Materials Science
Coursework and Investigations for KS3 and KS4
May 2007
1 Contents
Ammonia Diffusion______3
Enthalpy of Crystalisation______7
Viscous Goo Challenge______9
Reactivity Series and Corrosion______13
To determine a reactivity series for a range of metals using a voltmeter______15
Passivity – Demo / A’ level______17
An Electrifying Experience______19
Tensile Testing______21
Hardness Testing______33
Creep______37
Ferrofluids______43
These experiments are intended as a framework to be adapted for the specific needs of a class.
All teachers should carry out their own safety assessment for all experiments.
2 Ammonia Diffusion This Sc1 should follow a demonstration of the diffusion of ammonia and hydrogen chloride along a tube. The position and shape of the “smoke ring” so produced indicates…
Different gases diffuse at different speeds. This links in to the rate of diffusion being related to molar mass. The shape of the ring shows ammonia to be less dense than hydrogen chloride, since it slants in such a way as to indicate that ammonia floats over the top of the hydrogen chloride. Leaving the ring to develop illustrates this clearly. Also, observing the ring closely shows small convection currents which can be stimulated by holding two fingers below the ring and waiting for heat to diffuse through the glass.
This gives pupils imagination to tackle the Sc1…
“ Does Ammonia gas diffuse equally fast in all directions?”
The question prompts a discussion on the density of gases – brighter pupils can calculate that ammonia is less dense than air (approximates to nitrogen or oxygen nitrogen mix?) Others can look up data in a book, or simply be told. Diffusion works well in boiling tubes, typically taking 5 to 15 minutes to cover the distance from the neck to base, depending on conditions. Progress of the gas is measured by using thin, pink litmus sensors. (Long litmus strips give a diffuse boundary which makes taking measurements harder.) Diffusion (boiling) tubes can be set up along Cartesian axes, and brighter pupils may want to further divide the angles. This should not be encouraged too much, especially initially. There is scope for a reasonable, simple theory and prediction. Complicated or Simple plans to suit pupils abilities are soon ready allowing lots of results and graphs to be produced for examiners. As differences are random and not directional, there is plenty of scope for evaluations, explanations and suggested improvements. Some considerations for key factors include, amounts of ammonia solution used, water inside the tube reabsorbing ammonia gas, holding the tubes and making one warmer than another, ammonia already in the air…
In spite of the difficulties, this Sc1 provides a short simple assessment that even the most challenged can attempt and the brightest can score maximum marks in 2 – 3 weeks. It also teaches that experiments do not always work the way you expect, but for GCSE that does not matter providing it is realized that the experiment has not worked as expected – ideally with an explanation.
Pupils will assume the ammonia advances down the tube with a flat diffusion front perpendicular to the direction of diffusion. This is clearly not so. When difficulties arise, a demo of dropping a few cm3 of milk into a 250 cm3 of still water illustrates the misconception and answers some of the more confusing anomalous results.
The following represents a distillation from a few bright (A* grade) pupils - for your ideas.
3 Does Ammonia diffuse equally in all directions? Theory
Gases diffuse because their particles are moving very fast and are at very large distances apart compared to their actual size. As a result particles of one gas can move between and mingle with particles of another gas.
The particles spread out evenly throughout a container over time.
It is known that gases of low formula mass diffuse faster than gases of large molecular mass, as demonstrated by allowing ammonia
Ammonia, NH3 is a gas at room temperature with a molecular mass of 17 g mole-1.
Air is a mixture of gasses at room temperature. For the purpose of this investigation it is assumed that the composition of air is 80% N2 and 20% O2.
Therefore average molar mass of air = 0.8 x MN + 0.2 x MO
Where MN and MO represent formula masses of nitrogen and oxygen respectively,
= 0.8 x 28 + 0.2 x 32
= 28.8 g mole-1.
Therefore the molar mass of ammonia is much less than the average molar mass of air.
The molar volume of any gas = 24.0 dm3 at room temp. and press. (25C)
Therefore molar density of ammonia = 17.0 g mole-1 / 24.0 dm3 mole-1.
= 0.71 g dm-1.
Therefore molar density of air = 28.8 g mole-1 / 24.0 dm3 mole-1.
= 1.2 g dm-1.
Because ammonia gas is less dense than the surrounding air, I predict that flotation will assist diffusion in the vertically upward direction, oppose diffusion in the vertically downward direction and have no noticeable effect on the horizontal directions.
That is I predict diffusion upward will be faster than diffusion downward and that horizontal diffusion will be an intermediate speed.
4 Method - Plan Ammonia can be detected using damp, pink litmus paper that turns blue on contact with ammonia.
Ammonia can be controlled conveniently by using 0.1M ammonia solution on cotton wool. The gas escapes from the solution and diffuses through air to the litmus paper turning it blue. The time taken for the litmus to turn blue is recorded. By placing at least five papers at measured intervals in a boiling tube, the speed (or rate) of diffusion can be measured along the tube.
The ammonia is made to diffuse upwards, downwards and horizontally in separate experiments, each experiment being repeated to check measurements and to obtain an average value.
Cotton wool with ammonia solution
Strips of damp litmus paper stuck to the inside of the tube by surface tension.
cm scale (graph paper) taped to the glass
Upward Downward Horizontal Diffusion Diffusion Diffusion
Key Factors to Control
The experiment was tested several times to determine the optimum conditions. Based on preliminary tests, the following key factors to be controlled were noted.
Temperature should be the same for each test, since diffusion is due to the kinetic motion of particles and the speed of the particles is related to temperature.
The amount of ammonia solution used (4 drops from a dropper), as more solution means more ammonia evaporating per second and hence a higher rate of diffusion. (Diffusion from a point is concentration or partial pressure dependant, hence the need to control the amount of ammonia.)
Parallax, hence the need to stick a scale as close to the ammonia as possible.
Need to use dry boiling tubes, as ammonia dissolves readily in water. If the ammonia dissolves in water drops inside the tube, the measured rate of diffusion will be affected. (This may be discounted providing the tubes are ALL equally damp at the start.
Use thin strips of litmus, as litmus paper changes through several shades of blue if diffusion is slow. Thinner strips make the change sharper.
Handling of the tube needs to be minimal, as heat from the hand can cause localised warming and hence change the rate of diffusion.
5 Method – Procedure
A boiling tube was cleaned with a paper towel to make sure it was dry.
Thin pieces of damp, pink litmus were placed at approximately 1.5 to 2.0 cm intervals inside the boiling tube using a glass rod.
The tube was clamped in position and four drops of ammonia solution were placed on a piece of cotton wool that had been glued to a rubber stopper. The stopper was placed firmly into the boiling tube.
As soon as the first litmus turned blue, the stopwatch was started and the times taken for other papers to turn blue were recorded.
This method was repeated for diffusion in vertically up, vertically down and horizontal directions. Measurements for each direction were taken three times each and an average was calculated.
A graph of distance versus average time was plotted. Distance Predict - /cm Upwards Predict - Horizontal Predict - Downwards
Average time taken /sec
Results
NOTE: The diffusion of any gas is not so simple. Some pupils will achieve results that agree with their predictions by chance, most will not. A simple way of illustrating the point for pupils is to show a large tube of still water in a measuring cylinder. The cylinder represents the air in the boiling tube. By adding a few cm3 of milk, to represent the ammonia, the milk will be seen to swirl as it travels down the tube. Sometimes it will move down one side of the cylinder before returning up the opposite side. Pupils often get analogous results to this with the ammonia apparently diffusing the “wrong way”. This illustration of turbulent flow is enough for pupils to explain any anomalous results. The turbulence is random and can be influenced by many factors, such as holding the tube in one place, setting up convection currents, moisture inside the tube (including the moist litmus paper) absorbing ammonia from the air. This gives good opportunities for the student to suggest improvements and explain anomalous results. It also has the potential to develop into a discussion about the differences between a model of a situation and the reality, with the need to refine models to obtain more accurate predictions.
6 Enthalpy of Crystallisation Thermodynamic terms can be difficult for sixth formers to understand, especially when they are not directly measurable. This experiment does not provide an accurate method for determining the enthalpy of crystallisation of sugar, but does offer an (edible) illustration of it. Boiled sweets are essentially amorphous sugar and very soluble in water with an accompanying increase in temperature. Granulated sugar, being crystalline requires that enthalpy of crystallisation is added to the system to enable dissolution. Consequently, granulated sugar dissolves endothermically in water. Method Using a measuring cylinder, measure 100cm3 of water and leave it to reach an equilibrium temperature. Crush about three Fox’s Glacier mints in a mortar and pestle. Take care as large pieces take longer to dissolve and too much crushing may compact the mint into the base of the mortar. Weigh an empty polystyrene cup. Add the crushed mint to the cup. Reweigh the polystyrene cup and obtain the mass of the mint used. Take the temperature of the water using a 0 to 50C thermometer (easily readable to 0.2C). Add the water to the mint, in the polystyrene cup, and stir vigorously to dissolve the mint. Take care not to let any solution splash out of the cup. Record the highest temperature reached. Repeat the experiment using granulated sugar. For the purpose of approximation, assume that both materials are chemically identical and the mint is both anhydrous and completely amorphous.
Questions 1. Explain the observed difference in the change of temperature. 2. If left exposed to the atmosphere, the mint becomes sticky on the surface, but granulated sugar does not. Explain this observation. 3. Of the two forms of sucrose, which has the highest energy level? 4. Make an estimation of the difference in energy of the two forms of sugar. 5. You may make very precise measurements in this experiment, but it cannot be accurate. Explain the meaning of the terms accuracy and precision in this context. 6. Briefly discuss the accuracy of this experiment. Assume specific heat capacity, s, of the solution is 4.3 J g-1 C-1.
Thermal energy change = ms(T2-T1), Where m = mass of solution and (T2-T1) = Temperature change of the system.
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8 Viscous Goo Challenge Notes
This simple experiment is fun for everyone and is often used as “entertainment” in science clubs, Open Days or end of year activities. However in this format it is used as a science investigation by linking observation to numerical values.
This is not intended to be a thorough piece of investigative work, but is used to illustrate scientific principles in an entertaining way. There are several levels at which this can be approached depending on the ability of the class that will carry out the investigation.
Very young / low ability: This can be simply a bit of fun tied in with data collection
More able KS3 / KS4: The experiment can be used to reinforce ideas about…
Solid, liquids and gases and their basic properties
Polymers, their properties and cross linking
Viscosity
Discussions about accuracy and precision of data and what level is appropriate
Top GCSE and A’ level
Hydrogen bonding
Curing (properties will change with time)
Creep (and an analogy to convection in the mantle of the Earth or metals)
…and more besides.
PVA adhesive is non toxic, but is not edible.
Borax is poisonous if swallowed and may cause skin irritation where students have sensitive skin or an allergy. Borax has been used as a domestic water softener a mild antiseptic and domestic cleaner, though, today, applications are generally superseded by more commercial products.
9 Viscous Goo Challenge - Teachers’ Notes
PVA adhesive (wood glue / white paper glue) will vary in composition depending on its source, so it is necessary to try quantities for yourself. White paper glue mixed with water in a 1 :2 ratio (glue to water) has been found to give consistent results. PVA could stand for Poly Vinyl Alcohol or Poly Vinyl Acetate. In fact glues are probably a mixture of the two, but since both are polar and can interact with borax, this is not important for the experiment.
PVA is non toxic; Borax would be poisonous if consumed but (other than occasional skin irritation) is harmless. Uses include mild anti-septic, fridge and kitchen surface cleaner, water softener. Until recently borax was readily available from chemist shops. The solution used is made as a saturated (4%) solution prior to use.
The behaviour of Goo is useful as a discussion on solids and liquids (stretches if pulled slowly, snaps if pulled quickly). The flow of goo on a bench surface illustrates creep. Creep is common in polymers and metals at about 1/3 mpt (lead is a good example). Also convection currents in the solid mantle of the earth can be describes as creep in a ceramic material. Although not the same process, there are similarities with bonds being broken and reforming in a different position. Whereas Goo will creep by several cm in a few seconds, lead will take centuries and the mantle takes millennia.
Experiments can be aimed at KS2 to A-level, from purely fun activities to simple experiments illustrating changes in physical properties of polymers due to cross linking and intermolecular forces (Hydrogen Bonds).
It is possible to develop investigations based on the above experiments, particularly the sinking coin by linking this to the amount of borax used on a fixed amount of PVA.
10 Viscous Goo Challenge
Plastics (polymers) are large chains of repeating molecules. We can sometimes change the properties of polymers and make them into more useful materials by joining up the separate chains using chemical bonds. This is called cross-linking. Chains of P.V.A. (Poly Vinyl Alcohol) can be (loosely) cross-linked by adding borax solution. This results in the P.V.A. changing from a liquid to a rubbery slime. Polymer chains can slide past each other under the effect of a force. Liquid polymers are viscous (thick, not very runny liquids) because of forces between the chains. We can change the viscosity by changing the way that the chains slide past each other – effectively changing the friction between them. More force/friction = harder for chains to slide past = more viscous liquid.
Cross linking can Polymer chains tangled Forces make the chains slide prevent the chains together past each other and line up. from untangling. PVA molecules and how they interact Polymer Chain Alcohol Group (Covalent –OH )
The alcohol groups interact weakly and the chains tangle. This slows the molecules down when they try to slide past each other, making the polymer behave as a viscous (treacly) liquid.
PVA molecules with Borax Polymer Chain Alcohol Group (Covalent –OH ) Cross-Link: Strong Attractive Force (Hydrogen Bond can be broken and re-formed easily) Borate Anion: [B(OH) ]2- 4 The cross-link causes a MUCH stronger interaction, making the material MUCH more viscous and can even give the appearance of being a solid. (Note: if the cross link was made using a stronger, full covalent bond, then the material would be a solid: e.g. rubber and thermosetting polymers.
11 Your challenge:
Design and test: The slime with the greatest bounce. The slime that can stretch the most without breaking. Investigate the viscosity of slime as a function of the amount of borax used
Your time limit is 30 minutes. 5 minutes to make a plan. 10 minutes to carry out your plan. 10 minutes to carry out your tests. 5 minutes to clean up the mess.
The basic recipe for the slime is given below.
Chemical and Apparatus Lists
250 ml bottle of 25% P.V.A. solution (shared). 250ml beaker of saturated Borax solution (shared). Borax is a >>>- POISON -<<< Wash your hands after using it!!! Plastic dropper (each). 100 ml beaker (each). Food dyes (optional). Stirring rod (each).
Procedure Use the scale on the side of the 100ml beaker to measure 20m1 of the P.V.A. solution. You may need to estimate from the scale. Ask for help if you are not sure about this. 1. OPTIONAL – you can stir in one (or two) drops of food colouring at this point. 2. Add between 1 and 10ml of the 4% borax solution. (Note: It’s best if each member of the team agrees to investigate a different amount of Borax – organise this between yourselves e.g. 2, 4, 6 etc ml.) 3. Stir vigorously for several minutes until the solution has gelled. You may find that you have to scrape the GOO from the rod several times before you get an even mixture. The more stirring you give, the better the GOO. 4. Wash your hands before eating or drinking anything. Borax is a poison if swallowed.
Testing viscosity – this is one possible method: feel free to test your own. 1. Prepare samples of slime made with different amounts of borax (2, 4, cm3 etc.) 2. Leave to settle in a small beaker. 3. Place a coin on the surface and time how long it takes to sink. 4. Draw a graph of time to sink (viscosity) against volume of borax solution used.
12 Reactivity Series and Corrosion
Introduction – Teacher notes (Strong GCSE or A-Level) For this piece of work, it is assumed that a basic introduction to corrosion has been given and that pupils are aware of the following… Rusting applies to iron only. Corrosion is a (electro)chemical process. Some metals are more reactive than others – reactivity series / displacement reactions. If two dissimilar metals are joined in a corrosive environment, the most reactive metal corrodes, the least reactive metal does not. Structural metals can be protected by coating them / connecting them with a more reactive metal – sacrificial protection. Differences in reactivity between two dissimilar metals can also be used in an electrochemical cell as a portable source of electricity. The driving force for the electrochemical reaction can be measured in terms of the emf of a cell using a voltmeter when no current flows between the electrodes.
The following work can be set as a class practical exercise or as a Sc1 investigation. There are many difficulties with measuring the emf of the cells which must be tackled by the pupils allowing for detailed discussion of accuracy, reproducibility and experimental error. However, if this can be done then there is the possibility of good marks. The major problem is in obtaining reproducible results; since the measured voltage will not be a true emf (some current will flow). There is a major problem with fluctuating results due to many causes (changes on the electrode surface, diffusion, convection and polarisation). On top of this there is the problem of the measured potential decaying along an exponential curve. Pupils must realise that this is part of the experimental problem which they have to overcome and is not because apparatus does not work or they have “done something wrong”. A technique to obtain acceptable results would include… Always start with a fresh electrode, preferably abraded just before use (fresh abrasive paper to avoid particles of another metal being scratched into the surface. Connect electrodes before inserting into the solution. Leave electrodes for a predetermined time before taking a reading, e.g. 10 or 15 sec. Always record which metal is connected to –ve terminal of the voltmeter.
This exercise takes the form of a Sc1 investigation which assesses the ability measure data with an awareness of experimental error and to evaluate those results. There is scope for more than one investigation along these lines.
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14 To determine a reactivity series for a range of metals using a voltmeter
Background You have performed displacement reactions and have concluded that some metals are more reactive than others. You have also learned that these displacement reactions involve the oxidation of a reactive metal and reduction of the ions of the least reactive metal. Since oxidation and reduction in these cases involves the transfer of electrons and that moving electrons constitute an electrical current, it should be possible to use electrical methods to determine numerically if a metal is more reactive than another. We need to determine the energy driving the reaction and the term associated with electrical energy is the volt. We will, therefore, set up some simple cells and connect the metals via a voltmeter to measure the electrical potential (an energy related term measured in volts). It is predicted that the order of the voltages measured will match the observations from displacement reactions.
Apparatus Stopclock (to measure seconds). Voltmeter (high resistance to minimise current flow). 2 plug to croc leads (to connect electrodes to the voltmeter). 100cm3 beaker. 3 Approximately 100cm of 0.5M Na2SO4 solution. (Other solutions could be used, preferably NOT including ions of the metals being tested or chloride ions.)
Diagram Electron flow circuit... Voltmeter V Leads Electrodes
Stopclock
Ion flow circuit Optional extras? Electrodes Magnetic stirrer / glass rod Electrolyte Thermometer
15 Method – Preliminary Work After following these instructions, you may think of ways to improve your experiment in order to obtain either better results or a better understanding of the experiment. Feel free to develop the experiment as you see fit. Abrade the electrodes’ surfaces using a fine paper. Take care not to leave fingerprints or other marks on the surface. Connect cleaned copper and zinc electrodes to the voltmeter. Make sure the voltmeter is switched on before placing the two electrodes in the electrolyte at the same time. Notice the behaviour of the voltmeter reading over a minute or so. Also note the sign of the voltage. What happens if the electrodes are swapped round?
Results Metal on +ve Metal on (+ve) More reactive terminal (Red) terminal (Black) metal Measurement Time / sec 10 15 20 30 60 Voltage Measured / V
Observations
You will notice at least two important features about the voltage reading… 1. The values “jump about” a lot, and 2. The values decrease, quickly at first but then more gradually.
This poses a problem of how to take an accurate measurement. In fact it will be impossible to get measurements which give the same value each time the experiment is repeated (except by chance). There are many variables which are affecting your results so we have to be very careful with our method so as not to disturb the system more than necessary and to make our measurements as consistent as possible. Always start with freshly prepared, clean electrodes. Keep electrodes as far apart as possible.
Possible further preliminary investigations… You have seen that the measurements decrease quickly. How can you make readings from separate tests compatible with each other? (Note: Do not expect them to be the same – what, in your opinion constitutes consistent results for this experiment?) Does stirring have any effect? The process is temperature dependant. What reasonable precautions should you take to make sure that measurements are as consistent as possible? What if you connect two electrodes of the same metal?
Investigation You are to select pairs of metal electrodes and determine a series for the metals. Compare this series with that of the reactivity series obtained from displacement reactions. In your evaluation, comment on difficulties experienced in taking measurements and how you tackled them. How accurate do you think your measurements are, and with this in mind, how precisely should they be recorded? Try to justify any assumptions made.
16 Passivity – Demo / A’ level Teacher Notes
(Safety note – Use conc. HNO3 (aq) in a fume cupboard. The use of gloves to protect skin is advised and eye protection must be worn.) Under normal conditions, iron corrodes to produce rust, which is flaky and does not protect the underlying metal from further chemical attack, as does the oxide layer on aluminium. However, in extreme oxidising conditions, a thin continuous and adherent oxide layer does form which does protect the substrate. This thin layer, microns thick, can protect steels from further attack but is easily damaged.
Method Place a small piece of clean steel in a 100 cm3 beaker and cover it with a few cm3 of dilute nitric acid. Observe what occurs. Now transfer the steel to another 100 cm3 beaker and cover it with a few cm3 of concentrated nitric acid. Observe what occurs. Next return the steel to the first beaker containing the dilute acid. Take care NOT to disturb the surface of the metal at this stage. If you avoid knocking the specimen passivity will continue. Tap the specimen with a glass rod and note what occurs. This demonstrates the formation and breaking of a thin protective (passive) oxide layer. Other oxidising acids (sulfuric and phosphoric have similar effects. Phosphoric acid is used in commercial applications to treat car bodies for rust. Identify any gases produced. A’ level extension. For comparison, add a small piece of copper to the concentrated acid. TAKE CARE USE A FUME CUPBOARD, GLOVES AND EYE PROTECTION. A BROWN, TOXIC GAS (NITROGEN DIOXIDE) IS PRODUCED. Record what you observe naming any gases or coloured compounds formed. Comment on why copper, which is less reactive than iron, reacts so much more vigorously. This explains why steel pipes, generally considered to corrode more than copper, are used instead of copper pipes when using oxidising acids. This is relevant to chemical engineering
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18 An Electrifing Experience
Aims: 1. To show that an electric current can flow when different metals are connected via an electrolyte. 2. To show that this leads to corrosion of the metals involved.
Apparatus:
Zinc foil, copper foil, powdered gelatine, 0.5M copper sulphate solution, 0.5M zinc sulphate solution, filter paper, multimeter, leads Zinc electrode
Filter paper containing zinc sulphate gel
Moist filter paper
Multimeter V Filter paper containing copper sulphate gel
Copper electrode 4 cm Method:
1) Preparing the metal electrodes. 5.0cm 3.5cm Each group will need 5 copper electrodes (cut from GP grade copper foil approx. 0.1mm thickness) and 5 zinc electrodes (cut from GP grade 1.0cm zinc foil approx 0.3mm thickness). Each electrode should be 3.5 cm 1.5cm 3.5cm square with a 1.5cm x 1cm tab.
2) Preparing the electrolyte gels a) Prepare a 0.5M solution of copper sulphate (12.5g/100ml water) using hot water. Add 2g of powdered gelatine per 100ml and stir until all dissolved. b) Prepare a 0.5M solution of zinc sulphate (14.4g/100ml water) using hot water. Add 2g of powdered gelatine per 100 ml and stir until all dissolved.
3) Preparing the filter paper gels a) Soak a piece of filter paper in the copper sulphate gel and hang it up to dry. Either use standard “Whatman No1” circular papers (90mm) or square 4.0cm sheets (460x570mm). b) Soak another piece of filter paper in the zinc sulphate gel and hang it up to dry. 4.0cm c) When the papers are dry, cut out squares 4cm x 4cm as shown.
4) Building a cell and a battery V a) Build the first cell as shown in the apparatus diagram above. Lay a copper electrode on the bench; place a moist copper sulphate Cell 1 filter paper gel on top of it, followed by a moist piece of filter V paper, a moist zinc sulphate filter paper gel and finally a zinc electrode. It will be easier to make connections if the tab on the Cell 5 zinc electrode points in a different direction. Connect leads to the two electrodes and measure the voltage with a multimeter. Cell 4 b) Create a battery by stacking up to 5 cells on top of each other. Cell 3 You can measure the voltage of each individual cell or Cell 2 combinations of them by connecting to different copper and zinc Cell 1 electrodes in the stack. Record your readings in the table. 19 Results:
A) Connect the multimeter to the electrodes of the single cell and record the readings in the table Positive terminal on meter Negative terminal on Reading on Multimeter connected to: meter connected to: (in volts) Copper electrode cell 1 Zinc electrode cell 1 Zinc electrode cell 1 Copper electrode cell 1 Q.1) Which electrode forms the positive side of your cell, and which forms the negative? Q.2) Which way is the current flowing around your circuit? Q.3) Which particles form the electric current, and which way are they flowing around the circuit? Q.4) Try making a cell with dry filter paper gels. Explain any difference in performance you observe.
B) Build a battery of 5 cells. Investigate how the cells work together by connecting the multimeter as shown in the table (and any other combinations you care to try).
Positive terminal on meter Negative terminal on Reading on Multimeter connected to: meter connected to: (in volts) Copper electrode cell 1 Zinc electrode cell 1 Copper electrode cell 1 Zinc electrode cell 2 Copper electrode cell 1 Zinc electrode cell 3 Copper electrode cell 1 Zinc electrode cell 4 Copper electrode cell 1 Zinc electrode cell 5 Copper electrode cell 2 Zinc electrode cell 5 Copper electrode cell 3 Zinc electrode cell 5 Copper electrode cell 4 Zinc electrode cell 5 Copper electrode cell 5 Zinc electrode cell 5 Copper electrode cell 2 Zinc electrode cell 4
Q.5) Sketch a graph showing the number of cells against the voltage read on the multimeter. Q.6) What is the relationship between the number of cells and the voltage output of the battery? Q.7) Explain why the output from the cells is not exactly the same. Q.8) How could the design of the battery be improved to obtain a better output? Q.9) What is happening at the interface between the copper electrode and the copper sulphate? Q.9) What is happening at the interface between the zinc electrode and the zinc sulphate? Q.9) What would you expect to happen to each of the electrodes if the cells were left connected for a long period of time? Q.10) Suggest two practical uses of the chemical reactions taking place in this experiment.
Reference:
You may find this link helpful: http://www.creative-science.org.uk/sea1.html
20 Tensile Testing
Notes
This refers to stretching a specimen at a constant strain rate and recording the measured load required for that strain.
A perfectly elastic material would be expected to produce a graphical plot of stress v strain which is a straight line, ending suddenly with a brittle failure. This occurs as bonds between neighbouring atoms are stretched to the point of breaking. When a bond does break, it puts extra load on the remaining bonds, making them more likely to break. A crack starts to run across the material at this point and the material fails.
A ductile material produces, by contrast a curve due to plastic deformation taking place. As load increases, atomic planes slide past each other causing physical distortion of the specimen and simultaneously relieving the stress. Thinning of the specimen in one position (called necking) occurs. In this localised area, however, the stress does continue to increase (due to reduction of cross sectional area) which concentrates the plastic deformation in this area.
Most materials experience a combination of the two behaviours, with an elastic region followed by a plastic region. Variations can be introduced through different heat treatments and alloying.
The following describes a simple experiment which can be used at any age from an investigation at KS3 to an introductory demonstration at higher levels.
An excel Spreadsheet is provided for a number of metal samples. Although there is one workbook in this spreadsheet determining the Stress / Strain characteristics, it is expected that this workbook would be deleted for sixth formers so that they can calculate this for themselves from the data provided.
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22 Plastic Deformation in Plasticine
Background
This experiment can be used as a simple Sc1 investigation or class practical from KS 2 to A-level. The practical is suitable for students of all abilities, allowing the least able to do something and get results, while the most able can apply a high degree of understanding and link phenomena such as necking with the onset of plastic flow, the decrease in load to maintain a constant plastic flow, effects of inclusions on ductility and reduction in cross sectional area, influence of temperature. At A-level this would be linked to stress-strain curves and although there is a reduction in load, the decrease in cross-sectional area actually means that stress continues to rise (difference between engineering and actual stress). This simple experiment is useful as an introduction to stress-strain curves of metals.
Use different coloured samples of plasticine to represent a material containing different amounts of precipitated particles.
Technical - Preparation of samples before the experiment
Make three lumps of plasticine, each of a different colour and each with a mass of 35g. The colour provides a colour code to identify which specimen is which, later on, e.g. green, red and blue.
Mix 2g of coarse sand thoroughly to the red plasticine and 4g of the same course sand to the blue. These represent precipitates or particles within the microstructure of a metal.
This process takes time to thoroughly mix the sand and is best done for the pupils rather than by them.
Different amounts of sand may be needed to suit the grain size. It is better to use sand from a builder’s merchant than from a fire-bucket, as this sand invariably contains a wide range of particle sizes and is also usually dirty.
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24 To investigate the effect of inclusions (sand) in a ductile material (plasticine)
Instructions
Roll the plasticine for about a minute to get it to hand temperature.
Shape each of the specimens into a cylinder of roughly the same size and shape – below…
Approx 1.5 cm Approx 6 cm
Grip the ends of the cylinder firmly and pull the ends of the plasticine test pieces apart, slowly and steadily care is needed or you may find your hands flying apart! This will constitute a failed test. You may find that you have to ease off a little once the specimen starts to break.
Repeat for each specimen in turn, pulling with about the same force.
Examine the fracture surface of the plasticine using a magnifying glass.
The diameter of the test pieces started about the same size. Are the fracture surfaces about the same size or is there a pattern? Describe the pattern. o What happened to the plasticine before it broke? o What did you notice about the force needed to pull the ends apart at a steady rate before the plasticine broke? Use a magnifying glass to look closely at the surfaces. What do they look like? Predict what would happen if you added 6g of sand to another 35g lump of plasticine. Write what you think you would see and draw a diagram to help. Now make the extra specimen with 6g of sand and put your theory to the test. Describe what you found out. Were you right or was it different in some way? Comment on your results.
25 Stress-Strain diagrams recorded on a Tensometer
The shape of the curves can be misleading due to a combination of the design of the Tensometer, which is designed to provide a constant strain rate, and the behaviour of the ductile specimen being tested. The following refers to specimen charts illustrated in the Excel file, Tensile Tests.xls, on the OxMAT CD ROM (Reprinted for reference in the Appendix).
An overview of the properties of a ductile metal can be explained below. This is most closely represented by Copper in the data provided in the spreadsheet, but all ductile metals show some of these characteristics. Difference corresponding to carbon steel are considered later.
In the case of the three metals shown in the chart, all follow the following general patterns:-
In the diagram (right) Engineering Stress (Nm-2) O = Initial origin OO1 = Permanent elongation C A = Elastic limit OA = Linear region B B = General point beyond A BO = Relaxation if stress removed X A (Note: O1B is parallel to OA) C = Maximum Strength AC = Region of plastic Deformation X = Failure CA = Plastic Flow (Note: This is the region where “necking” or localised thinning of the specimen takes place.) O O 1 Strain (No dimensions) The overall shape (not values) of the curve would be the same for a plot of Force v length. However, plotting (Engineering) Stress v Strain standardises the curve for different cross sectional areas and different lengths.
Problems: The plot assumes that the initial cross sectional area and length are constant throughout. This is not the case. The volume of material remains constant, therefore as the length increases, the cross sectional area decreases. This effect is minute and is generally ignored over the elastic region. Once necking takes place, however, reduction in the cross sectional area becomes marked. This is not normally allowed for in these plots and can give lead to misleading observations. Since the Stress is determined by Load/Area, as localised area decreases, the stress will, in fact increase at this point. As the Tensometer cannot measure the decrease in cross sectional area, it is not actually measuring the “True stress”, but the stress assuming a constant cross-sectional area, referred to as the “Engineering stress”. Engineering stress is generally used for simplicity, since once this starts to happen, any safe loading has been exceeded by far, and few structures are designed to fail in such a precise way. Exceptions may include safety bolts that are designed to fail under dangerous loading conditions.
26 Line OA: The line is linear and the material follows Hooke’s Law. Relieving stress returns the specimen to its original, unstretched length.
Beyond A: the stress is such that dislocations (later) in the crystal start to move. This changes the spatial arrangement of some of the atoms relative to each other, hence a permanent change in length is observed and the line is no longer linear. Relaxing the specimen of all stress at this point allows the Stress v Strain line to follow a path parallel to the line OA, as the slope is a property of the material (Young’s Modulus = Stress/Strain), but returns to point O1 where OO1 indicates the permanent extension resulting from the movement of dislocations.
Carbon Steels The above applies to carbon steels also, but the alloying element carbon affects the behaviour of the dislocations.
Carbon atoms are small compared to iron and tend to fit between iron atoms in the crystal lattice (interstitial alloy), as opposed to occupying the site of an iron atom in the crystal as happens with metal-metal alloys (substitutional alloys).
The position of carbon atoms increases the local stress (same dimensions as pressure) in the crystal. If the atoms could be positioned in a region where the crystal lattice is imperfect, and greater gaps between iron atoms are found, then the stress will be reduced. Placing the interstitial atoms in these spaces (see dislocations, later) forms a lower energy situation than having the carbon distributed within a perfect crystal lattice. Such imperfections occur at grain boundaries (where crystals of different orientation meet) and at dislocations. Natural systems adjust from high to low energy systems, therefore, to minimise the energy of the system, carbon atoms diffuse to these sites – that is, grain boundaries and dislocations.
Carbon atoms occupy the larger than usual gap at a dislocation. This prevents the dislocation from moving and is said to “pin” the dislocation.
If the dislocations in iron are pinned, then they cannot move, preventing plastic deformation at Stress stress values that would otherwise result in plastic deformation. This extends the useful range of the iron by extending the elastic region of the stress v strain curve. There is a C Steel limit, however, and once it is reached, the Pure Iron dislocation is forced past the carbon atom that has been pinning it. This results in a slight relaxation and an initial peak at the end of the elastic region. More dislocations are pinned, NOT TO SCALE preventing further deformation, hence the line rises again. This stress and relaxation pattern repeats a number of times giving the series of Strain peaks observed in the graph.
27 Dislocations
…occur when an otherwise perfect Consider crystal planes, viewed edge-on, crystal has an “extra” plane of atoms so they appear as straight lines… inserted.
The extra plane displaces adjacent planes, increasing local stress. Bonds between metal atoms are strained and therefore of higher energy than those within the bulk of the perfect crystal and can, therefore break more easily.
Note: in the diagram, the three atoms a. A perfect crystal b. A dislocation (a. and b.) represent a plane of atoms perpendicular to the plane of the paper and the “extra space” forms a “tunnel” 1 2 3 4 into which carbon (and other atoms) can diffuse, thereby forming a lower 1 2 3 4 energy system.
Under the influence of a stress (tensile 1 2 E 3 4 or compressive), metallic bonds between the metal atoms become 1 2 E 3 4 strained and break, reforming with atoms of the adjacent “extra” plane (figure c.). Effectively, the dislocation c. Under stress, bonds “flip” moves, disturbing the arrangement of atoms and giving rise to a permanent change of shape (ringed in figure d.) 1 2 3 4
Insertion of interstitial atoms, such as 1 2 3 4 carbon, into the dislocation makes “bond flipping” more difficult. Higher energy is needed to make the 1 2 E 3 4 dislocation move. Once the dislocation has moved, the material continues to 1 2 E 3 4 deform until held back by more pinned dislocations, resulting in greater d. The dislocation “moves” changing the resistance to plastic deformation: this spatial arrangement of the atoms around it: is indicated by further rising of the that is a slight, permanent change of shape curve. The process repeats several occurs. times resulting in a series of peaks before all dislocations have become unpinned.
Please note. This is a simplified explanation of the influence of carbon, or other interstitial alloy elements on the behaviour of steel. The whole process is also affected by heat treatments (which can produce a variety of different microstructures and crystal allotropes within the steel), other alloy materials and actual strain rates of the test. However, it should be more than detailed enough for any A- Level treatment of the subject.
28 Appendix
Chart showing stress v strain charts for different metals.
Tensile Specimens
12000
Steel P Little plastic flow - UTS for relatively brittle. Steel K Little necking. 10000
Elastic Limit for Steel K More plastic Flow. Steel K Some ductility. Some necking. 8000 N
Significant plastic flow. / Copper
d 6000 Very ductile a
o Significant necking L
4000
2000
0 0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50 4.00 Extension / mm
Engineering and True Stress / Strain
In tensile testing, the load cell in the tensometer detects load only. The tensometer is designed to test specimens of fixed dimensions and hence of fixed and constant cross sectional area and original length. Using these pre-defined values, output is read in terms of stress and strain.
F Stress is defined as , where F and a are the applied load (N) and cross sectional area (m2) a e Strain is defined as , where e and l are the linear extension (m) and original length (m) l
As the test proceeds and elastic the limit is exceeded, the specimen necks and permanent extension takes place, hence both the cross sectional area and “original” length of the specimen change. The Tensometer cannot take these changing values into account and continues to assume the initial values, resulting in error values of the absolute stress and strain. In the absence of any cost effective way of correcting this defect in the experimental design, the output is accepted, but is qualified by referring to “Engineering” stress and strain.
If the true stress and true strain could be plotted, the lines of the charts would continue to rise, as the stress in the region of necking continues to rise as a result of decreasing cross sectional area.
29 Elastic Limit and Limit of Proportionality
In the detail of a stress strain diagram… EL = Elastic Limit Stress EL LP = Limit of Proportionality From origin to LP, the metal obeys Hooke’s Law. That is:- Stress Strain LP Furthermore, extension is not permanent, and removing the load will return the metal to its original size and shape Beyond the point LP, but NOT coincident with it, is a point at which plastic flow starts to occur, EL. Path taken on unloading Beyond EL, there are two components to the extension, Path taken on BEFORE EL unloading such that if the load is removed and elastic component will AFTER EL result in the material trying to regain its original size and shape, but also there will be a plastic deformation, resulting in a permanent increase in length. (This was discussed in the last email.) Strain
The region BETWEEN LP and EL is also purely elastic, with full recovery on the unloading of the metal, but is NOT linear. To explain this feature, we need to consider the Potential Energy between metal atoms as the separation of the atoms changes. As the atoms approach, the PE changes:- 1. Decreases (-) due to electronic / bonding effects (inverse square law). a. Over relatively large distances, the attractive force dominates. b. Approaching from infinity (moving right to left in figure 2a), the PE increases to a large negative value (- for attraction) as the extent of electronic / orbital overlap increases. c. The PE increases as the square of the distance. 2. Increases (+) due to repulsive forces between nuclei (inverse cubic). a. Over very short distances, the repulsive force dominates b. As the nuclei approach (moving right to left), electrostatic repulsion increases the PE (+ for repulsion) between the atoms c. The PE varies as the cube of the distance. 3. There is an equilibrium position where the forces balance and the PE is at a minimum value referred to as the Potential Energy well. This corresponds to the bond length, or lattice parameter in the case of a cubic metal crystal. 4. The PE well is not symmetrical. To the left, repulsive forces (varied as 1/distance cubed) dominate and to the right, attractive forces (varied as 1/distance squared) dominate. 5. Compressing the material is, therefore harder than extending the material. 6. In extending the material, the atomic distance is increased. Over a short period of time, this approximates to a linear relationship. 7. Larger distances reduce the attraction between neighbouring atoms making it easier for bonds to break and reform, e.g. at a dislocation, where a strained bond breaks but reforms with a closer neighbour. Hence we get plastic deformation. 8. Between the linear, elastic region and the onset of plastic deformation is a region in which a. The relationship between force and distance is NOT linear, but… b. The attractive forces do not allow for bonds to break, hence… c. A non-linear, elastic region is recognised.
30 Rapid rise in repulsive PE PE between atoms – hence compression is PE due to Attractive forces very difficult Beyond recovery, some bonds PE due to Repulsive forces break and new ones form between neighbouring atoms. PE due to Combined forces (Dislocations move) i.e the above two lines added together.
Inter-atomic distance Elastic, but non-linear
Relatively linear over a short region. (Up to LP on - plot)
Equilibrium condition The Bond Length 2a: Potential Energy well for 2b: Linking the energy well to the adjacent atoms in a metal Stress-Strain plot crystal.
NOT drawn to scale
During Plastic Deformation – an extension and “worked” example Dislocations provide the energy for strained bonds to break, but these bonds reform in a less strained position. Other bonds become strained etc. as the dislocation moves through the metal. It should be noted at this point that work is being done, since effectively, planes of atoms are moving through the crystal under the influence of an applied force. Doing work generates heat, hence the metal warms up. This is illustrated clearly by… 1. Straighten out a trombone paper clip. 2. Place against the lips to notice the temperature. (This may not be necessary) 3. Bend back and forth rapidly until the clip breaks. 4. Place against the lips – and notice the very large rise in temperature. This may be noticeable through the fingers, but the lips are more temperature sensitive. Furthermore, the clip bends easily at first, but it soon gets harder to bend. This is due to 1. The generation and movement of many dislocations. The dislocations are limited to sliding along specific crystal planes. 2. Bottlenecks of dislocations jam up the crystal planes used by the dislocations. 3. In turn this restricts / prevents dislocations from moving and hence limits plastic flow / ductile behaviour. 4. The material is hardened, as steel is with carbon, by the pinning of dislocations but via a different mechanism. 5. This mechanism is referred to as “Work hardening”.
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32 Hardness Testing
Background
Hardness is the resistance of a material to plastic deformation. This property can be used to measure the hardness of a material by deliberately indenting the surface with a known load and an indenter of known shape and size. The experiment / investigation is relevant to reinforce terminology such as plastic, elastic, ductile, brittle, hardness, toughness and could be applied in science or DT at Key stages 2, 3 or even 4 (where higher assessment marks are not an issue).
The principle behind this home made apparatus is that a fixed, pointed shape is pressed into a surface by a consistent force.
Plasticine can be tested at different temperatures using ice-water and ice-salt-water mixes allow two values below room temperature, the use of a water bath at 50C allows a safe maximum value to be used, hot water in a beaker, allowed to slowly cool to a test temperature and room temperature itself allow five test temperatures.
Plasticine is a very poor thermal conductor, so temperature variations will be minimal if the pupils test as soon as the plasticine leaves its heat source.
If left in water at 50C for several minutes, anomalous results may be obtained indicating that the plasticine has been hardened.
Testing different woods (parallel, perpendicular and end-on to the grain), different plastics, different metals, the effect of using composites could link with DT.
Small indentations can reasonably be observed with a magnifying glass and metal rule.
Notes: Could be used as Sc1 at KS3. Dropping masses from greater heights and/or increasing the size of the mass are ways of adjusting to harder materials. Only materials which are softer than the indenter can be measured. The point of the indenter is likely to wear with time if hard materials are tested. The thickness of the material to be tested must always be such that neither energy nor indention is transferred to the substrate. Beware testing brittle materials, such as glass.
33 To investigate the hardness of plasticine at different temperatures.
Theory Many substances soften and become more ductile when they are heated.
Prediction Indentations in plasticine will increase as the temperature of the plasticine increases, because as it softens it becomes more ductile and is, therefore, more easily deformed.
Method Place equally thick discs of plasticine in different temperature environments for about 15 minutes to allow the temperature of the plasticine to become even throughout.
Place the base of the tester on the plasticine disc.
Place the point of the indenter on the plasticine, through the central hole.
Fit the tube in place
Drop a weight from the top of the tube.
Measure the indentation.
Record results and repeat to obtain three values at each of five temperatures.
Results (Typical temperature values are given) Environment Temperature / C Indent / mm Ice-salt-water mix -5C Salt-water mix 0C Room temperature 25C Cooling water 40C Water bath 50C
Conclusion To be decided…
34 All measurements are approximate and to fit 24 mm Approx. 45 Wood / Cork
20 mm Nail
40 mm
The diagram is intended to suggest a possible construction, only. Materials and dimensions can be chosen to suit the needs of the class and available materials. Any suitable and available materials can be used e.g. Length to suit e.g. Tube: Al, Cu, plastic, cardboard 200, 300, 400 mm Base: Al, Fe, wood or plastic Weight: Fe, brass, lead Indenter: Fe, Sharpened wood (pencil in simple cases) , Nail in wood
3 mm 20 mm
Weights in simple ratios that can be dropped onto the indenter. 20 mm 20 mm 20 mm 40 mm 30 mm 20 mm
150 mm
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36 Creep Teachers Guide The following is not intended to be a full or detailed explanation of Creep, but sufficient to introduce to pupils at school and give the non-specialist teacher a background that is greater than that which would be required for schools. The mechanism of creep is not discussed and it should be noted that the text is very general since different metals will have different characteristics. Background
Engineering Stress (Nm-2) Stress-Strain Diagram In the diagram (right) C O = Initial origin YP = Elastic limit YP X OA = Linear, elastic region A C = Maximum Strength Past C = Region of necking X = Failure
O Strain (No dimensions)
Plastic deformation occurs in metals via a mechanism of dislocation movement in which a linear defect slips one atom position at a time giving the same effect as a plane of atoms gliding through the crystal. The relative position of atoms moves one position per dislocation.
Under shear forces, a dislocation moves along a crystal (slip) plane until it reaches the edge. Here the stresses are relieved and a “step” is left at the crystal surface.
Many millions of these dislocations move through the crystal along different planes, frequently their paths intersect. Dislocations cannot occupy the same space nor pass through each other, so they “lock together”. This is called a JOG. Because the dislocations block free movement to each other, the material is “hardened” and exhibits increased resistance to further deformation. This hardening due to the effect of applying forces (doing work on the material) is termed WORK HARDENING.
37 Now, consider a dislocation under compressive forces. The dislocation is “squeezed” out of the crystal, perpendicular to the compressive forces but to allow this, there must be some diffusion of atoms away from the dislocation.
Because the dislocation is not sliding in a direction perpendicular to the intersecting plane along a plane, but is moving along the intersecting plane, we have a different mechanism. The former is effected by flips of metallic bonds, and the latter by diffusion processes. This latter process is referred to as CLIMB.
Combining the above mechanisms, we can imagine a crystalline metal in which dislocations are jogged (that is locked together, preventing further movement). If there is sufficient temperature to allow diffusion processes to become significant, then the jogged dislocations may climb (out of the way of each other” such that they no longer affect each other. Once the dislocations are no longer aligned, then the dislocation may continue on its journey under the influence of applied stress.
The temperature at which the diffusion processes becomes significant is about 1/3 of the melting point of the metal on the Kelvin scale. Lead is usually chosen to illustrate creep experimentally, since it has a melting point of about 327ºC (600K) and a room temperature of 27ºC (300K) places it in excess of this temperature. This explains why lead used to cover a sloping roof tends to flow down the slope of the roof over time, such that the lead is thinner at the top than at the bottom.
During creep, the metal will thin uniformly. This decreases the cross sectional area of the metal and as a direct consequence, the stress increases. Eventually the stress increases to a point where normal ductile failure occurs and there is then evidence of necking in the final stages.
38 Since creep is diffusion controlled, it is a time dependent process. The process has been noted to have three distinct phases as shown in the diagram.
Both primary and tertiary stages are relatively short times compared to the steady state phase. The diagram on the right greatly distorts the time axis to show the shape of the primary and secondary phases. These may be a matter of seconds or minutes whereas the steady state may be hours, days or years.
In creep experiment described here, from specimen data, it will be noted that two different mechanisms may be detected.
1. High stresses – ordinary ductile failure – short failure times. 2. Low stresses – a diffusion based creep mechanism – long failure times.
In a simple loading experiment in which load is recorded along with time to failure, a plot of load v log(10)(time/sec) produces data (specimen data collected with apparatus as described in the following pages) that graphs as… Creep in Lead Wire
350 340 330 320 310 300 290 280 270 260 250 240 g / 230 R2 = 0.9894 d a
o 220 L 210 200 190 180 170 160 150 R2 = 0.9996 140 130 120 110 100 0 0.5 1 1.5 2 2.5 3 3.5 4 log(time /min)
The existence of the two slopes allows for students who make careful observations to identify these changes in slope and for the more able student to suggest that this may be due to different mechanisms. Without being prompted, there is no reason why students up to and including A’ level should know any detail about the different mechanisms – simply to note the evidence and deduce the fact would be commendable.
It will be noted that, in the experiment, the treatment of data requires that load be plotted against log(10)(time). This should not be beyond A’ level candidates, and GCSE pupils should be able to use the log function of a calculator. To make the analysis simpler, the teacher could provide a spreadsheet into which pupils enter data and a graph is automatically plotted. Needless to say, where assessment of ICT is required, teachers could set a separate exercise for pupils to prepare their own spreadsheet.
39 Apparatus
The apparatus can be made from cheap easily obtained materials.
Eyelet
Lead Wire Support Bar Retort Stand Dowling (5/16Dia)
Eyelets 3-windings around the dowling and then fasten to the eyelet Kite String
Weights
Fishing Line Guide (Taut)
Wooden Block (Base)
+
- Battery
Alarm clock
40 Method 1. Measure about 20 cm lead wire taking care not to crease it or bend it sharply, as this will introduce mechanical working that will affect the results. 2. Attach the wire to the eyelet on the top support bar. 3. Wrap the wire around the support bar 3 times – this provides a friction grip that prevents excess load being placed on the eyelet connection. 4. Attach the lead wire to the lower load bar in a similar way, making sure that the length of wire to be loaded is consistently the same each time (about 15 cm). 5. Set the timer to -1 min. 6. Connect the timer lead to the top eyelet. 7. Attach the timer lead to the lower bar: the clock will start. 8. Attach the weights to the sling below the lower bar BUT SUPPORT THEM UNTIL THE MINUTE FINGER IS ABUT TO PASS ZERO. 9. Carefully apply the weights onto the lead wire. The weights must NOT be released suddenly as this may cause a premature failure. 10. Leave the apparatus, ideally in a draft free and constant temperature place until the wire fails 11. Record the time to failure and the weight applied.
Extension 12. Use a micrometer to measure the diameter of the wire before and after the experiment. a. What is the initial stress? b. What is the final stress? c. Does the wire thin uniformly over its length, or only at the site of failure? 13. Repeat the experiment for loads in the range 150g to 350g. This type of experiment will produce what appears to be a large “scatter” in the results – especially for the longer times. It may be useful to repeat each experiment at least once and calculate an average. This is because on the microscopic scale, and below, where the failure mechanism applies, each and every piece of wire is individual and has its own microscopic defects – no matter how carefully you handle the wire. 14. Plot the results of Load (or stress) against log10(time / seconds). Your teacher may provide a spreadsheet to help with this. 15. What do you notice about the slope of results with high loads compared to those with lower loads (above / below about 320g)? Can you suggest an explanation?
Note: The lower loads may take in the region of 104s (27 to 30 hours) and the higher loads may result in a failure of only a few minutes or even seconds.
Challenge Set up a creep test that, according to your results, will fail during your NEXT science lesson. The group with the test that fails closest to the middle of the lesson is the winner!
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42 Ferrofluids Technology Overview
A ferrofluid is a stable colloidal suspension of sub- domain magnetic particles in a liquid carrier. The particles, which have an average size of about 100Å (10 nm), are coated with a stabilizing dispersing agent (surfactant) which prevents particle agglomeration even when a strong magnetic field gradient is applied to the ferrofluid. The surfactant must be matched to the carrier type and must overcome the attractive van der Waals and magnetic forces between the particles. The colloid and thermal stabilities, crucial to many applications, are greatly influenced by the choice of the surfactant. A typical ferrofluid may contain by volume 5% magnetic solid, 10% surfactant and 85% carrier. In the absence of a magnetic field, the magnetic moments of the particles are randomly distributed and the fluid has no net magnetization. When a magnetic field is applied to a ferrofluid, the magnetic moments of the particles orient along the field lines almost instantly. The magnetization of the ferrofluid responds immediately to the changes in the applied magnetic field and when the applied field is removed, the moments randomize quickly. In a gradient field the whole fluid responds as a homogeneous magnetic liquid which moves to the region of highest flux. This means that ferrofluids can be precisely positioned and controlled by an external magnetic field. The forces holding the magnetic fluid in place are proportional to the gradient of the external field and the magnetization value of the fluid. This means that the retention force of a ferrofluid can be adjusted by changing either the magnetization of the fluid or the magnetic field in the region. A Ferrofluid is designed as a component of a device and therefore it must meet specific performance objectives of the device. The selection of ferrofluid depends on many factors such as environments, operating life, etc. There are many different combinations of saturation magnetization and viscosity resulting in a ferrofluid suitable for every application. The operating life of the product depends on the volatility of the ferrofluid. Products that require long life must use ferrofluids with low evaporation rate or vapor pressure. Also, seals operating at high vacuum must incorporate low vapor pressure fluids. On the other hand, ferrofluids for domain observation must evaporate quickly so that the process time can be minimized. The lower the volatility, the higher the viscosity of the ferrofluid. Thermal conductivity of a ferrofluid depends linearly on the solid loading. Fluorocarbon based ferrofluids have the lowest thermal conductivity of all commercial ferrofluids, therefore they are the least desirable materials for heat transfer applications. In devices, ferrofluids come in contact with a wide variety of materials. It is necessary to ensure that ferrofluids are chemically compatible with these materials. The fluids may be exposed to hostile gases, such as in the semiconductor and laser industries; to liquid sprays in machine tool and aircraft industries; to lubricant vapors in the computer industry; and to various adhesives in the speaker industry. Furthermore, ferrofluids may be in contact with various types of plastics and
43 plating materials. The surface morphology can also affect the behavior of the fluid. The selection of ferrofluid is carefully engineered to meet application requirements. Additionally, ferrofluids may be expected to perform at temperature of 150°C continuously or 200°C intermittently, in winter conditions (-20°C) and space environments (-55°C). They may also be required to withstand nuclear radiation without breakdown. The thermal stability of a ferrofluid is related to particle density. The particles appear to behave like a catalyst and produce free radicals, which lead to cross linking of molecular chains and eventual congealing of the fluid. Catalytic activity is higher at elevated temperatures and, therefore, ferrofluids congeal more rapidly at these temperatures. Oxidation is another mechanism that contributes to congealing of ferrofluids, and again the higher the temperature, the faster the rate of reaction. The ferrofluids in sealed environments stay in a liquid state significantly longer than those in open air. High magnetization ferrofluids are of interest as they produce volumetric efficiencies of magnetic circuit designs leading to lightweight and lower cost products. They can also be used to reduce reluctance of magnetic circuits and fringing field thus increasing useful flux density in the air gap. The domain magnetization of magnetite ultimately limits the maximum magnetization value that can be realized in a ferrofluid. In summary we can say that Ferrofluids are a unique class of material. Ferrofluid technology is well established and capable of solving a wide variety of technical problems. There are many successful applications of this engineering material and there is immense future potential. In many applications, ferrofluid is an active component that contributes towards the enhanced performance of the device. These devices are either mechanical (e.g., seals, bearings and dampers) or electromechanical (e.g., loudspeakers, stepper motors and sensors) in nature. In other cases, ferrofluid is employed simply as a material for nondestructive testing of other components such as magnetic tapes, stainless steels and turbine blades. When correctly applied, Ferrofluid can produce dramatic improvements in a products' performance; or achieve a level of performance unattainable by any other technology or product. Ferrotec Corporation (formerly Ferrofluidics) has led the development of Ferrofluidic® technology since 1968 and has worked closely with many companies as their new product teams incorporate ferrofluids in next-generation products. With a comprehensive fluid development and applications laboratories in both the US and Japan, and an experienced staff of scientists and engineers available to assist you, Ferrotec is well placed to help you solve your engineering challenges using ferrofluid.
Acknowledgement: The above text is taken from the Ferrotec Web site… http://www.ferrotec.com/usa/ferrofluid_technology_overview.htm …where much more information about Ferrofluids can be found. Much of the information on the web site could be useful for school projects.
44 Ferrofluid Demonstrations Ferrofluid in a bottle Ferrofluids can be obtained from Ferrotec (expensive) or from… www.ebay.co.uk http://www.scuddlebutt3.co.uk (seller on ebay) http://www.mutr.co.uk/products.aspx?catID=18 The ferrofluid is the dark brown fluid and is hydrocarbon based. The colourless liquid is a support medium intended to reduce the surface tension of the ferrofluid and provide some buoyancy – hence improving the “spikey” effect due to a magnetic field. 1. Experiment with the magnet. Note the spikey effect as magnetic nanoparticles attempt to follow the magnetic lines of force and are confined by the surface tension of the support medium. Note the effect of slowly withdrawing the magnet. 2. Effect of a shaped magnet Pour a small (0.5cm3) ferrofluid into the aluminium container. Place an aluminium container on top of the “A” magnet. Take care to approach the magnet from ABOVE – the ferrofluid will jump out of the container if the magnetic field is strong enough. Observe the shape of the ferrofluid. Move the magnet and observe. 3. Separation technique: Pour ferrofluid into a container to a depth of about 0.5cm. Add a 5p piece (Copper-nickel alloy and NOT magnetic whereas some 1p pieces are magnetic. Do not use magnetic coins). The coin is seen to sink. Place the container, ferrofluid and 5p piece over a strong magnet. Approach from above the magnet. The 5p piece floats. This is due to magnetite nanoparticles drawing closer together in the magnetic field, thus increasing the ferrofluid’s density. Add more ferrofluid. The coin does NOT rise to the surface. Probe the ferrofluid with a thin rod. The coins is found to be floating just above the base of the container. As distance from the magnet increases, the magnetic field decreases. Hence the magnetite particles are not so close together and density decreases through the fluid from bottom to surface. The coin floats at the point where it is neutrally buoyant. 4. Application – fractional flotation of precious metals etc using a variable electromagnet.
Other useful web addresses for information about ferrofluids and how they can be used… http://www.ferrolabs.com/ http://www.teachersource.com/catalog/index.html
45 For further information about class experiments, school visits and courses (pupils and teachers), please contact:
Martin Carr (Schools Liaison Officer) Department of Materials University of Oxford Parks Road Oxford OX1 3PH
T. 01865 273 710 F. 01865 273 789 E. [email protected] W. http://www.materials.ox.ac.uk/undergraduate/schools.html
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