Strong Materials, or Why You Don't Fall Through the Floor(Penguin)

Gordon J E 1978 Structures, or Why Things Don't Fall Down (Penguin)

All these books will take you on into other texts and convey to you the wonderful story of metalstechnologies.

Cakes, confectionery and chocolateReading 80T: Text to Read

Teaching Notes | Key Terms | Resources

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This information sheet will get you started on a case study of chocolate and related foodstuffs. Itcontains a brief introduction, which explains why this is a worthwhile topic to study, followed by a listof resources which will point you in the direction of the information you need.


Fats, sugar and a large measure of materials scienceWhatever your view on the nutritional value of chocolate, sweets, cakes and biscuits, they are allfascinating to study from a materials point of view. The main component of chocolate, for example, iscocoa butter – a fat which is extracted from the cocoa bean. Unlike other natural fats, cocoa butter isremarkably uniform in its composition and has a sharp melting point of 34 C (just below bodytemperature). These simple physical facts underpin the whole of the chocolate industry; chocolate iseither solid or liquid – with no in-between stages (compare butter) – and it melts in the mouth, giving apleasing cooling effect that adds to the taste sensation.

Sweets are based on sugar, and food scientists can produce a wide range of different textures andmechanical properties in confectionery just by controlling the size of the sugar crystals as a product ismanufactured (not unlike the manufacture of different steels, in fact). And we should also mentionice-cream – an impressive material made from just fat, sugar and air. Biscuits and cakes rely on theaction of heat to create interesting materials from simple ingredients – flour, sugar and eggs.

Many food materials are composites (and manufacturers are dreaming up new concoctions all thetime). Whatever the latest recipe or process, though, it must produce a material with a tensile strengththat gives a pleasing texture in the mouth. We expect our food to break down into pieces as we chew– but how it does this is important, for it must release its flavour at a rate compatible with our rate ofchewing – otherwise the experience would be distinctly unpleasant. These are the kinds of questions


that material scientists specialising in food are currently researching.

Resources to useMcGee H 1997 On Food and Cooking: the Science and Lore of the Kitchen (Collier Books). This bookcontains all you need to know about the history and science of chocolate, ice cream, confectioneryand baked goods – see chapters 1, 6 and 8.

Getting started:Questions to think about1. Can you describe, say, ten confectionery, cake or biscuit products that could be classed as

composite materials and give their main components?

2. What are the mechanical properties of a chocolate bar?

3. Find out how one particular product in this category is manufactured from its raw materials.

4. How big is the chocolate / confectionery / biscuits and cakes industry in the UK?

5. Why do we find these foods appealing? What aspects of material properties are involved?

6. Dream up a new type of chocolate bar or sweet, based on what you have learned about the basicingredients and the kind of materials they can form when processed together – then make a salespitch to your company outlining the materials science involved.

Environmentally friendly plasticsReading 90T: Text to Read


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This information sheet will get you started on a case study of how plastics which are easier to disposeof are being developed, and why this is important for the environment. It contains a brief introduction,which explains why this is a worthwhile topic to study, followed by a list of resources which will pointyou in the direction of the information you need.


Reducing waste with new polymer materialsThe properties that make plastics such excellent materials for so many applications are their downfallwhen it comes to their disposal. They are physically and chemically inert, which means they withstandcorrosion and wear during their lifetime. But when it comes to throwing them away, they linger formany years, taking up valuable space in landfill sites and generally having a negative effect on theenvironment. In Europe, over 7% by mass of household waste is composed of plastics. But thevolume taken up in landfill dumping by plastic packaging and related products is larger than this,because plastics have low density (another of their apparently desirable material properties).

Over the last 20 years or so, materials scientists have become more environmentally aware, and tendto think in terms of a 'cradle to grave' (lifetime cycle) analysis of their products. This means that


disposal of a material becomes a vital part of the cost/benefit equation. With any material there arethree main disposal options: landfill (dumping), incineration (burning) or recycling. For plastics, thelatter two options have not, to date, proved to be particularly economic or practical.

That is why there have been developments in creating polymers which degrade more easily oncethey are disposed of in landfill. There are four basic categories of these new materials, which arelisted below:

1. Biodegradable polymers, like Biopol; these are made by bacteria and can easily be broken down(by bacteria) in the environment, because of their chemical structure.

2. Photodegradable polymers. These are synthetic materials with chemical bonds which are brokenby sunlight, rendering the polymer chain more accessible to degrading bacteria in the soil wherethe material is dumped.

3. Synthetic biodegradable plastics. Like Biopol, in principle, these have starch granules embeddedin polymer chains. This means that bacteria degrade the material into tiny particles which are, inturn, more easily broken down.

4. Water-soluble plastics. These contain hydoxyl groups, which are water soluble.

Resources to useCallister W D Jr 1997 Materials Science and Engineering: an Introduction 4th edn (Wiley) chapter 24A university textbook, but don't be put off; chapter 24 is all about environmental considerations andthe use of materials. It is a fairly easy read and will give you all the background you need for this casestudy.

Emsley J 1994 The Consumer's Good Chemical Guide (W H Freeman)

Emsley J 1998 Molecules at an Exhibition (W H Freeman)Two chemistry books with plenty of useful material on plastics manufacture and disposal.

Getting started:Things to think about1. What is meant by the term biodegradable?

2. What types of plastics can be incinerated or recycled?

3. Why are biodegradable polymers expensive?

4. Describe the recycling symbols given on the labels of plastic products.

5. What are the facts and figures for plastics waste in the UK? World-wide?

6. Is plastics waste worse for the environment than metal or glass waste?

Disaster!Titanic and Challenger


Reading 100T: Text to Read


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This information sheet is intended to help you get started researching a case where a material hasfailed with disastrous consequences – either the sinking of the Titanic or the destruction of the SpaceShuttle Challenger . There is a brief introduction that explains why each topic may be interesting tostudy, followed by a listing of resources to point you in the direction of the information you need.

Before you start work, study the questions in the final section of each topic; these could help youfocus your thinking.

TitanicOn 15 April 1912, during its maiden voyage, the White Star Line's Titanic sank with the loss of 1500lives. This disaster has gripped the public imagination ever since, and there are many questionssurrounding the tragedy. There have been suggestions that the ship was incomplete when it sailedand that there were poor decisions about the construction and design of the vessel. You could focuson two aspects of the incident: either the ship itself or the properties of the iceberg that caused itsdestruction.

The shipShips are made of steel; the plates are riveted together to make a hull which is (more or less)watertight. In the Titanic complete sections of the hull and the structure were designed so that theycould be isolated from each other to prevent seawater in one section from spreading throughout theship. This design failed because the water was able to move over the top of the watertight sections.But there have also been questions about the quality of the steel used in the hull – its metallurgy.Materials science in the 1910s was less advanced than it is today and you will be able to find articlesthat discuss some of the conclusions reached following the sampling of steel taken from the hullduring recent salvage work.

There are a wide variety of articles that refer to the Titanic disaster in both printed and electronic form.This interest has been re-kindled following the making of an epic film on the subject during the late1990s.

1. What recent evidence is there about the metallurgy of the ship's hull? There have beenunderwater examinations of the fracture itself and chemical analyses of damaged metal removedfrom the wreck.

2. What does this evidence show?

3. How would metallurgists treat the problem of designing the material for the hull today? Whatsolutions might they develop?

The iceThe ice tore a huge hole in the side of the ship, which on the face of it is surprising for we tend tothink of ice as a weak, plastic, rather slippery material. This is wrong. Ice – especially ice below themelting point – can be a strong and resilient substance. During the Second World War there waseven a proposal to construct an aircraft carrier from a composite of ice and wood pulp.

An Atlantic iceberg consists of old ice. Most icebergs calve from the ice pack and then move into theregion between northern Canada and Greenland. As many as seven years can elapse before theberg moves south and into the shipping lanes. During this time the ice will recrystallise and change itsmechanical properties as it does so. But this time span is as nothing compared to the age of the icecrystals which originally fell on Greenland to make the iceberg. This snow could have fallen in 1000


BC and be calving from the edge of the ice sheet today.

You could develop a case study that investigates the mechanical properties of ice either in its pureform or as berg ice. You might begin with the book The Physics of Glaciers by W S B Paterson(Pergamon Press), available in a number of editions.

1. What processes occur in the ice during the long interval between snowfall and iceberg calving?

2. How does the grain shape and the grain size change in the ice (i) before it leaves the ice sheet,(ii) once it has entered the sea?

3. What are the mechanical properties of the ice?

4. How do the mechanical properties of the iceberg and the steel of the Titanic's hull compare?

ChallengerIn 1986 the Space Shuttle Challenger was destroyed in an explosion shortly after take-off. Thistragedy stunned the world and halted space flights by NASA for a significant time. A subsequentboard of enquiry found that the probable cause of the accident was faulty O-rings in the fuel tanks.The story of the discovery of the problem is an interesting one and features one of the greattwentieth-century physicists, Richard Feynman. In a famous televised session he demonstrated theproblem with the O-rings to his fellow committee and a huge television audience using some simplebut effective apparatus. He wrote about this in his book (1989), What Do You Care What OtherPeople Think? (Bantam Books). You may also find it interesting to scan the New Scientist CD-ROM ifyou can obtain access to it (New Scientist 5 August 1995). Finally, there is a 20 minute sequenceintroducing the problem in a video package produced for schools in the Teaching Pack ofExperiments in Materials Science published by the Institute of Materials. You may wish to use part ofthis video to illustrate an oral presentation.

1. What was the problem with the O-ring material? How was the problem cured?

2. Could you, in your presentation, demonstrate the O-ring problem in a similar way to RichardFeynman? (Hint: the temperature difference between the compartment of a domestic freezer andboiling water is about 120 ºC.)

3. Were there other problems relating to the decision-making process at NASA?

4. Have there been further O-ring failures in the space programme?

Paper versus plasticReading 110T: Text to Read


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This information sheet will get you started on a case study of how to assess the overall cost/benefitratio of two different materials – paper and polystyrene – as used for making a disposable coffee cup.It contains a brief introduction, which explains why this is a worthwhile topic to study, followed by a listof resources which will point you in the direction of the information you need.

Before you start work, study the questions in the final section; these should help you focus your


thinking on this topic.

Cradle to grave:A full environmental auditSo far, you have looked at materials in use, appreciated how their properties match them to their joband seen how designer materials have been created from insights into the structure of everydaymaterials like materials, plastics and ceramics. But that is only part of the story. Increasingly, asnatural resources dwindle under the demands an increasing human population puts on them, it hasbecome necessary to consider materials in the context of their life cycle. Cradle to grave or life cycleanalysis means just what it says; the costs to society and the environment of the manufacture of amaterial, and the corresponding costs of its disposal, are now seen as important as the cost/benefitanalysis of the material in use.

There is a tendency to assume that natural materials – paper, wood and so on – are always best, atleast when it comes to manufacturing and disposal costs. So you may be surprised that a life cycleanalysis of the costs and benefits of a paper cup compared to a polystyrene cup shows that theplastic squares up better to the analysis than its common image may suggest. But the main lesson totake away from this case study is that such comparisons are complex and you can rarely give aclear-cut decision on which material is best.

The important thing, however, is to put materials into this broad context – considering their whole lifecycle – so that we can, in the future, make wise choices of materials that will benefit both theincreasing human population and the global environment.

Resources to useHere are some references which you may find useful:

PapersHocking M B 1991 Paper versus polystyrene: a complex choice Science 251 504–5. This is the major resource for this case study; although the figures are a little out of date, it is anexcellent example of how to analyse the various factors in a life cycle assessment of two materials forcomparison. Although Science is an academic journal, you should not have much difficulty ingrasping Hocking's arguments.

Emsley J 1991 Degradable plastics. Inside Science New Scientist (19 October). This paper, designed for A-level students, puts Hocking's analysis into context.

BooksCallister W D Jr 1997 Materials Science and Engineering: An Introduction 4th edn chapter 24. A readable and up-to-date introduction to the ideas of life cycle analysis and environmental audit.

Emsley J 1994 The Consumer's Good Chemical Guide (W H Freeman) chapter 6. All about the environmental impact of plastics.

Getting started:Questions to think about1. What is meant by life cycle analysis?

2. What are the potential adverse environmental impacts of the main classes of materials: metals,


polymers, glasses and ceramics?

3. Are natural materials always best?

4. Construct a flow chart showing the 'cradle to grave' analysis of a material of your choice.

Toughened glassReading 130T: Text to Read


Quick Help

This information sheet will get you started on a case study of toughened glass. It contains a briefintroduction, which explains why this is a worthwhile topic to study, followed by a list of resourceswhich will point you in the direction of the information you need.

Before you start work, study the questions in the final section; these could help you focus yourthinking on this topic.

Car windscreensMost of us have seen the sobering aftermath of a serious accident – shattered glass sometimes onthe road, but surprisingly often, still intact as a windscreen but now totally opaque. Car windscreenswould be very dangerous if they fractured like ordinary window-glass. So how are they treated tomake the glass more safe when broken?

The mechanical strength of glass is impaired by the presence, in the interior of a glass and on itssurface, of very small cracks (known as 'Griffith cracks'). These cracks may vary in size between 1m and 1 nm. Their effect is to distort the stress pattern in a material, concentrating the stress in theregion round the tip of the crack. The cracks thus act as stress-raisers, and the local stress aroundthe crack can reach the theoretical fracture stress, while the general stress level is still well below thebreaking stress for the material.

Clearly, a crack can grow only when the region about it is in a state of tensile stress, so that the wayto stop the Griffith cracks growing is to ensure that the surface is maintained in a state ofcompression. In thermal toughening, the glass is heated above the temperature at which it melts (itstransition temperature), and the surface is rapidly chilled.

Thermal tougheningIn practice this involves rapid cooling as the final stage in the manufacture of the glass article. Theoutside of the glass is cooled to room temperature by means of air jets, with the result that the surfacemolecules have little time to rearrange themselves. The interior cools more slowly; the moleculesrearrange themselves so that more shrinkage occurs than in the outer layers. Consequently thestructure becomes denser from the surface inwards, and this means that whereas the outer layersare in compression, the centre layers are in tension.

When such toughened glass does break, it shatters into small dice because the release of the highstress energy goes to creates many new surfaces. These dice are not cubes but have the shapeshown in the diagram, where the influence of the compressive surface forces and tensile inner forces


can be clearly distinguished.

centre plane of glass sheet

The deformation has the desirable effect of reducing the sharpness of the edges.

Glass cannot be further processed once it has been toughened in this way, since any disturbance ofthe surface destroys the balance of stresses and causes the glass to shatter. The process of thermaltoughening is therefore particularly well-suited to the strengthening of flat glass, and of articles ofsimple shapes, like car windscreens and tumblers. It is less applicable to complicated shapesbecause stresses tend to build up at irregular points, and may cause the glass to shatter.

Air-cooled toughened glass cannot be made thinner than about 3–4 mm, because the surfacecompressive stress is proportional to the temperature gradient created as the air jets play on thesurface. The gradient decreases with the thickness of the glass.

Chemical tougheningIn this process, a finished glass product is placed in a fused salt containing alkali ions larger than thealkali ions in the glass. The temperature is kept below the temperature at which the glass melts.Some of the surface ions in the glass are then replaced by larger ions from the fused salt, and thisproduces surface stresses which are retained on cooling to room temperature, to give the surface thedesired state of compression. Thus, if soda-lime-silica glass is placed in fused potassium nitrate, thecooled glass is found to be considerably strengthened.

Resources to useThe following references could be consulted for further information:

Chown M 1995 Why do teardrops explode? New Scientist 145 (11 February)

British Glass Manufacturers Federation 1992 Making Glass 3rd edn

Getting started:Questions to think about1. Minor collisions often result in fragments of glass on the roadway and in car parks. This glass will

not puncture the tyres of your bicycle. Collect some samples and see whether the shape of thefragments corresponds to that described in the text.

2. What exactly happens when a windscreen shatters? Consider this from a safety point of view.

3. Do some research on the composition of glass.

4. What other uses are there for toughened glass?

5. What are Prince Rupert's drops? New Scientist carried an article on this topic (see referenceabove). Can you account for the behaviour of Prince Rupert's drops in terms of thermaltoughening?


Revision Checklist I can show my understanding of effects, ideas and relationships bydescribing and explaining:the evidence we have for the sizes of atoms and molecules

A–Z references: electron microscopes and atomic microscopy

differences between the mechanical behaviour of different classes of materials - metals, glassand ceramics, polymers, composites – in terms of their structure and bonding, including effectsof dislocations and of crack propagation

A–Z references: crystals, metals, ceramics, polymers, glass, composite material, cracks,bonding

Summary diagrams: Cracks and stress, Stopping cracks, Fracture energy and tensile strength,Shaping and slipping, Metals and metal alloys, Ceramics versus metals, Explaining stiffness andelasticity

differences between the electrical behaviour of conductors, semi-conductors and insulators, interms of the number of free charge carriers

A–Z references: electrical conductivity and resistivity

Summary Diagrams: Conduction by metals and semiconductors, Free electron model of metal,Conduction in doped silicon

I can interpret:images produced by SEM (scanning electron microscopy), STM (scanning tunnellingmicroscopy), AFM (atomic force microscopy) and other images to obtain information about thestructure of materials

A–Z references: electron microscopes and atomic microscopy

Summary Diagrams: Looking inside wood, Looking inside metals and ceramics, Looking insidepolymers, Looking inside glasses

I can calculate or make justified estimates of:the size of a molecule or atominteratomic forces using the value of the Young modulus (e.g. in steel)

A–Z references: atom

Summary Diagrams: Explaining stiffness and elasticity, Fracture energy and tensile strength


I can show an appreciation of the growth and use of scientificknowledge by:giving examples of how the properties of a material are linked to its structure and so affect itsuse

A–Z references: metals, ceramics, polymers, glass, composite material

Summary Diagrams: Cracks and stress, Stopping cracks, Fracture energy and tensile strength,Shaping and slipping, Metals and metal alloys, Ceramics versus metals, Explaining stiffness andelasticity

In giving a presentation I have shown that I can:use resources to gather, analyse and communicate information about the properties and uses ofa material

e.g. textile fibres, building materials, designed materials, semiconductor materials

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