As Physics TSM

GCE Physics Guidance for the AS practical assessment

September 2008

Tutor support materials

Edexcel, a Pearson company, is the UK’s largest awarding body, offering academic and vocational qualifications and testing to more than 25,000 schools, colleges, employers and other places of learning in the UK and in over 100 countries worldwide. Qualifications include GCSE, AS and A Level, NVQ and our BTEC suite of vocational qualifications from entry level to BTEC Higher National Diplomas, recognised by employers and higher education institutions worldwide.

We deliver 9.4 million exam scripts each year, with more than 90% of exam papers marked onscreen annually. As part of Pearson, Edexcel continues to invest in cutting-edge technology that has revolutionised the examinations and assessment system. This includes the ability to provide detailed performance data to teachers and students which helps to raise attainment.

Acknowledgements

This specification has been produced by Edexcel on the basis of consultation with teachers, examiners, consultants and other interested parties. Edexcel would like to thank all those who contributed their time and expertise to the specification’s development.

References to third-party material made in this specification are made in good faith. Edexcel does not endorse, approve or accept responsibility for the content of materials, which may be subject to change, or any opinions expressed therein. (Material may include textbooks, journals, magazines and other publications and websites.)

Authorised by Roger Beard Prepared by John Crew

All the material in this publication is copyright © Edexcel Limited 2008

This document should be read in conjunction with the GCE Physics specification - issue 3 (Publication code UA018902).

Contents

Introduction 4 How Science Works 4

General considerations 4

Preparing students for the practical assessment 6 Introduction 6

Safety 6

Planning: Identifying equipment 6

Planning: Identifying techniques to use 7

Implementation: measurements 9

Accuracy and precision 9

Implementation: Recording results in tables 10

Analysing: Graphs 10

Analysing: Limitation of results 11

Evaluating 12

Advice for students 13 Summary of the case study or visit 13

Plan 13

Implementation and measurements 14

Analysis 14

Uncertainties in measurements 16 What are uncertainties? Why are they important? 16

Calculating uncertainties 16

Calculating percentage uncertainties 17

Compounding errors 17

Guidance for visits 20

Guidance for case studies 21

Some ideas for practical assessments 22 Visits 22

Case studies 22

Conducting the AS assessment 23 The summary of the case study or visit 23

Tutor support – AS Physics – Draft 1 – September 2008 © Edexcel Limited 2008

2

The plan 23

Carrying out the practical work 23

Providing guidance to students during the practical session 24

Carrying out the analysis 24

Returning work 25

Exemplar of assessed work: Refractometry 26 Briefing sheet 27

Case study on refractometry 28

AS Marking grid for case study on refractometry 35

Examiner’s comments on refractometry 38

Exemplar of assessed work: Geophysics 40 Visit report for Geophysics 41

AS Marking grid for visit to an archaeological site 49

Examiner’s comments - geophysics 52

Exemplar of assessed work: Focal length 54 Visit report for an optician’s shop 55

AS Marking grid for visit to an optician’s shop 65

Examiner’s comments for a visit to an optician’s shop 68

Exemplar of assessed work: Solar cells 70 Visit report for solar cells 71

AS Marking grid for solar cells 78

Examiner’s comments for solar cells 81

Frequently asked questions 82 Questions relating to the visit 82

Questions relating to written work 82

Questions relating to the practical session 83

Questions relating to marking work 84

Other questions 84

Further advice 86 Plagiarism and collusion 86

Annotation of student work 87

Glossary 89

Appendix 1: Briefing sheets for exemplars based on visits 91 Introduction 91

Briefing sheet for the geophysics case study 92

Briefing sheet for the optical case study 93

Briefing sheet for the case study on solar cells 94

Appendix 2: Precision, accuracy and sensitivity 95


4

Introduction

All AS students are required to carry out one piece of assessed practical work that is based on either a case study or a visit that involves an application of physics. This book provides guidance and examples for the practical work. It includes a section that discusses how students should be prepared for this assessment, advice for students and some notes on uncertainties that may be issued to students, and suggestions and exemplars of practical assessments.

How Science Works

The practical assessment gives students the opportunity to address some of the “How science works” themes. These themes are about how scientists go about investigating the world about us using the scientific method. It has nothing to do with content and is a development that builds on and extends the science skills in Key Stages 3 and 4. Students can use this opportunity to demonstrate:

• their knowledge and understanding to pose scientific questions, define scientific problems, and to present scientific arguments and ideas

• their ability to use appropriate methodology to answer scientific questions and solve scientific problems

• their ability to carry out experimental and investigative activities, including appropriate risk assessment

• their ability to analyse and interpret data to provide evidence, recognising correlations and casual relationships

• their ability to evaluate methodology, evidence and data • their ability to communicate information and ideas in appropriate ways using

appropriate technology • a consideration of ethical issues • an appreciation of the ways in which society uses science to inform decision-making • a consideration of applications and implications of science.

General considerations

It is important to ensure that all students have the opportunity to gain marks for all the assessment criteria for unit 3 when selecting the visit or case study.

The practical work must relate to either the visit or case study and students must point out this relationship. It would be beneficial to the students to be given a practical on a topic within the AS course but this is not a requirement of the assessment criteria (however it is expected that this work will show progression from GCSE). The case study or visit should be undertaken at an appropriate time during the course so that it integrates into the teaching of the subject matter and coincides with the teaching of the relevant topic.

The practical work needs to involve the variation of two interdependent quantities which can be measured. Students need to be able to produce a graph which will usually be a straight line and derive the relationship between the two variables or derive a constant. For example this might involve one variable plotted against the square root of the other. It is not envisaged that AS students will plot log graphs.

Edexcel does not specify a list of equipment that should be made available to students and therefore the practical assessment may be achieved by using basic laboratory apparatus; this

5

does not preclude students from using more complex equipment, such as signal generators, oscilloscopes and data logging devices where these are available.

The practical work has been designed to be flexible so that centres may use their existing resources. If many students in large centres require the use of expensive equipment then different groups of students may have to do the practical assessment at different times of the year. If a staggered approach is taken then different groups of students should do different experiments to avoid collaboration.


6

Preparing students for the practical assessment

Introduction

The practical work will assess each student’s ability to:

• plan

• implement

• analyse and

• evaluate.

Centres should devise and implement a suitable course of practical work throughout the AS course to ensure that they acquire the skills and experience that will be needed for them to succeed in each of these aspects of the practical assessment. The specification suggests experiments that students could carry out to enable them to experience a wide range of practical skills. The suggestions are not exhaustive and centres could use different experiments to those suggested to reflect the equipment that they have available.

Students should be encouraged to calculate percentage uncertainties (discussed in another section) whenever possible in experiments that they do throughout the course.

Safety

Teachers should emphasise the importance of safety in all practical work throughout the course as a matter of good practice.

Planning: Identifying equipment

Students should be able to identify apparatus and materials that are needed to achieve a particular aim. This includes the identification of the most appropriate measuring instruments for a particular task. For example, if a student needs to measure the width and thickness of a rule then they would be expected to select vernier callipers for the width and a micrometer for the thickness (or a suitable digital device for both).

Students should be aware of the precision of instruments, in general:

mm scale 0.50 mm

vernier 0.10 mm

micrometer 0.01 mm

If measuring a mass such as the mass of a coin students should identify an appropriate instrument to use. Different digital top pan balances have different ranges and different precisions. The student should select the most appropriate top pan balance to use.

Where appropriate, students should calculate / estimate the values of equipment needed, eg, resistors and their power rating in electrical circuits or suggest a range of values, eg weights, that will be needed for their experiment.


7

Planning: Identifying techniques to use

Students should develop their knowledge and understanding of a variety of techniques in order to produce results which are as accurate and reliable as is reasonably possible. Experience shows that students who do this are more likely to gain higher marks for the better results that this achieves. The following list (which is by no means exhaustive) contains some common techniques that should be experienced several times during normal practical work:

• zero error checks

• repeat measurements (at different places if appropriate)

• difference methods (eg for extension of a spring)

• eye level to avoid parallax error

• use of marker at centre of oscillations to aid timing

• use of set square for checking vertical or horizontal arrangements

• interpolation of analogue scales

• trigonometric methods for measuring angles


8

Technique for measuring the diameter of a cylinder that is several cm across

Using a trigonometric method for measuring angles

Using a marker at the centre of an oscillation to aid timing

D

Pin

Cork

Tan θ = y/x

θ y

x


9

Implementation: measurements

During the course, students should develop their skills for making valid, reliable measurements using appropriate techniques. Students should provide written evidence when writing up their assessed practical work to show the techniques that they have used to ensure that they get the appropriate credit; it is recommended that students be encouraged to do this with the normal practical work that they do throughout the course so that it becomes a habit.

Students should realise that a liquid must be stirred before using a thermometer to record its temperature and this should be mentioned in the notes that the students produce.

Before taking measurements, students should check instruments for zero error and record that this was done.

If measuring a fixed quantity, eg diameter of a rod, then students should take repeat measurements in at least 3 different places at different orientations (recording all these measurements to provide evidence they have done this).

Students should make and record sufficient relevant observations over a suitable range of values with appropriate precision. What is a “sufficient” number of observations cannot always be defined - it depends on the nature and context of the experiment and is in itself a “skill” which is acquired through experience. For example, for a mass oscillating on a spring with a period of about 1s it might be appropriate to time, say, 20 oscillations and then repeat this measurement. However, with a heavily damped motion it might not be possible to count more than a few oscillations, in which case it might be necessary to repeat 5 oscillations at least 4 times. Students should be prepared to modify their planned procedures in response to their experimental observations.

Students should realise that in some experiments (eg, plotting a cooling curve) it is not possible to take extra measurements after obtaining a set of readings and therefore they should plan to take as many readings as possible (eg by taking readings every 30 s rather than every minute). It may actually be counter productive to take repeat readings in some cases, for example in an electrical experiment a component may heat up and so a repeat set of readings would be completely different from the first set of readings.

Where it is difficult to make a precise measurement, eg timing a ball rolling down a slope (which is likely to be in the order of 2 seconds and subject to considerable subjective error) then several readings should be taken and averaged.

Accuracy and precision

Students should be aware of the difference between the accuracy and precision of measurements, for example although a stopwatch can read to high precision (0.01 s) timings will be subject to error because of the reaction time in starting and stopping the stopwatch. This will give rise to random errors, which can be reduced by taking several readings. When measuring the resistance of a length of wire the contact resistance can lead to a systematic error. Repeat readings might not do anything about this but plotting a graph of resistance against length of wire should reveal a value of the contact resistance when length is zero.

Thermometers are notoriously inaccurate: although 0 – 100 ºC thermometers can be read (by interpolation) to a precision of 0.5 ºC or better they are unlikely to be accurate (due to their manufacture) to within 1 ºC, or even worse. This has less effect when measuring a temperature difference (eg determining the rise in temperature when a beaker of water is heated) and so students should still be trained to attempt readings to 0.5 ºC or better.

Students should recognise that even though an instrument is capable of high precision (eg digital meter, electronic balance, digital stopwatch), its accuracy may well be in doubt


10

(particularly if the student hasn’t checked for any zero error) or there may be a further uncertainty due to human error.

Implementation: Recording results in tables

Students should present work appropriately in written, graphical or other forms. In particular, results should be tabulated with data columns headed by the corresponding units with the data expressed to the appropriate precision, eg:

h1 / mm h2 /mm x/mm 20T / s 20T / s T ² / s²

327.5

327.5

321.0

314.5

6.5

13.0

19.52

27.64

19.64

27.50

0.96

1.90

All readings should be shown and recorded to the precision of the instrument. It is not essential to record “intermediate” calculations (of, for example, the mean value of 20T and T), but the required quantity, T 2, should be expressed to a suitable number of significant figures. The number of significant figures is deemed to represent the precision of the value, eg 0.96 s2 indicates a value of 0.96 + 0.005 s2.

Analysing: Graphs

Graphs should be drawn using a large scale, but avoiding “awkward” scales, particularly scales of three. A rule-of-thumb definition of “large” is that the points should occupy at least half the grid in both the x and y directions (or else the scale could be doubled!); this may include the origin if appropriate. The axes should be labelled with the quantity being plotted (or its symbol) and its units (if applicable), eg T 2 / s 2, ln (V / cm 3)1, l / D 2 / m-2. Points should be plotted with precision (interpolating between grid lines) and denoted by a dot with a small circle round it or a small cross. Error bars are not expected, although students could be made aware of them. Students should be taught to draw the line of best fit, whether it be a straight line or a smooth curve, preferably with a sharp pencil.

If a straight line graph is anticipated, it is appropriate initially to take 6 measurements over as wide a range of values as possible. Having plotted the graph it might be necessary to take extra measurements, perhaps in a region where there is some doubt as to the nature of the line. This is particularly so in the case of a curve where more points are generally required, especially in the region of a maximum or minimum.

1 Bold type indicates that this is an A2 requirement


11

X X

X X

(i) (ii)

Does graph (i) curve to the origin, or continue as a straight line and give an intercept? More readings would be needed (if possible) to decide. In graph (ii) extra readings in the region of the maximum would help to define its shape more precisely.

At A2, and where appropriate at AS, students are expected to relate linear graphs to

y = mx + c and to understand that a straight line graph must pass through the origin to confirm a proportional relationship. They should, however, bear in mind that not all relationships in physics are linear! A2 students are expected to be able to plot logarithmic graphs in order to test for exponential relationships or power laws.

Students should be able to interpret information from a graph, allocating units where appropriate to the gradient, intercept and area under the curve where these represent physical quantities. When a gradient is being determined, whether from a straight line or by drawing a tangent at the appropriate point on a curve, as large a triangle as possible should be used and its co-ordinates should be recorded in the calculation of its value.

The student’s graph may not pass through the origin, from which s/he might infer that there could be a systematic error, eg there may be an additional constant term in the expression that they are using.

Analysing: Limitation of results

In analysing their observations, students should be aware of the limitations of their experimental measurements. They should understand that certain types of measurement are more reliable than others. For example, finding the period of a mass oscillating on a spring from 20 oscillations (say 20 s) should be a reliable, reproducible measurement, whereas the time for a ball to roll down a slope is likely to be fairly unreliable for a number of reasons: human error in measuring a time of about 2 s, the ball may not roll in a straight line and the ball might skid. Simple electrical measurements using digital meters should be reliable, whilst thermal experiments may be less so due to thermal energy losses and inaccurate and insensitive thermometers.

They should understand how repeat measurements and graphical methods can reduce random and systematic errors and how such techniques can invariably improve the reliability of their data.

Students should be aware of the precision of instruments as discussed previously. They should recognise that if a measurement is the result of the difference of two readings (eg the depression of a cantilever as measured by a metre rule), it would be unreasonable to quote an uncertainty of better than 1 mm (ie 0.5 mm for each reading).

X


12

Evaluating

In drawing their conclusions, students should be aware that as well as possible instrument errors (even with high precision devices such as digital meters and electronic balances), values stated on components (eg masses, resistors and especially capacitors) are only “nominal” values, subject to manufacturers’ tolerances. For example, electrolytic capacitors may have a tolerance of 10% or more.

They should also be aware of factors inherent within their apparatus or experimental arrangements which limit the reliability of their measurements, eg friction, air resistance, contact resistance, fluctuating power supplies and change of temperature during the experiment.

Students should assess the reliability of their data by considering the uncertainty of their measurements. In general terms this should be taken to be half the range of their measurements if several readings are taken or else the precision to which the instrument can be read if only a single reading is taken. However, if human error is likely to exceed this (eg reaction time starting and stopping a stopwatch) then this should be taken into consideration (eg although a stopwatch can read to a precision of 0.01 s, a more realistic uncertainty when using it to time oscillations might be 0.1 s to reflect reaction time). Uncertainties are usually of little value unless expressed as a percentage, eg a 0.1 s uncertainty in timing 20 oscillations (say 20 s) would give rise to a percentage uncertainty of only 0.5%, whereas a realistic uncertainly of 0.2 s in timing a ball rolling down a slope (say 2 s) would result in a 10% uncertainty.

Conclusions, wherever possible, should be based on quantitative evidence. For example, in an experiment to determine acceleration of free fall, the student might get a value for g of 10.4 ms-2. A valid conclusion would be that the experiment confirms the relationship within experimental error because the value of g obtained is within about 4% of the accepted value and the experimental uncertainty is 10% from just the timing. Comments such as “close to the right value” get no credit!

Finally, students need to apply their knowledge and understanding of physics, together with common sense. For example if in an experiment to determine a value for the density of a golf ball it was found it to be 140 kg m-3 they should stop and think “but doesn’t a golf ball sink in water?” A check of their calculations might enable them to discover, perhaps, that they had used the diameter of the ball instead of its radius and hence found a volume that was 8 times too large (“is the volume really 320 cm3?”). If a careful check does not reveal such an error, then a suitable comment should be made to indicate that the student is somewhat surprised by the result.


13

Advice for students

Summary of the case study or visit

You will need to produce a summary of either a visit or a case study based on an application of physics.

If you are doing a visit then you should include brief details of the venue that you visited.

If you are doing a case study, you should include at least three different types of sources of information for writing your summary. These could be your text book, a website and a magazine. Three different textbooks do not count as three different types of sources of information. You should ensure that you list all the sources of information that you have used.

If you quote from any sources of information then you must ensure that you indicate this clearly in your work and ensure that the source of each quote is acknowledged.

Describe the application that uses physics and relate it to the physics principles involved, ensuring that you use specialist terminology correctly – use a textbook or website to help you if necessary.

Discuss a social, environmental, historical or other relevant context in your summary.

Comment on the benefits of the physics used in the application that you are studying (eg ultrasonic scans provide a quick, cheap method for detecting small cracks in aeroplane wings) or risks involved (eg X-rays can harm the body and therefore special precautions need to be taken to protect both the operator and patient, for example the operator usually wears a lead apron).

Include at least one piece of information that has not been mentioned in any documents that you received for the briefing or case study. This could, for example, be a diagram to illustrate a point that you are making.

Plan

List all the materials that you require for your experiment.

State how you will measure two different types of quantities using the most appropriate instrument. For example, you could write:

• I will use a voltmeter to measure the voltage across the resistor.

• I will use a thermometer to measure the temperature of the water.

Explain why you have chosen two of the measuring instruments that you have listed. For example, you could write:

• I will use a micrometer to measure the thickness of the ruler because this allows me to measure to the nearest 0.01mm giving me a more precise measurement than vernier callipers.

• I will use a data logger because I need to take several readings over very short time intervals. It would be difficult for a human to take so many readings that are close together.

Describe at least two measuring techniques that you have used to make your measurements reliable. For example, you could write:

• I will use a pin to mark the position of the spring at the centre of its oscillation.


14

• The angle that I need to find is about 5o. This is too small to measure accurately with a protractor so I will measure the height and length of the slope and use trigonometry to calculate the angle.

Write down which is the independent variable and which is the dependent variable in your experiment. You need to identify other variables that could affect your results and state how these were controlled to ensure that you carried out a fair test. For example, you could write:

• I kept the weight at the end of the string constant throughout the experiment so that its tension was the same for each measurement that I recorded.

If you will not be taking repeat readings you should explain why. For example, you could write:

• I will be recording the temperature of the liquid as it cools down, so it will not be possible to repeat readings. However, I will take many readings that are close to each other in case I misread the thermometer.

Identify any safety hazards in your experiment and any precautions you may take. For example, you could write:

• I will wear safety goggles because the wire is under a lot of tension and could break while I am taking a reading.

Indicate how you intend to use the data that you collected. For example, you could write:

• I will plot stress against strain and use the gradient of the linear part to find the Young modulus.

Include a diagram showing the arrangement of the apparatus that you will use. Mark important distances on this diagram and, in particular, mark any distances that you will measure.

Finally, remember that your plan should show logical thought by describing what you intend to do in sequence. The plan should be written in the future tense but this is not essential.

Implementation and measurements

Record all your results in an appropriate table.

If you take the average of, say three readings, then you should ensure that you write down each individual reading, not just the average value to show the examiner that you have taken an appropriate number of measurements.

If you are plotting a graph then you should aim to take at least 6 readings and repeat these if necessary. It is a good idea to draw a rough graph as you are taking the measurements so that you can investigate anomalous readings or to take extra readings near any turning points in any curves that you obtain.

Make sure that you take measurements over as wide a range as possible. For example, if you are determining the distance between two nodes that are separated by a few centimetres then you should not measure the distance between two nodes only. Instead, measure the distance occupied by several nodes and then calculate the average distance between two of these nodes.

Analysis

When you draw your graph, you should use more than half the graph paper in both the x and y directions. The graph need not necessarily include the origin; this depends on the measurements that you are carrying out.

Use a sensible scale; for example avoid the use of a scale that goes up in steps of three as this will make it difficult for you to process any readings that you take from your graph.


15

Make sure that you label each axis with the quantity being plotted (or its symbol) and its units if it has any, eg T 2 / s 2.

Plot points accurately, using either a dot surrounded by a small circle or a small cross.

Make a brief comment on the trend shown by your graph, eg as temperature increases, resistance increases linearly. Remember that a straight line graph must pass through the origin to confirm a directly proportional relationship.

If you need to obtain the gradient of your graph you should draw as large a triangle as possible on your graph paper to show how you worked out the gradient. State the units of the gradient if it has any.

Briefly list sources of error and calculate the uncertainties that these contribute to the result(s) of your experiment.

Suggest at least one realistic non-trivial modification that you could make to reduce the errors in your experiment or to improve your experiment. Trivial suggestions such as if I had more time I would have taken more readings will not score this mark!

Briefly mention any physics principles that you use in your calculations and/or conclusion.


16

Uncertainties in measurements

What are uncertainties? Why are they important?

When you repeat a measurement you often get different results. There is an uncertainty in the measurement that you have taken. It is important to be able to determine the uncertainty in measurements so that their effect can be taken into consideration when drawing conclusions about experimental results.

Calculating uncertainties

Example: A student measures the diameter of a metal canister using a ruler graduated in mm and records three results:

66 mm, 65 mm and 61 mm.

The average diameter is (66 + 65 + 61) / 3 = 64 mm.

The uncertainty in the diameter is the difference between the average reading and the biggest or smallest value obtained, whichever is the greater. In this case, the measurement of 61 mm is further from the average value than 66 mm, so the uncertainty in the measurement is:

64 – 61 = 3 mm.

Therefore the diameter of the metal canister is 64 3 mm.

Even in situations where the same reading is obtained each time there is still an uncertainty in the measurement because the instrument used to take the measurement has its own limitations. If the three readings obtained above were all 64 mm then the value of the diameter being measured is somewhere between the range of values 63.5 mm and 64.5 mm.

In this case, the uncertainty in the diameter is 0.5 mm.

Therefore the diameter of the metal canister is 64.0 0.5 mm.

+ -

+ -

+ -


17

Calculating percentage uncertainties

The percentage uncertainty in a measurement can be calculated using:

The percentage uncertainty in the measurement of the diameter of the metal canister is:

The radius of the canister = diameter/2 = 32 mm.

The percentage uncertainty for the radius of the canister is the same as its diameter ie 1%.

Compounding errors2

Calculations often use more than one measurement. Each measurement will have its own uncertainty, so it is necessary to combine the uncertainties for each measurement together to calculate the overall uncertainty in the result of the calculation. The method for combing uncertainties together depends on how the measurements are used in the calculation:

The total percentage uncertainty is calculated by adding together the percentage uncertainties for each measurement.

Example 1: Calculating the percentage uncertainty for the area of a square tile.

A student using a rule to measure the two adjacent sides of a square tile obtains the following results:

Length of one side = 84 0.5mm

Length of second side = 84 0.5mm

Show that the percentage uncertainty in the length of each side of this square tile is about 1%.

Calculate the area of the square.

(The above two calculations are left as an exercise for the student.)

[Area of square = 84 x 84 = 7100 mm]

2 This section applies for the A2 practical only

+ - + -

Uncertainty of measurement Measurement taken

x 100%


x 100% = 0.5 64

x 100% = 1 %


18

The percentage uncertainty in the area of the square tile is calculated by adding together the percentage uncertainties for its two sides.

Percentage uncertainty in the square tile is:

1% + 1% = 2%

Example 2: A metallurgist is determining the purity of an alloy that is in the shape of a cube by measuring the density of the material. The following readings are taken:

Length of each side of the cube = 24.0 0.5mm

Mass of cube = 48.230 0.005g

Calculate (i) the density of the material (ii) the percentage uncertainty in the density of the material.

Solution 2:

(i) Density of alloy = mass/volume = 48.230 x 10 -3 kg/ (24.0 x 10-3)3 = 3500 kg m-3.

(ii) Percentage uncertainty in the length of each side of the cube

Percentage uncertainty in mass of cube

Therefore total percentage uncertainty = 2% + 2% + 2% +0.1% = 6.1%

We normally ignore decimal places in calculating uncertainties so the percentage uncertainty in the density of the material is 6%.

Example 3: Calculating the percentage uncertainty for the cross sectional area of a canister.

If the student determines that the radius of the metal canister is 32 mm with an uncertainty of 1% then the cross sectional area of the canister is:

= π r 2

= π (32) 2

= 3200 mm2.

The cross sectional area was calculated by squaring the radius (ie multiplying the radius by the radius). Since two quantities have been multiplied together, the percentage uncertainty in the

+ -

+ -

0.5 24

x 100% = 2 % =

0.005 48.2

x 100% = 0.1 % =


19

value of the cross sectional area is found by adding the percentage uncertainty of the radius to the percentage uncertainty of the radius:

Percentage uncertainty in cross sectional area

= 1% + 1%

= 2%


20

Guidance for visits

The visit may be related to any practical application of physics; it need not involve an industrial visit as some of the exemplars in this book demonstrate. The visit should, if possible, be integrated with the teaching programme so that it becomes a natural part of the course of study. Teachers should ensure that they are familiar with their centre’s policy for taking students off site before arranging a visit. The teacher should make a preliminary visit to the organisation that students will visit and discuss its purpose with the employer (or other contact) before students go on the visit. The teacher should identify the type(s) of practical work that students could undertake as a result of the visit and ensure that the visit will provide students with the opportunity to achieve all the requirements of the assessment criteria (see the specification for the assessment criteria). Some visits will provide students with the opportunity to do different types of practical work; other visits may provide the opportunity to do one type of practical work only. Health and safety issues should be discussed with the employer at this stage. The organisation may provide some documentation about the physics involved in the visit; teachers should check that this documentation is at an appropriate level for all the students in the group. Teachers may produce their own documentation or supplement any documentation provided by the organisation with their own notes. A copy of any documentation provided by the organisation and/or the centre that is issued to students should be included in work that is sent to Edexcel for moderation or marking. Teachers should brief students before they go on the visit. The briefing should include an outline of what the students are expected to achieve. Students could each make up a list of questions to ask when they do the visit. Teachers may wish to review in class the physics that students will need to gain maximum benefit from the visit. Alternatively, teachers may require students to review the necessary physics concepts for homework and possibly give them a test on these concepts before the visit commences. If the organisation has its own website then students should be encouraged to look at it, and possibly compare this to similar sites. This may help them to formulate questions to ask when doing the visit and also give them some preliminary background information. Teachers should remind students that when writing their report, they may refer to material on websites but they should not merely copy large chunks of text into their own work; instead they should use their own words to convey their understanding of what they have read. However, short quotes may be used provided that the source of the quotes are clearly identified. Centres with a large cohort of students are unlikely to be able to take every student on the same visit at the same time. For this reason, such centres may arrange staggered visits for different groups of students. It is good practise to take students to different organisations when visits are staggered over a long period of time to reduce the opportunities for students to collaborate with each other. Alternatively, students in different classes could do different practicals that are based on the same visit. If a student misses a visit, or if a student produces a poor piece of assessed work for the visit then the centre may allow the student to do a case study as an alternative to the visit. Centres could produce a briefing for a case study that relates to the visit for students who miss the visit. This document contains examples that illustrate how case studies may be based on visits.


21

Guidance for case studies

Case studies may be based on any practical application of physics. The case study should be integrated with the teaching programme so that it becomes a natural part of the course of study. Alternatively, students may express an interest in an application that appears in the media, possibly in a scientific magazine, eg Focus, Scientific American or New Scientist and this could provide the opportunity to develop a case study that will capture their interest and thereby provide a high motivation factor.

It is not necessary for all students to do the same case study; this is at the centre’s discretion, although centres may find it convenient for all students in the same class to do the same practical to make it easier to organise the resources required for the practical session. Teachers could provide students with a selection of different briefs so that students can chose the one that they find the most interesting. Teachers could build up a bank of case studies over time for this purpose.

Case studies require a briefing paper. This could include general information such as the use of the marking grid to ensure full coverage of all the assessment criteria, use of good English and the importance of working individually. Exemplars are included in this book.

The briefing paper for the case study should identify an aspect of physics that has a broad practical application. A specific application of this aspect of physics should not necessarily be given in the briefing document, as this is something that students could determine for themselves, providing greater scope for variety in their summaries. A statement such as “Many industrial situations require an accurate measurement of the refractive index of liquids and solids.” would be a sufficient introduction to set the scene for the work that is to be carried out. This statement shows the aspect of physics that is to be at the focus of the practical work (ie refractive index) has industrial applications although these are not specifically identified. The briefing document should then instruct students to research the application(s) of this aspect of physics and to explain how relevant physics principles are used. The document should also indicate the type of experiment they will be doing.

A copy of the briefing document must be included in work that is sent to Edexcel for moderation or marking.

If students in different groups do the practical assessment at different times then they should do different case studies to reduce the risk of collaboration. In particular, this is likely to apply to centres with large numbers of students.

Students may refer to material on websites but they should not merely copy large chunks of text into their own work; instead they should use their own words to convey their understanding of what they have read. However, short quotes may be used; the source of any quotes must be clearly identified in the text.


22

Some ideas for practical assessments

Some examples of possible practical assessments that relate to different learning outcomes are listed below. The list is not exhaustive – there are many other opportunities for incorporating practical assessments into the course.

Visits

• Theme park: Experiment involving conversion of potential energy and kinetic energy (learning outcome 53).

• Diggerland: Experiment involving the Young’s modulus of materials used (learning outcome

24). • Church: Experiment involving the length of organ pipe and frequency of note (learning

outcome 35). • Construction site: Experiment involving a property of a material used in construction or

safety clothing (learning outcome 26). • Local garage: Experiment involving viscosity of oil or the properties of materials used in a

car (learning outcome 21). • Concert hall: Experiment involving the length or tension of guitar string and frequency of

note (learning outcome 35). • Food manufacturers: Experiment involving a property of a material used in food production

(learning outcome 26).

Case studies

Case studies may be based on all the suggestions above. Further suggestions for case studies include: • Fishing rods: Experiment involving Young’s modulus (learning outcome 24). • Cameras: Experiment involving the focal length of lenses (although lenses are not mentioned

on the specification, this does not prohibit students from using them). • Historic development of cells: Experiment to determine the emf of a lemon cell (learning

outcome 59). • Crashes: Experiment investigating the crumple zone in a car (learning outcome 16). • Solar cells: Experiment on efficiency of energy conversion (learning outcome 70). • Lifts: Experiment on the efficiency of an electric motor when raising different weights

(learning outcome 53).


23

Conducting the AS assessment

The summary of the case study or visit

Students may produce the summary of the case study or visit either in class or at home; this part of the assessment need not be conducted under supervised conditions.

Students should complete the summary of the case study or visit report before they produce their individual plans since the summary should be used as the basis of the practical work..

Teachers should collect in the summary of the case study or visit from students and return them when they are producing their plan, doing their experiment and analysing their results under supervised conditions.

The plan

If students in different classes will be doing the same practical experiment then all students should produce their plan for the experiment before any students carry out practical work. This will ensure that students in some classes will not produce plans that are informed by practical experience gained by students in other classes.

Students should be given, in advance, a brief description of the experiment that they will be planning and its title so that they can review the physics that may be needed. The experiment must have a clear relationship to the case study or visit.

Students should be able to produce the plan for the practical work in one normal practical session.

The plan must be produced under supervised conditions to ensure that students do not help each other. Students should be advised that they will need to ensure that the practical work that they are planning can be completed in one normal practical session; they will need to gain sufficient practical experience throughout the course to judge the timing of practical work. It may be helpful to give them some planning exercises for practice before they each produce their own plan for the unit 3 assessment.

Teachers should return the summary of the case study or visit to students, issue a copy of the assessment criteria and issue a copy of briefing documents at the start of the practical session; students may not bring their own copies to the session as there is a risk that students may annotate these. Teachers may provide students with any formula that are needed during the session without penalty.

The teacher should collect the plans and summaries of the case study or visit at the end of the planning session. Plans must be checked for health and safety issues before the students carry out the practical aspect of this assessment. The student may have identified health and safety issues and provided comments on how to deal with these in their plans. However, if a student has not identified a relevant health and safety issue, then the teacher should raise this issue with the student before beginning any practical work and the student will lose the mark for P10: Comments on safety.

Carrying out the practical work

Students must carry out the practical work individually under supervised conditions.

It is advisable to have spare parts available, particularly for vulnerable components.


24

It should be possible for students to set up their equipment and record all necessary measurements in one normal practical session. If it is not possible to complete the practical in one session the n the teacher may decide to use the following session to complete the practical.

The unmarked plan and summary of the case study or visit should be returned to students at the beginning of the lesson. Teachers may give students a copy of the assessment criteria (marking grids) from the specification and briefing documents at the start of the session; students must not bring their own copies of any documents to the session to prevent them from accessing annotated versions that they may produce. Teachers may provide students with any formula that are needed during the session without penalty.

Teachers should remind students of health and safety issues before they begin the practical work and advise students to have, for example, electrical circuits checked before the power is switched on. Relevant warnings should be given, eg warning students that a component may get very hot during the course of the experiment.

Students must work individually.

Teachers must collect in all the work that the student has produced at the end of the lesson including the summary of the case study or visit.

Providing guidance to students during the practical session

The specification states that “Teachers may provide guidance to students without penalty. Guidance is feedback that a teacher might reasonably be expected to give to a student who asks questions about the work that they are carrying out. In effect, the teacher is being used as a resource.” For example, the student may ask the teacher to check whether apparatus has been set up correctly if the apparatus does not appear to be working correctly. For example, a student carrying out an experiment using an electrical circuit might sensibly ask the teacher whether the circuit is correct before switching on the power supply. The teacher should check the circuit and tell the student if it is incorrect. The error still needs to be identified and corrected by the student and this advice would carry no penalty. If however after several attempts the teacher feels the error needs to be explained and corrected then this should be noted clearly on the Candidate Record Sheet.

The specification continues: “Students may require assistance whereby the teacher needs to tell the student what they have to do. Assistance in this respect carries a penalty. The teacher should record details of any assistance provided on the report.” It may be necessary to tell a student how to connect up a circuit so that they can carry out the experiment and record some measurements. In this situation, students will be penalised. If the teacher has to explain how to use an instrument, eg micrometer, then the help given should be recorded and the student should lose the mark for P4: States how to measure a second relevant quantity using the most appropriate instrument. However, if the student provides a satisfactory reason for the choice of this measuring instrument they will not lose the mark for P5: Explains the choice of the second measuring instrument with reference to the scale of the instrument as appropriate and/or the number of measurements to be taken.

Carrying out the analysis

The analysis may be carried out in a separate lesson under supervision.

At the beginning of the lesson, teachers should return the summary of the case study or visit and other work that students produced for the experiment. Teachers may also give students a copy of the assessment criteria, briefing documents and formula that may be needed.


25

Working individually under supervised conditions, students should analyse their results and write up their conclusions. Teachers must not assist students with the analysis or presentation of their results, or provide any hints about possible conclusions.

At the end of the session the teacher should collect in all the documents that students have in their possession.

Returning work

Teachers must not return work to students to improve. However, students may do more than one case study or visit. Their best piece of work should be submitted to Edexcel for assessment purposes.


26

Exemplar of assessed work: Refractometry

Introduction In this refractometry example a case study topic was given (see student brief on the next page) to the student. The students were expected to research the topic and present a report. The experiment which followed modelled one of the two refractometry techniques that the student had researched. The student was expected to establish a clear link between the experiment and case study. The same experiment could have been based on a visit to a food manufacturing company. The practical which follows was planned in approximately 45 minutes. The student then carried out the practical, took measurements, produced a graph for the results and discussed conclusions within a one and a half hour lesson. The practical report was originally hand written.

Important note

This report and all the other exemplars have been word processed for ease of reproduction in this book and in this respect they do NOT exemplify actual reports as students are required to hand-write their experiment reports.

Specification links

Unit 2 Physics at Work

Concept-led approach: Topic 2 Waves, outcomes 36, 37 and 38

Context-led approach: Chapter 1 The sound of music, outcomes 36, 37 and 38


27

Briefing sheet

The following briefing explains what you must do for the assessment for unit 3. You should refer to the marking grid for this unit to ensure that you cover all the requirements of this assessment. Remember that some marks are awarded for the use of clear English. You should work by yourself for this assessment. Background Many industrial situations require an accurate measurement of the refractive index of liquids and solids. Two main types of refractometers are used for measuring refractive index – the Abbe refractometer and the Pulfrich refractometer. For this assessment, you are going to identify applications that require an accurate measurement of refractive index, describe each type of refractometer, explain how they operate using relevant principles of physics, and find the concentration of a sugar solution by measuring its refractive index. What you should do

1. Outline applications that require the accurate measurement of refractive index. 2. Describe and explain the principles of:

(a) an Abbe refractometer (b) a Pulfrich refractometer.

You will be planning an experiment that uses one of these principles to measure the refractive index of a sugar solution. The title of the experiment is: using refractive index to determine the concentration of a sugar solution.


28

Case study on refractometry

Refractometry can be used to find the refractive index of a particular liquid. This is useful because knowing its refractive index should enable us to

• Identify the liquid if it is unknown (this could be useful as a forensic aid)

• Check whether it is a pure sample ie not polluted in any way (for instance checking a water supply)

• Check the concentration of something which has dissolved in the liquid (for instance checking how much sugar is in a fruit juice)

Using refraction provides manufacturers with a fast method to measure the concentration of a sugar solution. The diagram below shows the main components of one type (Abbe) of refractometer. This type relies solely on refraction. Light passes into a sample of the liquid from the illuminating prism. (Source:http://www2.ups.edu/faculty/hanson/labtechniques/refractometry/theory.htm) Light is refraction as it passes the boundary between the illuminating prism and the sample. Light is further refracted as it passes between the sample and the refracting prism. Look at ray AB. It is refracted BC. This is the widest angle of light so anything further to the right of C will not have any light and will be dark.

Sample

A

Light Dark

Refracting Prism

θi

θr

B

Illuminating Prism

C


29

The longer the dark strip the greater the refraction at B. This gives an indication of the refractive index between the sample and the refracting prism. The more the refraction the bigger the difference in speed of the light between the two media. The larger the refraction the less the speed of light is in the sample and therefore the smaller the refractive index between the sample and air. The other type of refractometer (Pulfrich) relies on total internal reflection. A liquid sample is placed along the side of a prism.

(source: Edexcel Physics examination paper psa2 January 2001) A light beam is introduced into the prism. It internally reflects off the first surface between glass and air providing the angle is bigger than the critical angle ie about 42 degrees. At the top (second) surface some of the light will totally internally reflect and some will refract because the critical angle is larger than with air - typically 66 degrees between glass and liquid. This is because the refractive index (n) is closer to 1. This is because the speed of light in liquid is closer to the speed of light in


30

glass. Finally critical angle = sin -1 1/n , as the fraction 1/n will typically be in the range between 0.9 and 1 then the critical angle will be large compared to glass-air. Any light more than the critical angle totally internally reflects and arrives at the scale. So the more dense the liquid – the closer in speed the speeds of light in glass and liquid, the ratio is nearer to 1, the greater the critical angle, the less light arrives at the scale. Sources:

1. http://en.wikipedia.org/wiki/Refractometry 2. Physics Review Volume 15 No 1 Article on refraction pages19/20 3. AS Salters Horners Advanced Physics (ISBN: 0435 628 909) Heinemann

pages 257/259


31

Experiment to determine an unknown Sugar Concentration I am going to use the Pulfrich method. This experiment uses the principle of the pulfrich refractometer to determine the critical angle at the interface between a prism and liquid. I first have to calibrate my equipment – that is obtain a calibration graph. I am going to use a semi-circular prism. Ray box. Protractor. Sugar paper and various strips of different known concentrates of sugar/water to calibrate my equipment. Method I will arrange the apparatus as shown. I can use a white board and pens which wipe clean after use on which to draw the outline of the prism and the path of the light rays. I can soak the sugar paper in the liquid then stick it to the edge of the prism as shown.

I will determine the critical angle by rotating the prism until the ray totally internally reflects. I will measure the critical angle c – shown with the protractor – this is the dependent variable. A protractor can measure to the nearest degree and as I am expecting to measure angles of about 60o this should give an appropriate level of accuracy. I will then calculate refractive index n using n = 1/sin c. I will start with pure water. Then I will make up some different sugar concentrations. I will do this by mass and express it as a percentage. For instance a 50% solution will be made up of 100 grams of water and 50 grams of sugar. I can use a top-pan balance to measure the masses. I will start by measuring the mass of an empty beaker. The top pan balance I am going to use will measure to the nearest 0.1 gram which is an appropriate level of accuracy for the typical masses (50 grams) I am going to measure out.

Incident ray Reflected ray

Sugar paper

Normal


32

Repeat for several different sugar solutions – this is the independent variable. I will plot a graph of refractive index vs sugar concentration. I have asked the technician to make up an unknown concentration for me. I will then find c for the unknown solution. Calculate n. Then use the graph to find concentration. I expect to be able to measure an angle to the nearest degree, I will repeat the rotation of the block and retake the angle a second time to check. I will look vertically down onto protractor and lines to avoid parallax errors. I will do the experiment in as dark a laboratory as possible. Safety Ray boxes can get hot – take care not to burn fingers etc. I may use the razor kit rather than the ray box which uses a small laser. Lasers: do not look directly at these. Ensure always flat on bench for other peoples sake. Results Solution Concentration / %

Critical Angle / degrees

Critical Angle/ degrees

Refractive Index

0 63 63 1.12

15 65 64 1.10

30 67 67 1.09

45 70 70 1.06

60 74 74 1.04

Unknown 69 69

The refractive index of the unknown solution is 1.07 . Note that the angle is measured to 2 sf. Refractive index to 3 sf. If rounded off too much then graph loses sensitivity.


33

Conclusions The refractive index is linearly related to % concentration of Sugar. The gradient of the graph is – 0.08 /60 = 1.33 x 10-3 So the equation of the line is n = - 1.3 x 10-3 . concentration + 1.12 over this range of concentrations. The points do not lie perfectly on the straight line. This represents lack of uncertainty in the experiment. I measured the angles to the nearest degree. The change in critical angle was only a few degrees so this represents quite a wide range of inaccuracy. The accuracy could be improved by using a larger protractor to measure the angle.


34

My unknown concentration has a refractive index of 1.07 – from my graph this has a concentration of 39%. The technician told me afterwards that she made it 40 %. My line of best fit may not have been drawn in the right place. My points are a little scattered due to the reasons given above and this means my straight line is possibly not quite in the right place but my result is quite close.


35

AS Marking grid for case study on refractometry

A: Summary of case study or physics-based visit Ref Criterion Mark Ref Criterion

Mark

S1 Carries out a visit OR uses library, consulting a minimum of three different sources of information (eg books/websites/journals/magazines/case study provided by Edexcel/manufacturers’ data sheets)

1

S2 States details of visit venue OR provides full details of sources of information

1

S3 Provides a brief description of the visit OR case study

1

S4 Makes correct statement on relevant physics principles

1

S5 Uses relevant specialist terminology correctly

1

S6 Provides one piece of relevant information (eg data, graph, diagram) that is not mentioned in the briefing papers for the visit or case study

1

S7 Briefly discusses context (eg social/environmental/historical)

0

S8 Comments on implication of physics (eg benefits/risks)

1

S9 Explains how the practical relates to the visit or case study

0

Marks for this section

7


36

B: Planning Ref Criterion

Mark

P1 Lists all materials required

1

P2 States how to measure one relevant quantity using the most appropriate instrument

1

P3 Explains the choice of the measuring instrument with reference to the scale of the instrument as appropriate and/or the number of measurements to be taken

1

P4 States how to measure a second relevant quantity using the most appropriate instrument

1

P5 Explains the choice of the second measuring instrument with reference to the scale of the instrument as appropriate and/or the number of measurements to be taken

1

P6 Demonstrates knowledge of correct measuring techniques

0

P7 States which is the independent and which is the dependent variable

1

P8 Identifies and states how to control all other relevant variables to make it a fair test

0

P9 Comments on whether repeat readings are appropriate in this case

1

P10 Comments on safety

1

P11 Discusses how the data collected will be used

1

P12 Identifies the main sources of uncertainty and/or systematic error

0

P13 Draws an appropriately labelled diagram of the apparatus to be used

1

P14 Plan is well organised and methodical, using an appropriately sequenced step-by-step procedure

1


11

C: Implementation and Measurements Ref Criterion

Mark

M1 Records all measurements using the correct number of significant figures, tabulating measurements where appropriate

1

M2 Uses correct units throughout

1

M3 Obtains an appropriate number of measurements

0

M4 Obtains measurements over an appropriate range

1


3


37

D: Analysis

E: Report Ref Criterion

Mark

R1 Summary contains few grammatical or spelling errors

1

R2 Summary is structured using appropriate subheadings

1


2

Total marks for this unit

32

Ref Criterion

Mark

A1 Produces a graph with appropriately labelled axes and with correct units

1

A2 Produces a graph with sensible scales

1

A3 Plots points accurately

1

A4 Draws line of best fit (either a straight line or a smooth curve)

1

A5 Comments on the trend/pattern obtained

0

A6 Derives relation between two variables or determines constant

1

A7 Discusses/uses related physics principles

1

A8 Attempts to qualitatively consider sources of error

1

A9 Suggests realistic modifications to reduce error/improve experiment

1

A10 Calculates uncertainties

0

A11 Provides a final conclusion

1


9


38

Examiner’s comments on refractometry

A: Summary of case study The student has provided five different sources, including one text book, two web sites and one journal. The two sources from which the diagrams were obtained are also identified. The description of the application is clear and the physics is reasonably well explained. There are two relevant diagrams to illustrate both techniques and the context/purpose is discussed. The report indicates various uses of this technique in industry and states the benefit that it provides a fast method for measuring the concentration of a sugar solution, therefore the mark for S8: comments on implication of physics (eg benefits/risks) was given. How the practical relates to the case study is stated at the start of the practical write-up. Although the student has listed three applications of refractometry in the summary, the student should have linked the experiment to one of these applications and therefore the mark for S9: Explains how the practical relates to the visit or case study was not awarded.

B: Planning The equipment is listed. Measuring instruments for the two variables are stated and which is dependent etc. There is a comment about repeat readings and safety. The student doesn’t appreciate that by measuring the angle between the incident and reflected ray and then dividing by two a more accurate value of the critical angle C can be determined and therefore no mark were awarded for P6: Demonstrates knowledge of correct measuring techniques. The mark for P8: Identifies and states how to control all other relevant variables to make it a fair test is not awarded as the student does not mention control of other variables. The student has repeated readings as shown in the table of results and therefore the mark for P9: Comments on whether repeat readings are appropriate in this case is awarded. There is no real attempt to discuss the main source of uncertainty. The student doesn’t appreciate that the range of differences in angles is going to be quite small losing the mark for P12: Identifies the main sources of uncertainty and/or systematic error = 0. The apparatus set-up is drawn out. Although the student has not included the ray box or labelled the incident/reflected rays, the diagram can be understood and, importantly, the quantity to be measured (critical angle) has been clearly marked and therefore the mark for P13: draws an appropriately labelled diagram of the apparatus to be used is awarded. The method is clear. The plan explains how the results will be used to calculate refractive index and the graph that will be plotted.

C: Implementation and measurements Correct units and measurements have been used but there are only 5 sets of results; the student could easily have made up an extra concentration to give six points on the graph so M3: Obtains an appropriate number of measurements did not receive a mark but the range is satisfactory and therefore M4: Obtains measurements over an appropriate range scored a mark.

D: Analysis The graph is sensibly scaled. The points are correct and the best line fit seems appropriate. Although the student has obtained a straight line graph there is no comment on the trend obtained eg as the sugar concentration increased, the refractive index decreased so the mark for A5: Comments on the trend/pattern obtained is zero. The equation relating refractive index and concentration is derived using the graph. The student has used physics principles – the relation between n and critical angle – and therefore scores the mark for A7: discusses/uses related physics principles. The student has considered sources of error, suggested an improvement but has not quantified the sources of error and therefore the mark for A10: Calculates uncertainties


39

could not be awarded. There is a conclusion in the form of an equation and the concentration of the unknown solution has been identified so the student gains the mark for A11: Provides a final conclusion. The student has included the minus sign with the gradient when stating the equation (this was omitted when calculating the gradient) so the student has not been penalised for omitting it in the calculation.

E: Report The report had few spelling or grammar errors, although the presentation could be improved. It was understandable and sensibly organised.


40

Exemplar of assessed work: Geophysics

Introduction The suggested case study and visit address the same topics, and lead to similar practical work suggestions. Although the student exemplar is from a visit, a very similar report and practical could result from the case study (see appendix 1 for a more detailed briefing sheet that could be issued to students).

Visit to an archaeological site, museum with geophysical links, university or local

authority department.

A visit to one of the above should provide opportunities to explore geophysical techniques in practice. Students could note the importance of geophysical surveys for planning decisions as well as archaeological purposes.

Suggestions for practical work.

Any standard method could be used by students to determine the resistivity of a given wire. The brief could be linked to the visit by asking students to use the resistivity to identify the material of the wire. In this case a copy of a table of metal resistivities should be provided for each student, as consulting textbooks in the laboratory during the planning and implementation of the experiment is not permitted.

Specification links


Concept-led approach: Topic 2 DC Electricity, outcome 57

Context-led approach: Chapter 3 Digging up the Past, outcome 57


41

Visit report for Geophysics Visit summary report

We went to visit a museum of archaeology attached to a local university. While at the museum we had a talk on the latest geophysical survey of nearby Roman sites and a demonstration of metal detection. The main speaker came from the local university’s Department of Archaeology which has an archaeological services section. Planning applications are now checked by local authorities as archaeology is now an integral part of the planning process, and planning applications can be refused if there is any doubt that archaeological implications have not been taken into account. For this summary I am going to explain resistance surveying and metal detecting, although we saw other applications of physics including ground penetrating radar and magnetometry.

Geophysical surveying

The first stage in archaeological surveys is a site walk, although sometimes there is evidence from aerial photographs too, sometimes nowadays from Google Earth. On a photograph you can see dark lines and circles which suggest building foundations or pits. The next stage is often a magnetometry survey, which looks at changes in the direction of the magnetic field which can be produced for example by forges or iron working. At two of the sites we heard about there had been a ‘Time team’ investigation and a picture of one of these is shown below.3 [Photograph of the time team removed for copyright reasons.] The resistivity survey uses the idea of measuring the resistance between two electrodes stuck into the ground. In 1916 Frank Wenner started using four electrodes4, as shown in the diagram below. Two electrodes, a fixed distance apart, are connected to the supply and a current is passed through them. To avoid polarization an alternating supply is used. Another two probes are pushed into the soil and connected to a voltmeter: the voltmeter readings are built up into a map of the resistance of the ground.

3http://www.channel4.com/history/microsites/T/timeteam/episode_guides/pastprogs/index.html Accessed 5/8/8 4 Science Education Group, University of York (2000) Salters Horners Advanced Physics:

Student Book AS Level (Salters Horners Advanced Physics) Oxford, Heinemann


42

Multi-probe resistance surveying

Ditches are often wet so they have a low resistance. Stone foundations are more usually of high resistance.

Theory

If the original current is known, as well as the distance the electrodes go into the earth, their separation and their width, then as V = IR, the resistivity of the ground can be calculated from ρ = RA/l.

Advantages

Geophysical surveying is non-destructive, so saves time, money and damage to any remains. Although techniques haven’t changed very much in recent years, the developments in computing have made geophysical surveying much easier and quicker. So much so that there are now community ‘digs’ which even involve children.

Limitations

The resistivity of soil varies according to how wet the soil is, so it is important that surveys are done on the same day. Geophysical resistivity plots can’t sort out changes over time and can’t survey under tarmac, although ground penetrating radar can. Interpretation of results needs an expert!

Metal detection

Metal detection is often a hobby, but sometimes the finds can alert archaeologists to sites that were previously unknown, as happened at one of the sites that the ‘Time team’ visited in the county. We saw a demonstration of metal detecting. A metal detector involves magnetic coils, induction and

v

Power supply

probes

Electrodes

air

soil


43

oscillations. An alternating current is passed into a coil which induces a magnetic field. If this is disturbed by passing over a piece of metal a change in tone is heard by the detectorist. [Illustration removed for copyright reasons.]5

Economic and environmental factors Planning implications have already been mentioned but there are more reasons for using physics. The Treasure Act 1996 and Treasure (Designation) Order 2002 says that ‘All coins from the same find provided they are at least 300 years old when found (but if the coins contain less than 10 per cent of gold or silver there must be at least ten of them)’, and ‘Any metallic object, other than a coin, provided that at least 10 per cent by weight of metal is precious metal (that is, gold or silver) and that it is at least 300 years old when found’ will be counted as treasure trove.6 If you find any ‘treasure’ it has to be reported to the coroner of the district within 14 days, so it is important to know what metal your find is. The experiment we are going to do is to identify a metal by finding its resistivity. Resistivity is important both for metal detectorists and for people doing resistivity surveys of sites. The latest discoveries we were told about were said to have changed historians’ views of Roman sites in the county, so that now they know there were civilian as well as military sites. Word count 766 Other sources Briefing materials from the visit Clark, A. (1996) Seeing beneath the soil London, Routledge

5http://www.ukdetectornet.co.uk/andy1.htm Accessed 5/8/8 6 http://www.finds.org.uk/treasure/treasure_summary.php Accessed 5/8/8


44

Name that wire Plan After our visit we were asked to identify a metal by measuring its resistivity ρ. Resistivity is the resistance of a 1 m3 cube of the material measured between opposite faces. Resistivity is different for different materials, and unlike resistance is independent of the size of the sample.

ρ = RA/l R = resistance A = area = π r2

r = radius of wire l = length of wire Area will be constant. The resistance will change with length. The length will be changed as follows: 1.00 m to 0.10 m in 100 mm steps. I will measure current, keeping the p.d. constant at 6.00 V, using multimeters. I will then calculate resistance for each length. I will measure the diameter of the wire with the micrometer at three different places and orientations to ensure the diameter is uniform. From this I will calculate the radius, r, and then the cross-sectional area, A, of the wire using A = π r2. It is important to reduce the uncertainty in the measurement of diameter as much as possible as it is a small wire and any uncertainty will be doubled in the final calculation as the radius is squared to give the area which doubles the uncertainty. I will also need to measure the lengths of the wire. A metre rule will be suitable for this as it has sensitivity of 1 mm and for a length of 1.0 m, this will give a percentage uncertainty of about 1 %. Unfortunately the percentage uncertainty in length will rise as the lengths get shorter. I will draw a graph of resistance against length. Length is the independent variable and resistance is the dependent variable. Ρ = gradient x area. Once I have found resistivity I will check the table of resistivities I have been given to identify the metal. Ideally the experiment should be repeated to increase reliability but I don’t think I will have time. Apparatus The apparatus I will be using will be as follows. wire 2 multimeters power supply variable resistor micrometer screw gauge metre rule crocodile clips leads


45

Circuit diagram

Take care with electrics and do not allow any liquids near the area. Be careful not to short circuit the Wire, and be aware that the wire will get hot due to electric currents. Also be careful with wire cutters. Accuracy To improve the accuracy I will use the same ruler/meter scale/wires /etc throughout and assume that the contact resistance will be negligible. The ammeter and voltmeter are both digital and so will easily measure to 2 decimal places. The accuracy of such devices is generally good; however the final digits tend to flicker. Method

1. Cut the wire (1 m) and measure with the ruler 2. Set up the equipment shown in the circuit diagram, use the variable

resistor to keep the p.d. at 6.0 V. 3. Measure current and record this in the results table. 4. Using the wire cutters cut 10 cm off the wire and repeat the steps

above. 5. Calculate R. 6. Draw a graph as explained and take the gradient. 7. Calculate resistivity. 8. Retake any anomalous results if time permits.

A

V

Wire - unknown Crocodile clip

Variable resistor

Power supply


46

Results Diameter of wire: 0.45 mm, 0.44 mm, 0.46 mm average 0.45 mm Area = 1.59 x 10-7 m Length of wire/cm Potential

difference/V Current/A Resistance/Ω

100.00 6.0 1.43 4.20 90.00 6.0 1.62 3.70 80.00 6.0 1.76 3.40 70.00 6.0 1.94 3.10 60.00 6.0 2.22 2.70 50.00 6.0 2.50 2.40 40.00 6.0 2.86 2.10 30.00 6.0 3.16 1.90 20.00 6.0 3.87 1.55 10.00 6.0 5.00 1.20 The graph was a straight line but did not go through the origin. This is probably a systematic error caused by contact resistance.


47


48

Analysis Gradient of first line = 3.4 Alternative gradient of second line = 3.15 Resistivity = 1.59 x 10-7 x 3.4 Ωm = 54.1 x 10-8 Ωm From alternative gradient, resistivity = 50.1 x 10-8 Ωm So resistivity = 52 ± 4 x 10-8 Ωm allowing for uncertainties in the gradient. The values I have found are most similar to that of constantan which has a resistivity of 47 x 10-8 Ωm. (Gold and silver are 2.44 and 1.59 x 10-8 Ωm.) So if this was a find I would not have to report it. The graph I drew had its anomalous points retaken. Modifications If I was to do this experiment again I would look for away of reducing the contact resistance. Thinking about the experiment, I didn’t need to cut the wire but could have moved the crocodile clips along to get the required length. This would also have made repeats easier.


49

AS Marking grid for visit to an archaeological site


Mark


1


1


1


1


1


1


1


1


1


9


50


Mark


1


1


0


1


0


1


1


0


1


1


1


1


1


1


11


Mark


0


0


1


1


2


51

D: Analysis


Mark


1


1


2


34

Ref Criterion

Mark


1


1


1


1


0


1


1


1


1


1


1


10


52

Examiner’s comments - geophysics

General The visit report is over the suggested minimum length and mainly descriptive. The experiment is one for which a variety of methods could have been chosen: the student has not chosen the best method but has carried it out well. A Summary of physics-based visit The student has produced a report including appropriate subheadings. She has used the reference facility in Word to insert footnotes which makes her use of sources very clear. Although the report is mainly descriptive it does link physics clearly to the visit. The report makes reference to legal and historical implications of the work and comments on the link between the visit and practical. All marks have been awarded in this section. B Planning The student could usefully have commented more on the choice of meters. The student correctly chose to use a micrometer to measure the diameter of the wire so the mark for P2: States how to measure one relevant quantity using the most appropriate instrument is awarded but this decisions is not justified, eg she could have discussed the precision 0.01mm in relation to the typical diameter of a wire and therefore P3: Explains the choice of the measuring instrument with reference to the scale of the instrument as appropriate and/or the number of measurements to be taken was not awarded. The second quantity is length and the student correctly identifies a rule so the mark for P4: States how to measure a second relevant quantity using the most appropriate instrument was awarded. However, she refers to sensitivity when they meant precision and incorrectly calculates this as 1% so P5: Explains the choice of the second measuring instrument with reference to the scale of the instrument as appropriate and/or the number of measurements to be taken was not awarded. The mark for P8: Identifies and states how to control all other relevant variables to make it a fair test was not awarded as the student has not commented on the temperature of the wire and this could lead to incorrect conclusions. C Implementation and Measurements M1: Records all measurements using the correct number of significant figures, tabulating measurements where appropriate as not awarded as lengths are given to 0.01 cm which is not possible with a rule. The student has not given the correct unit for area. She has also not given units for the gradient of the graph, so M2: Uses correct units throughout has not been awarded. D Analysis The student has made a good attempt at quantitative uncertainties by drawing two alternative lines. The mark for A5: Comments on the trend/pattern obtained was not awarded because the student could have stated that the relationship between the resistance and length of the wire is linear. The mark for A8: Attempts to qualitatively consider sources of error was awarded as the student commented on the systematic error and identified its source.


53

E Report The student has included subheadings and the report has few spelling or grammatical errors so both marks in this section have been awarded.


54

Exemplar of assessed work: Focal length

The exemplar experiment is not based on a topic directly in the specification but is linked to specification material and is accessible for AS students who will probably have met lenses in Key stages 3 and 4. Although the student exemplar is from a visit, a very similar report and practical could result from a case study based on astronomy or optometry (see appendix 1 for a case study briefing sheet that could be issued to students).

Visit to an optician, astronomical society or observatory

Introduction

Both venues offer opportunities to see physics at work, for example in telescopes, prescribing spectacles and eye testing. The optician would provide opportunities to link to the medical applications of science, while an observatory would link to topics that will be studied in A2.

Suggestions for practical work

Experiments could be designed to determine the refractive index of glass or plastic. However the exemplar material looks at the related topic of focal length.

Specification links


Concept-led approach: Topic 1 Waves: 36 -39

Context-led approach: Chapter 1 The sound of music: 36-9


55

Visit report for an optician’s shop

A visit to an optician’s We went on a visit to an optician’s shop in the local high street. In the shop we met the optometrist who tests a person’s eyes and a technician who makes up the glasses. There is a lot of physics used in the process from optics in refraction to the properties of materials both for lenses and frames Although we learnt some interesting facts about materials and polarisation, I am going to concentrate on refraction in this report. The optometrist As well as finding the right power of lenses to correct some-one’s eyesight the optometrist also has other health related roles in checking for diabetes and high blood pressure, both of which can affect sight. By shining a parallel beam of light into the eye the optometrist can find whether a person is long or short-sighted. A long sighted eye will focus parallel beams of light behind the retina, and will need a convex (positive) lens to bring light to a focus nearer the front of the eye. This can be caused by having too weak a lens or too short an eyeball. A short sighted eye on the other hand will focus light before the retina because the eyeball is too long or the lens is too strong in this case a (negative) concave lens will be needed.

Concave lens for short sight


56

Sometimes eyes are not perfectly curved so the optometrist has to check for astigmatism and prescribe lenses which are cylindrical or toric to correct this. Theory The equation for the power of a lens P is P = 1/f, where f is the focal length of the eye. Glasses are usually thin lenses and the formula linking object distance

u, image distance v and focal length is: vuf111

+= . Power is measured in

dioptres (D) which are 1/m. Powers of lenses can be added together: concave lenses have negative powers and convex lenses have positive powers. So if a person’s eye should have a power of 50 D and has actually a power of 56 D they would need a convex lens of –6 D to correct that eye. This would be a concave lens and the person is short sighted. The technician The technician selects and cuts the lenses according to the prescription and frames chosen. I was surprised at how much of this was done by machine. We looked at titanium and memory metal frames and learnt how strong and flexible these can be. We also saw polarised lenses which are used in sunglasses to prevent glare. Only transverse waves can be polarised, that is vibrate in one plane rather than all planes. As light is a transverse wave it can be polarised and it is often polarised horizontally by reflection. So a polarising lens can be used to cut out the glare from light reflected from the sea or shiny roads. We were surprised that the tints on sunglasses were applied by ‘cooking’ the plain lenses in a hot dye.

Convex lens for long sight


57

Limitations It is not always possible to balance red and green vision, as red light and green light have different wavelengths, different wave speeds in different mediums and therefore different refractive indexes as =21μ (speed in medium 1)/(speed in medium 2). A test which is used to try and check this for people is to look at alternating red and green circles and ask which is clearer, but compromises have to be made. Economic and environmental factors Checking peoples’ health is very important, and ensuring that people can see as well as possible is a very useful contribution to their health. Word count 608 Sources Briefing materials from the visit Lens formula from Anning, P et al (2004) Revise AS Physics for Salters Horners, Oxford, Heinemann Red and green information from http://www.sgoeyecare.co.uk/fAQ.htm Wikipedia


58

An experiment to determine how different focal length and hence refractive index is for red and green light. Discussion After our visit we were asked to find how different are the focal lengths of red light and green light. This has implications for the design of lenses, as different focal lengths would mean that correcting spectacles would not necessarily work for different colours of the white light spectrum. The following equation will be used.

vuf111

+=

f = focal length u = distance from the light to the source v = distance from the light to the image One way to do this is to find the intercepts of the graph lines 1/u and 1/v. Both intercepts are 1/(focal length).

We will be controlling distance u (the independent variable). We will increase it from 20-50 cm in 2 cm steps. After recording this measurement, we will move the screen until the light is focused. Then distance v will be recorded. The results in the result table will be calculated as 1/u or 1/v, than a graph will be drawn. The intercepts will give 1/f. The variables involved in the experiment are as follows: distance u, distance v and the lens size/strength. The distance u is the distance we are changing, distance v is being measured and the lens we use shall remain the same throughout.

u v

lens screen Light

source


59

Here is an example of the graph I will draw.

Apparatus The apparatus I will be using will be as follows. Red LED Green LED Clamp and stand Metre rule Convex lens White card screen and stand Lens holder Connecting wires Electricity supply Try to keep experiment area darkened. Red and green light are in the same area of the electromagnetic spectrum. This means that they have almost the same wavelength. However white light is split in a prism as refractive index is different for different wavelengths. Red light is not refracted as much as green light. Safety The LEDs may get hot and working with electricity needs special care. No liquids in the experiment area.

1/v

1/u

Intercept or 1/focal length

Graph line


60

Diagram

Accuracy All measuring will be done as accurately as possible. Apparatus which may cause slight errors is listed. metre rule: +/- 0.5 mm

lens: this depends on the sharpness of the imaging which will be as constant as possible

Method 1. Collect and set up equipment (see diagram). 2. Turn on the power supply, make sure the LED, lens and screen are all in line and the lens and LED are roughly half way up the screen. 3. Distance u is being controlled. Set this to 20 cm, then move the screen and lens until there is a red or green dot (depending on which colour LED you are using) on the screen. Focus the dot by moving the screen, make the image as sharp as possible. 4. Using the metre rule, measure the distance marked v and record it, along with the distance u (20 cm). 5. Repeat this for the other coloured LED. 6. Take the results for each LED every 2 cm (20 cm, 22 cm 24 cm, etc) up to 50 cm. 7. Turn off the LED. 8. Work out 1/u and 1/v for each result and record this alongside the other results. 9. Draw a graph of 1/u against 1/v. 1/u should be on the x axis. 10. Retake any anomalous results and take results for any gaps in the graph. 11. Put away equipment making sure all sockets are turned off.

u v

lens

screen

Lens holder

Metre rule Clamp and stand

Power supply

LEDs

mains


61

12. Draw graph lines for red and green light. Take the intercept for each and record this as 1/(focal length). 13. Work out the focal length as intercept-1. Results Red light u (cm) V (cm) 1/u (1/cm) 1/v (1/cm) 20.0 46.5 0.050 0.022 22.0 43.0 0.045 0.023 24.0 40.0 0.042 0.025 26.0 36.0 0.039 0.028 28.0 32.0 0.036 0.031 30.0 30.0 0.033 0.033 32.0 27.5 0.031 0.036 34.0 26.0 0.029 0.039 36.0 26.0 0.028 0.039 38.0 25.5 0.026 0.040 40.0 25.5 0.025 0.039 42.0 23.5 0.024 0.043 44.0 23.0 0.023 0.043 46.0 22.0 0.022 0.045 48.0 22.5 0.020 0.044 50.0 22.0 0.020 0.045 Green light u (cm) v (cm) 1/u (1/cm) 1/v (1/cm) 20.0 47.5 0.050 0.021 22.0 43.0 0.045 0.023 24.0 40.0 0.042 0.025 26.0 36.0 0.039 0.029 28.0 32.0 0.036 0.030 30.0 30.0 0.033 0.032 32.0 27.5 0.031 0.036 34.0 26.0 0.029 0.039 36.0 26.0 0.028 0.036 38.0 25.0 0.026 0.039 40.0 25.5 0.025 0.040 42.0 23.5 0.024 0.041 44.0 23.0 0.023 0.042 46.0 22.0 0.022 0.044 48.0 22.5 0.021 0.045 50.0 22.0 0.020 0.044


62

Error boxes As shown in the ‘accuracy’ part of the plan, the closest I can read the ruler is to 0.5 mm. this means error boxes could be drawn on the graph. They will be calculated as follows. Error = 0.5 mm (0.05 cm) Example: 46.5 could be (A) 46.55 or (B) 46.45


63

The boxes would be extremely small and make little or no difference to the graph line. Therefore there is little point putting them on. Anomalous results There are few anomalous results on my graph. The ones which are slightly off the graph line could have been caused by an error when reading the ruler, or not measuring from exactly the middle of the lens. The ruler may not have been straight and the focusing may not have been as good as it could have been. I have circled 5 anomalous results on the graph - 2 red, 3 green – out of a total of 32 results. I think these results can be discarded as a good graph line can be drawn with the remaining 27 results. If I had more time I could retake the anomalous results several times and take an average for them and record these on to the graph. This may give a better and more accurate graph line. Red focal length 15.39 cm Green focal length 15.87 cm Difference 0.48 cm This difference is very small, and taking into account that the experiment error is quite large (relatively), it would not be wrong to say that there is no difference. Evaluation In general, this experiment went well. The only anomalous results have been explained and there were no problems on the practical side, except the focussing of the light which was done to the best of my ability, although it might not have been accurate enough or constant. I think that is the problem (along with the ruler uncertainty) which could have contaminated the results. If I could do this experiment again, I would use equipment which was more accurate and fix the light source to the bench so it could not wobble or move. I would use colours at the opposite ends of the spectrum (red and violet) so that any difference in focal length caused by wavelength would be larger and more noticeable. I would compare the red/violet difference. If the two differences were similar (within +/- 0.25 cm of each other), then I could state that there is no significant difference between the focal lengths of any colour light. Any larger difference between the two ‘differences’ would mean that there is a difference between the lengths of different colours of light and this could be investigated further. Conclusions From my graphs I can see that there is a difference between the focal length of red and green light and therefore in the refractive indexes. However this difference is extremely small (0.48 cm), and more or less unnoticeable, although as explained in the visit summary optometrists do try to allow for the differences. To determine whether or not these results are entirely accurate


64

another experiment would have to be done using more accurate equipment. Fixing the light source (on the clamp and stand) to the bench may help, as it would not wobble or move. (Other modifications can be found in the evaluation above.) So I don’t think that there is any great problem making spectacles from glass like this as the wearer would be unlikely to notice any difference between red and green parts of the image.


65

AS Marking grid for visit to an optician’s shop


Mark


1


1


1


1


1


1


0


0


1


7


66


Mark


1


1


0


0


0


0


1


0


0


1


1


0


1


1


7


Mark


1


1


1


1


4


67

D: Analysis


Mark


1


1


2


28

Ref Criterion

Mark


1


0


1


1


0


1


0


1


1


0


1


8


68

Examiner’s comments for a visit to an optician’s shop

General The visit report is just the recommended length. The experiment is very detailed in parts, but also repetitive and misses some important elements in planning. The student has used the word ‘we’ in places but it is clear both from the set of work submitted and the signed declarations that the student has worked independently as required by the specification. A Summary of physics-based visit The student has produced a report including appropriate subheadings. The student has only given the home URL and not the page used on Wikipedia, nor has the date on which the website was accessed been given but three sources have been used as required for S1: Carries out a visit OR uses library, consulting a minimum of three different sources of information (eg books/websites/journals/magazines/case study provided by Edexcel/manufacturers’ data sheets). This is a visit and details of the visit have been given, so S2: States details of the visit venue OR provides full details of sources of information has been awarded. S6: Provides one piece of relevant information (eg data, graph, diagram) that is not mentioned in the briefing papers for the visit or case study was awarded for the diagrams of the eye as these were not included in the original briefing materials. The word count has forced the student to limit the visit report to only one physics application but correct statements have been made about the relevant physics principles for the application chosen, so the mark for S4: Makes correct statement of all relevant physics principles has been awarded. Although some attempt has been made to discuss the context and implication of the physics involved neither is strong, so neither S7 or S8 have been awarded. The student has not commented on the link between the visit and the practical in the summary report, however this is mentioned in the practical plan so S9: Explains how the practical relates to the visit or case study has been awarded. B Planning There is a very clear explanation of the proposed data treatment. Although the student discusses the use of the metre rule, no mention is made of why this is the most suitable instrument so P3: Explains the choice of the measuring instrument with reference to the scale of the instrument as appropriate and/or the number of measurements to be taken has not been awarded. The experiment requires two measurements of length using a rule and therefore a second measuring instrument cannot be used. Consequently P4: States how to measure a second relevant quantity using the most appropriate instrument and P5: Explains the choice of the second measuring instrument with reference to the scale of the instrument as appropriate and/or the number of measurements to be taken have not been awarded. The student explains which is the independent variable and which the dependent although he does not use both words explicitly, so P7: States which is the independent and which is the dependent variable has been awarded.


69

P6: Demonstrates knowledge of correct measuring techniques has not been awarded as the student has not discussed the exact measurements to be taken, eg measuring from the centre of the lens. P8: Identifies and states how to control all other relevant variables to make it a fair test has not been awarded as the student could have commented on the alignment of the lens remaining consistent throughout the experiment. No comment has been made on whether repeated measurements are appropriate so P9: Comments on whether repeat readings are appropriate in this case has not been awarded. Although the student has incorrectly stated that the LEDs will get hot the mark for P10: Comments on safety is awarded for the other relevant comments. The mark for P12: Identifies the main sources of uncertainty and/or systematic error is not awarded as the student did not comment about the subjective aspect of obtaining a focussed image. C Implementation and Measurements All marks have been awarded in this section. D Analysis The student’s graph is of the minimum size acceptable. However, he was looking for a very small difference in gradient so a larger scale would have been better, and A2: Produces a graph with sensible scales has not been awarded. There is no comment on the trend obtained so A5: Comments on the trend/pattern obtained has not been awarded. A7: Discusses/uses related physics principles has not been awarded as the student did not explain why a graph of 1/u against 1/v gives a straight line with gradient -1 and both intercepts 1/f. The relationship with y=mx+c should have been discussed. The mark for A8: Attempts to qualitatively consider sources of error was awarded as the student offered one source of error. However, the student did not consider the main source of error – the subjective focussing of the image. The mark, A10: Calculates uncertainties, has not been awarded as there is no attempt to do this in the final results, although here is a qualitative comment so A8: Attempts to qualitatively consider sources of error has been awarded. E Report The student has included subheadings and the report has few spelling or grammatical errors so both marks in this section have been awarded. It is good to see that the student has referred to the visit in their final conclusion.


70

Exemplar of assessed work: Solar cells

Introduction The exemplar experiment is on a topic linked closely to the AS specification. The students were directed to find the internal resistance of a solar cell. Although the report is based around a visit the topic could easily be covered by a case study on alternative energy (see appendix 1 for a case study briefing sheet that could be issued to students). Visit to the Science museum to see a solar car or a site using alternative energy Both venues would offer opportunities to see physics at work and link to environmental issues. Suggestions for practical work Experiments could be designed to determine the relationship between distance from the light source and output from the cell as an alternative to the internal resistance determination used here. Specification links Unit 2 Physics at Work Concept-led approach: Topic 4 DC electricity 52, 59, Topic 5 Nature of light 72 Context-led approach: Chapter 2 Technology in space 52, 59, 72


71

Visit report for solar cells

Exemplar: visit and practical - Science museum Visit summary report In February we visited the Science Museum in London mainly to see the Durham University Solar Car (DUSC). The car is being built by a group of engineering students at the university ‘on a shoestring’, although they have got some sponsorship from local companies and the local branch of the Institute of Physics. The team building the car are going to compete in July 2008 in the North American Solar Challenge: a 2,400-mile race from Dallas to Calgary. The car [Photograph of car removed for copyright reasons.] The car has three wheels and is covered in 9 m2 of solar cells. The solar cells are supplied by RWE Schott Solar/Carl-Zeiss and are high efficiency silicon units. The cells are connected to an electric motor in the rear wheel via lead-acid batteries giving 5 kWh of battery storage. There are no gears but the car accelerates very quickly to a top speed of 62 mph. The car can travel for up to five hours when the batteries are fully charged, which even in the UK only takes nine hours. However when the car is working in sunny conditions it is being recharged and also the brakes are regenerative which means that energy is generated while the car is braking or freewheeling. It has no air conditioning and the driver can get very hot.


72

Solar cell theory

Solar cells are junction diodes usually made from silicon which work by using charged particles to transfer energy from incoming light radiation to an external circuit. The silicon has different impurities introduced on each side of the junction. The front of the cell has to be transparent to allow light to pass through to the material underneath. The top surface is coated with an anti-reflection coating so that as much light as possible is absorbed. Electrons naturally drift to one side of the junction so there is an excess of electrons on one side and a shortage of electrons on the other, giving

Absence of electrons

Excess electrons

- - - - - - - - - - -

Positive charge

Negative charge

Two different materials

R

Electrons

Electrons

Light


73

negatively and positively charged sides. When light falls on the cell the electrons get enough energy to move, either back across the junction or more usefully around an external circuit.

To get the maximum power from the cells it is important that the external load is matched to the internal resistance of the cell. However maximum power does not always mean maximum efficiency. The cells used by DUSC are 16% efficient. We are going to find the internal resistance of a solar cell for our practical. Economic and environmental factors Solar cells are being developed not just for cars like the one we saw but also for industrial and domestic use and also for powering satellites in space. Domestic uses can be ‘fun’ such as solar powered fountains and more serious as part of domestic electric supply when fixed to a roof. Even in the UK there is sufficient sun to provide houses and businesses with a useful amount of energy. In remote places, solar panels are increasingly being used for power and so help to reduce consumption of fossil fuels. Word count 520 Sources Discussion with the students at the museum and http://www.dur.ac.uk/dusc/ http://www.howstuffworks.com/solar-cell.htm http://www.soton.ac.uk/~solar/intro/tech0.htm Science Education Group, University of York (2000) Salters Horners Advanced Physics: Student Book AS Level (Salters Horners Advanced Physics) Oxford, Heinemann


74

The internal resistance of a solar cell

Plan Following our visit we were asked to determine the internal resistance of a solar cell. We were interested in the internal resistance as we had learnt that the maximum power is derived from a supply when the external load matches the internal resistance of the power supply. This is derived from rIR ==ε and P = IR2, which together give

( )2

2

rRrP

+=

ε.

ε is the EMF of the power supply, I the current, R the load resistance, r the internal resistance of the power supply and P the power in the external load. The circuit I will use is shown below. Circuit diagram

Apparatus wires solar cell ammeter variable resistor voltmeter lamp power supply for lamp ruler protractor Method

1. Set up the circuit as shown above 2. Using the protractor and ruler, place the lamp at right angles above the solar

cell, with a distance of 15 cm. This means the solar cell will have the maximum amount of light energy hitting it and it will be a constant distance from the cell,

A

V r

R

Solar Cell


75

so the intensity of light hitting the lamp will be constant. I hope the temperature of the circuit will remain constant throughout.

3. Set the variable resistor to its minimum value and set the power supply to 12 V. 4. Turn on the power supply and record the readings from the ammeter and

voltmeter. 5. Turn off the power supply and move the rheostat bar along a half centimetre. 6. Repeat stages 2 and 3 to get at least 7 results. 7. Calculate the power from P = VI and resistance from R = V/I. 8. Plot a graph of power against load resistance: resistance is the independent

variable and power the dependent variable. 9. Find the resistance at the maximum power.

Safety There aren’t many risks in this experiment. The greatest risk is from the lamp which will get very hot during the experiment and could burn it if I touch it. So I will need to be careful and will switch off if it is not in use. Choice of instruments I am going to use digital meters as they are more accurate. For a voltmeter the digital meter has a very high resistance (10MΩ) so very little current is drawn. The advantage of using a multimeter as an ammeter is that I can change the scale if necessary to get the best sensitivity. Results Potential difference across load/V

Current/mA Power/mW Load resistance/Ω

0.01 0.80 0.008 0.013 0.08 0.75 0.060 0.107 0.22 0.65 0.143 0.338 0.29 0.65 0.189 0.446 0.32 0.55 0.176 0.582 0.35 0.45 0.158 0.778 0.38 0.40 0.152 0.950 0.39 0.30 0.117 1.300


76


77

Analysis The graph was a curve which went to a maximum and then tailed off. The maximum power is at 0.46 Ω. The maximum power corresponds to the point at which the internal resistance is equal to the external load, so the internal resistance of the solar cell is 0.46 Ω. Looking at my curve the 0.35 Ω point appears to be anomalous. I think that I should also have taken more readings around 0.2 to 0.6 Ω as this is a turning point and the critical area of the graph. It was quite hard to get a variety of readings with the variable resistor and if I was doing this again I would try to find another way of varying the current.


78

AS Marking grid for solar cells


Mark


1


1


1


1


1


1


1


1


1


9


79


Mark


1


1


1


1


0


0


1


1


0


1


1


0


1


1


10


Mark


1


1


0


1


3


80

D: Analysis


Mark


1


1


2


33

Ref Criterion

Mark


1


1


1


1


1


1


1


0


1


0


1


9


81

Examiner’s comments for solar cells General

The visit report contains details of the physics of solar cells and discusses their implications. The student has been asked to use a method for finding internal resistance which involves maximum power and results in a curved graph. A Summary of physics-based visit

The student has produced a report including appropriate subheadings. The student has provided details of the venue for the visit so S2: States details of visit venue OR provides full details of sources of information, has been awarded, even although the list of sources does not include the date at which websites were accessed. S6: Provides one piece of relevant information (eg data, graph, diagram) that is not mentioned in the briefing papers for the visit or case study was awarded for the data given in the paragraph about the car

B Planning

There is a very clear explanation of the method to be used but little justification or discussion about techniques. The mark P5: Explains the choice of the second measuring instrument with reference to the scale of the instrument as appropriate and/or the number of measurements to be taken has not been awarded because the student refers to sensitivity instead of precision. P6: Demonstrates knowledge of correct measuring technique is not awarded as the student could discuss the use of different range settings on the digital multimeters to obtain sensible results. No comment has been made about repeat readings nor sources of uncertainty, so P9: Comments on whether repeat readings are appropriate in this case and P12: Identifies the main sources of uncertainty and/or systematic error have not been awarded. P13: Draws an appropriately labelled diagram of the apparatus to be used has been awarded even though the connection for the variable resistor is incorrect. C Implementation and Measurements

It is noted in the analysis that insufficient reading were taken around the critical turning point so M3: Obtains an appropriate number of measurements has not been awarded.

D Analysis

A7: Discusses/uses related physics principles is awarded as the student uses P=IV and V=IR. The marks A8: Attempts to qualitatively consider sources of error and A10: Calculates uncertainties have not been awarded as there is no attempt to discuss uncertainties.

E Report

The student has included subheadings and the report has few spelling or grammatical errors so both marks in this section have been awarded.


82

Frequently asked questions

Questions relating to the visit

Does the visit have to involve an industrial setting?

No. There are many applications of physics that are not found in an industrial setting that may be used for the visit (see exemplar visits in this document for examples).

Do students have to talk to a physicist employed at the site?

No, but they should be asking physics-related questions. The questions could be directed to, and answered by, their teacher.

Should the organisation provide documents for the visit?

Organisations need not provide documentation for the visit. If any documents are provided, then teachers should check that that they are at an appropriate level for the group of students who will be doing the visit. A copy of any documents that are issued to students must be submitted with the moderation sample or with work sent to Edexcel to be marked by examiners.

Can I give the students a worksheet for the visit?

Yes, but it should not give the ‘answers’. The worksheet must be submitted with the moderation sample or with work sent to Edexcel to be marked by examiners.

Do the students have to get and use data from the visit?

Students do not have to get and use data from the visit, although the use of data obtained from the visit is highly recommended. Students could for example, do an experiment to determine a constant that was mentioned during the visit and compare the value they obtained with that mentioned in the visit.

Questions relating to written work

Can students submit draft work for checking?

The assessment has been designed to enable students to show that they have appropriate skills, knowledge and understanding for this level of study. If, for example, a student does not use the appropriate number of significant figures and this is pointed out to them, then the assessment will not be a realistic measurement of the student’s own knowledge. Consequently, students must not submit draft work for checking.

How should sources be referenced?

The full address of websites must be stated in the text. To avoid errors, students could copy the address from the address line and past it into their work. The Joint Council for Qualifications provides guidance on the references of books and suggests that Harvard referencing be used (this uses the format Author, A., (Year of publication), “ Title of book”, publisher). However,


83

students will not be penalised if they do not use this format. Whatever format is used, it must be possible for the reader to identify the source.

Should students show all their workings?

If students enter numbers into a calculator and write down a result without showing their calculations then if the result is wrong it will not be possible to allow for the accidental pressing of the wrong button(s) on their calculator. For this reason, it is recommended that students show their workings in full.

Should the plan be written in the future tense?

Yes, but a student should not be penalised for using a different tense.

Do error bars have to be used on graphs?

Use of error bars could be encouraged where the variables plotted are simple. However, their use is not required by the assessment criteria and therefore students will not be penalised if they are not used.

Questions relating to the practical session

Can apparatus be set up for students?

No. Teachers will need to sign a form to verify that students have been able to handle equipment themselves. Consequently apparatus cannot be set up for students. However, teachers may check that the apparatus has been set up correctly before students use the equipment to take measurements. This will give teachers the opportunity to check, for example, that electrical circuits have been wired up correctly and to warn students of any health and safety risks, for example components that may become hot. If a student experiences difficulties with this aspect of the work then it should be noted on the Candidate Record Sheet.

Can students use a Physics simulator?

No. Teachers will need to sign a form to verify that students have been able to handle equipment themselves. If students use a software package to simulate an experiment, then they will not handle any laboratory equipment. Consequently the teacher will not be able to verify that students have been able to handle equipment and therefore the student will not pass the assessment for unit 3.

Can students have more time than is available in one lesson to complete the practical work?

Yes but in general this should not be necessary. Edexcel does not prescribe the amount of time that the practical work should take. It should be possible for the practical aspect of the AS assignment to be completed within one lesson; however, teachers may allow students to complete the practical in the following session.

Can work be done in pairs?

No, all aspects of the work that is produced for the practical assessment must be done individually.


84

Can I give the student any help?

If the student is doing something dangerous the teacher must intervene. If a student requests a formula then this may be given without penalty. If apparatus is being used incorrectly and the student is unlikely to obtain any measurements, help may be given in order to ensure that the student will have some data to process. Any help given of this nature must be noted on the Candidate Record Sheet.

Questions relating to marking work

Will I receive class sets of the marking sheets?

No, the templates provided by Edexcel should be copied for each student.

Will the marking grids be returned?

No. Please note that since they are removed during the moderation process it is essential that the actual work of the student be clearly marked with the centre and student details.

How much annotation is needed?

Brief annotation only. It is highly recommended that you make use of the codes given on the Edexcel marking grids.

Do I need to use the Edexcel marking grids?

Yes, this has been seen to lead to more accurate marking.

Can work for one skill be credited in another?

Yes.

Can I award a half mark if a criterion has not been fully met?

No. If the criterion has not been fully met then no mark should be awarded.

Other questions

How do I know if an experiment is AS standard rather than GCSE?

Does the experiment use AS physics theory? Does the experiment use measuring techniques that are post GCSE eg micrometers? Does it lend itself to some mathematical analysis of errors (but note that combining errors is not required at AS level)?

What is the maximum number of case studies/visits that a student can attempt?

Edexcel does not specify the maximum number of case studies or visits that a student can attempt. However, time restrictions are likely to limit the number of assessments that are attempted by students during the course. Students may attempt several case studies (or visits) under supervised conditions and the best one submitted for assessment.


85

Can students use ICT?

Students can word process their summary of the visit or case study, although they will not gain any extra marks for doing so. The report of the experiment must be hand-written and graphs must be hand-drawn. ICT may be used for collecting data, eg the use of data loggers is permitted. ICT must not be used for processing results. If a student use a spreadsheet package to produce a graph then it will be assumed that the student has used its facilities for automatically selecting an appropriate scale, drawing the best line through the points, etc, and hence the student will lose the relevant marks.


86

Further advice

Plagiarism and collusion

Teachers must be able to declare that the work submitted by the student is solely the work of that student. Any work submitted which does not have a record sheet signed to that effect will be returned for such authentication.

Edexcel is likely to penalise any student that deliberately copies information and attempts to pass it off as original work of their own. Since 2006, Edexcel has been using new software to identify any potential cases of plagiarism.

Plagiarism is defined by the Joint Council for Qualifications as “The failure to acknowledge sources properly and/or the submission of another person’s work as if it were the student’s own.” For example, this would apply if the student has included an extract copied from an internet site without suitable identification of the material and acknowledgment of its source. The Joint Council publish very useful leaflets for teachers and for students, which are available on the JCQ website www.jcq.org.uk. This includes advice on how to detect plagiarism:

Keeping watch on content

• Varying quality of content is one of the most obvious pointers. Well-written passages containing detailed analyses of relevant facts alternating with poorly constructed and irrelevant linking passages ought to give rise to suspicion.

• Another practice is for candidates to write the introduction and conclusion to an assignment to make if fit the question, and then fill in the middle with work which has been lifted from elsewhere.

• If the work is not focused on the topic, but presents a well-argued account of a related matter, this could be a sign that it has been used elsewhere. The same applies if parts of the work do not fit well together in developing the response to the assignment.

• Dated expressions, and references to past events as being current can also be indications of work which has been copied from out-of-date sources.

Keeping watch on vocabulary, spelling and punctuation

• The use of a mixture of English and American vocabulary or spellings can be a sign that the work is not original.

• If the piece contains specialised terminology, jargon, obscure or advance words, the internal assessors should ask if this is typical of this level of candidate and reasonable, or if it is because the candidate did not write the passage.

• Is the style of punctuation regular and consistent?

Keeping watch on style and tone

• Look for differences in the style or tone of writing. If a candidate uses material from textbooks alongside items from popular magazines the change of tone between the two should be marked.

• Look at level of sophistication of the sentence structure. Is this the sort of language that can be expected from a typical student? Is the use of language consistent, or does it vary? Does a change in style reflect a change in authorship at these points?


87

Keeping watch on presentation

• Look at the presentation of the piece. If it is typed, are the size and style of font uniform? What about use of headers and sub-headers? Are the margins consistent throughout? Does the text employ references and if so is the style of referencing consistent? Are there any references, for example, to figures, tables or footnotes, which don’t make sense (because they have not been copied)?

• Lack of references in a long, well-written section could indicate that it had been copied from an encyclopaedia or similar general knowledge source.

• Look out for quotations that run on beyond the part which has been acknowledged.

Other techniques

• Type in phrases or paragraphs into ‘Google’ (use the ‘advanced search’ option) and see if this comes up with a website that matches closely, if not entirely.

• Search parts of the bibliography for suspicious websites that are too closely matched to the title.

• Use free software as described on www.plagiarismdetect.com, www.turnitin.com, www.plagiarism.com, www.wordchecksystems.com or www.canexus.com/eve/index.shtml.

Remember that the centre, as well as the student, is liable for any plagiarism because the teacher will have signed a declaration ensuring that the student’s work is their own.

Collusion

Collusion includes excessive help from teachers or parents or collaboration with other students.

A student must not work with another student to carry out an assessed experiment.

Annotation of student work

The QCA Code of Practice requires that internal assessors show clearly how credit has been assigned in relation to the criteria defined in the specification. The codes in the marking grids provided by Edexcel are designed to facilitate this. Some teachers also send commentaries, which are useful but are not required. The annotation codes should be placed in the margin of the work at the point where it was decided to award that criteria.


88


89

Glossary

Accuracy The degree to which a measurement matches the true value of the quantity that is being measured. This is a qualitative term only.

Error An offset or deviation (either positive or negative) from the true value.

Dependent variable

A variable physical quantity, the values of which are not chosen by the person doing the experiment, but change with another variable ie the independent variable.

Independent variable

A variable physical quantity, the values of which are chosen by the person doing the experiment.

Percentage uncertainty

Precision of an instrument

This is a term meaning 'fineness of discrimination'. In practice, it is the smallest scale division on an instrument that can be read.

Random error An unpredictable error that has no pattern or bias. To reduce the effects of random errors when measuring a quantity it is necessary to take the mean of several values.

Range The difference between the smallest value and the largest value of a set of readings.

Reliability The extent to which a reading or measurement gives the same value when a quantity is measured several times under the same conditions.

Sensitivity The change in response of an instrument divided by the corresponding change in stimulus. For example, the sensitivity of a thermometer is expressed in mm/oC

True value The value that would be obtained if there were no errors in the measurement of that value.

Systematic error An error that has a pattern or bias, for example, errors caused by background lighting. This type of error adds or subtracts the same value to each measurement that is taken.

Uncertainty A range of values which are likely to contain the true value.


x 100% Percentage uncertainty =


90

Validity The level of confidence that is associated with a measurement or conclusion.

Zero error An error that is caused when an instrument does not read true zero, eg a spring balance may not read zero when there is nothing hanging from it. This type of error is a form of systematic error.


91

Appendix 1: Briefing sheets for exemplars based on visits

Introduction

This book contains exemplars of materials that were used for visits to a museum of archaeology, an optician’s and the Science museum.

A case study brief can be based on each of these visits. This appendix shows how case study briefs may be produced for each of these visits. The briefs are written for students.


92

Briefing sheet for the geophysics case study

The following briefing explains what you must do for the assessment for unit 3. You should refer to the marking grid for this unit to ensure that you cover all the requirements of this assessment. Remember that some marks are awarded for the use of clear English. You should work by yourself for this assessment. Background Geophysics is used extensively to explore areas of ground before new building work is carried out and for archaeological sites. For this assessment, you are going to identify two methods which are used to explore ground areas, discuss the relevant principles of physics that they use and explore potential uses of these methods. What you should do

1. Identify two methods that are used to explore areas of ground before new building work commences and/or for archaeological sites.

2. Discuss the two methods that you have identified, including the physics principles involved and potential applications.

You will be planning an experiment to identify a given metal wire by determining its resistivity. The title of the experiment is: Identifying a metallic material using its resistivity.


93

Briefing sheet for the optical case study

The following briefing explains what you must do for the assessment for unit 3. You should refer to the marking grid for this unit to ensure that you cover all the requirements of this assessment. Remember that some marks are awarded for the use of clear English. You should work by yourself for this assessment. Background Lenses have a wide variety of different applications. For example, they are used to make telescopes for astronomical observations and they are used to correct poor eyesight. However, lenses do have their limitations.

For this assessment, you are going to explore how lenses may be used to correct long and short sight, the limitations of a lens to focus red and green light at the same point and the implications for this when correcting eyesight. You will do an experiment to investigate this limitation. What you should do

1. Identify the type of eye defects that may be corrected by lenses.

2. Discuss how lenses may be used to correct short and long sight, and the limitations of a lens to focus different wavelengths of light at the same point. You must include relevant physics principles in your discussion.

You will be planning an experiment to investigate the ability of a lens to focus red and green light at the same point. The title of the experiment is: Measuring the focal length of a lens for red and green light.


94

Briefing sheet for the case study on solar cells

The following briefing explains what you must do for the assessment for unit 3. You should refer to the marking grid for this unit to ensure that you cover all the requirements of this assessment. Remember that some marks are awarded for the use of clear English. You should work by yourself for this assessment. Background Solar cells provide an alternative source of electrical energy to, for example, traditional coal stations. They may be used in a variety of different industrial and domestic applications.

For this assessment, you are going to identify applications that use solar cells, discuss how they work and determine the internal resistance of a typical solar cell. What you should do

1. Briefly outline applications that use solar cells.

2. Explain how a solar cell works, using relevant physics principles. You will be planning an experiment to measure the internal resistance of a solar cell. The title of the experiment is: Measuring the resistance of a solar cell.


95

Appendix 2: Precision, accuracy and sensitivity

Precision is a term meaning 'fineness of discrimination' but is often used erroneously to mean 'accuracy' or 'uncertainty'. It relates to the smallest division that can be read from an instrument. A thermometer that is marked in 1oC steps is less precise than one that is marked in 0.1oC steps because the latter has a more finely divided scale.

Accuracy relates to the difference between the measured value of a quantity and its ‘true’ value. Suppose that the temperature of a boiling liquid is actually 60oC and it is measured with two mercury-in-glass thermometers, one of which reads 59oC and the other reads 57oC; the first thermometer is the most accurate of the two because its reading is the closest to the actual value of the boiling liquid. Accuracy is a qualitative term only.

Accuracy can be improved by removing or compensating for the cause of a systematic error eg checking an instrument for a zero reading error and either adjusting the instrument to eliminate the error or noting the error and deducting its value from readings.

Sensitivity is defined as the change in response of an instrument divided by the corresponding change in stimulus. So for example, the sensitivity of a thermometer is expressed in mm/oC.


96

A note on precision and accuracy

Precision and accuracy are often confused with each other. One instrument may be more precise than another, but it may not be as accurate.

The diagrams show two thermometers that are being used to measure room temperature. The first thermometer is marked in 1oC steps and reads 22oC. The second thermometer is marked in 0.1oC steps and reads 20.2oC. If the room temperature is actually 23oC then the first thermometer gives the more accurate reading because it is closest to the true temperature. The second thermometer is more precise because the scale has finer divisions.

22°C 20

25

More accurate

20.2°C 20

21

More precise


97

Further copies of this publication are available from Edexcel Publications, Adamsway, Mansfield, Notts NG18 4FN

Telephone 01623 467467 Fax 01623 450481 Email: [email protected] August 2008 For more information on Edexcel and BTEC qualifications please visit our website: www.edexcel.org.uk Edexcel Limited. Registered in England and Wales No. 4496750 Registered Office: One90 High Holborn, London WC1V 7BH. VAT Reg No 780 0898 07

Documents

As Physics TSM