GCE Physics Guidance for the AS practical assessment

September 2008

Tutor support materials

Updated March 2012

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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 Updated by Simona Ondruskova All the material in this publication is copyright © Edexcel Limited 2008

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This document should be read in conjunction with the GCE Physics specification - issue 4 (Publication code UA024825).

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Contents Introduction

How Science Works 6

General considerations 6

Use of ICT 7

Preparing students for the practical assessment 8

Introduction 8

Safety 8

Planning: Identifying equipment 8

Planning: Identifying techniques to use 9

Implementation: measurements 11

Accuracy and precision 11

Implementation: Recording results in tables 12

Analysing: Graphs 12

Analysing: Limitation of results 13

Evaluating 14

Advice for students 16

Summary of the case study or visit 16

Plan 16

Implementation and measurements 17

Analysis 18

Uncertainties in measurements 19

What are uncertainties? Why are they important? 19

Calculating uncertainties 19

Calculating percentage uncertainties 20

Compounding errors 21

Guidance for visits 23

Guidance for case studies 25

Some ideas for practical assessments 26

Visits 26

Case studies 26

Conducting the AS assessment 27

The summary of the case study or visit 27

The plan 27

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Carrying out the practical work 28

Providing guidance to students during the practical session 28

Carrying out the analysis 29

Returning work 29

Exemplar of assessed work: Geophysics 30

Visit report for Geophysics 31

AS Marking grid for visit to an archaeological site 39

Examiner’s comments - geophysics 42

Exemplar of assessed work: Solar cells 44

Visit report for solar cells 45

AS Marking grid for solar cells 52

Examiner’s comments for solar cells 55

Frequently asked questions 57

Questions relating to the visit 57

Questions relating to written work 58

Questions relating to the practical session 59

Questions relating to marking work 60

Other questions 60

Further advice 61

Plagiarism and collusion 61

Annotation of student work 62

Glossary 63

Appendix 1: Briefing sheets for exemplars based on visits 64

Introduction 64

Briefing sheet for the geophysics case study 65

Briefing sheet for the optical case study 66

Briefing sheet for the case study on solar cells 67

Appendix 2: Precision, accuracy and sensitivity 68

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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.

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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 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.

Use of 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.

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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

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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.

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

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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

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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

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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 (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

1 Bold type indicates that this is an A2 requirement

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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.

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

X

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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).

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

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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.

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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.

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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.

• 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.

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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.

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.

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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.

+ -

+ -

+ -

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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%.

Uncertainty of measurement Measurement taken

x 100%

Uncertainty of measurement Measurement taken

x 100% = 0.5 64

x 100% = 1 %

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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]

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.

2 This section applies for the A2 practical only

+ - + -

+ - + -

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(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 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%

0.5 24

x 100% = 2 % =

0.005 48.23

x 100% = 0.1 % =

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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

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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.

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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.

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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).

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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.

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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.

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

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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.

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.

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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

Unit 2 Physics at Work

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

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

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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

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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 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

5http://www.ukdetectornet.co.uk/andy1.htm Accessed 5/8/8

v

Power supply

probes

Electrodes

air

soil

33

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

6 http://www.finds.org.uk/treasure/treasure_summary.php Accessed 5/8/8

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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

35

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

36

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.

37

38

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.

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AS Marking grid for visit to an archaeological site

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)

1

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

1

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

1

Marks for this section

9

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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

0

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

0

P6 Demonstrates knowledge of correct measuring techniques

1

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

1

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

Marks for this section

11

C: Implementation and Measurements Ref Criterion

Mark

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

0

M2 Uses correct units throughout

0

M3 Obtains an appropriate number of measurements

1

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M4 Obtains measurements over an appropriate range

1

Marks for this section

2

D: Analysis

E: Report Ref Criterion

Mark

R1 Summary contains few grammatical or spelling errors

1

R2 Summary is structured using appropriate subheadings

1

Marks for this section

2

Total marks for this unit

34

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

1

A11 Provides a final conclusion

1

Marks for this section

10

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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.

43

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. 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.

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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

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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.

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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 negatively and positively charged sides. When light falls on the cell the electrons get enough

Absence of electrons

Excess electrons

- - - - - - - - - - -

Positive charge

Negative charge

Two different materials

R

Electrons

Electrons

Light

47

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

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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

A

V r

R

Solar Cell

49

the maximum amount of light energy hitting it and it will be a constant distance from the cell, 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

50

51

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.

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AS Marking grid for solar cells 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)

1

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

1

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

1

Marks for this section

9

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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

0

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

1

P9 Comments on whether repeat readings are appropriate in this case

0

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

Marks for this section

10

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

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M4 Obtains measurements over an appropriate range

1

Marks for this section

3

D: Analysis

E: Report Ref Criterion

Mark

R1 Summary contains few grammatical or spelling errors

1

R2 Summary is structured using appropriate subheadings

1

Marks for this section

2

Total marks for this unit

33

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

1

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

0

A9 Suggests realistic modifications to reduce error/improve experiment

1

A10 Calculates uncertainties

0

A11 Provides a final conclusion

1

Marks for this section

9

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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

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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.

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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.

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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, 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.

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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. 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.

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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, except for planning marks which can only be awarded in section B: Planning. 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.

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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?

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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?

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. The annotation codes should be placed in the margin of the work at the point where it was decided to award that criterion.

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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.

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.

Uncertainty of measurement Measurement taken

x 100% Percentage uncertainty =

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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.

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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.

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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.

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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.

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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.

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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

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

Telephone 01623 467467 Fax 01623 450481 Email: publications@linneydirect.com 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