Guidance for the A2 Practical Assessment

8/9/2019 Guidance for the A2 Practical Assessment

1/87

Tutor support materials

Edexcel GCE in Physics

Guidance for the A2 practical assessment

February 2009

GCE


2/87

Edexcel, a Pearson company, is the UKs 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 guide 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 its development.

References to third-party material made in this specification are made in good faith. Edexceldoes not endorse, approve or accept responsibility for the content of materials, which may be

subject to change, or any opinions expressed therein. (Material may include textbooks, journals,magazines and other publications and websites.)

Authorised by Roger Beard

Prepared by John Crew

All the material in this publication is copyright

Edexcel Limited 2009


3/87

This document should be read in conjunction with the GCEPhysics specification Issue 3 (publications code UA018902)


4/87


5/87

Contents

Introduction 1How science works 1

General considerations 1

Preparing students for the practical assessment 3

Introduction 3

Safety 3

Planning: General 3

Planning: Identifying equipment 3

Planning: Identifying techniques to use 4

Implementation: Measurements 6

Accuracy and precision 6

Implementation: Recording results in tables 7

Analysing: Graphs 7

Analysing: Limitation of results 8

Evaluating 9

Advice for students 10

Plan 10

Implementation and measurements 11

Analysis 11

Conclusion 12

Uncertainties in measurements 13

What are uncertainties? Why are they important? 13

Calculating uncertainties 13

Calculating percentage uncertainties 14

Compounding errors 14

Using error bars to estimate experimental uncertainties 16

Carrying out the practical work 16

Providing guidance to students during the practical session 17

Carrying out the analysis 17

Returning work 18


6/87

Exemplar of assessed work: Interacting magnetic fields 19

Briefing 19

Exemplar for an able student using own plan for interacting magneticfields 20

A2 Marking grid for interacting magnetic fields able student 24

Exemplar for a less able student using own plan for interactingmagnetic fields 27

A2 Marking grid for interacting magnetic fields less able student 29

Examiners comments for interacting magnetic fields 32

Exemplar of assessed work: Guitar strings 33

Briefing 33

Exemplar for an able student using own plan for guitar strings 34

A2 Marking grid for guitar strings able student 39

Exemplar for a less able student using own plan for guitar strings 42

A2 Marking grid for guitar strings less able student 45

Examiners comments for guitar strings 47

Exemplar of assessed work: Linked oscillators 48

Briefing 48

Student exemplar using own plan 49

A2 Marking grid for linked oscillators 53

Examiners comments for linked oscillators 55

Exemplar of assessed work: Temperature control 56

Briefing 56

Student exemplar using own plan 57

A2 Marking grid for temperature control 60

Examiners comments for temperature control 63

Training Exercise: Safety in hospital 64Introduction 64

Safety in hospital 64

Frequently asked questions 65

Questions relating to written work 65

Questions relating to the practical session 65

Questions relating to marking work 66

Other questions 67


7/87

Further advice 68

Plagiarism and collusion 68

Annotation of student work 69

Glossary 70

Appendix 1: Exemplar centre devised plans for candidates 71

Plan for experiment for interacting magnetic fields 71

Plan for experiment for guitar strings 72

Plan for experiment for linked oscillator 73

Plan for experiment for temperature control 74

Plan for experiment for safety in hospital 75

Appendix 2: Precision, accuracy and sensitivity 77


8/87


9/87

Tutor support materials Edexcel GCE in Physics Guidance for the A2 practical assessment

Issue 1 February 2009 Edexcel Limited 2009

1

Introduction

All A2 students are required to carry out one piece of assessed practical work that is based on an

application of physics. This book provides guidance and examples for the practical work. Itincludes 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. It has nothing to do with content and is a development that builds on and extends the science

skills from Key Stages 3 and 4 through to AS. 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 solvescientific problems

their ability to carry out experimental and investigative activities, including appropriate riskassessment

their ability to analyse and interpret data to provide evidence, recognising correlations andcasual 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 theassessment criteria for Unit 6 when selecting the practical work.

It would be beneficial to the students to be given a practical on a topic within the A2 or AS

course but this is not a requirement of the assessment criteria (however it is expected that this

work will show progression from AS). The practical work can be completed at any time during

the A2 course but it would be more appropriate to administer the assessment near the end of the

course. The practical work should take no more than two hours to complete.

The practical work needs to involve the variation of two interdependent quantities that can be

measured. Students need to be able to produce a graph that 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 a development from AS

that students will often plot log graphs.


10/87

2 Tutor support materials Edexcel GCE in Physics Guidance for the A2 practical assessment Issue 1 February 2009 Edexcel Limited 2009

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 and centres are encouraged to

use equipment such as signal generators, oscilloscopes and data logging devices, where these

are available, to reflect the improving skills of the candidates.

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.


11/87



3

Preparing students for the practical assessment

Introduction

The practical work will assess each students ability to:

plan implement analyse and evaluate.Centres should devise and implement a suitable programme of practical work throughout the A2

course to ensure that students 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. They should be

given the opportunity to combine uncertainties and carry out more complex error analysis than

at AS.

Safety

Teachers should emphasise the importance of safety in all practical work throughout the course

as a matter of good practice.

Planning: General

The plan should include all aspects of the practical from selection of the apparatus through

methods employed to how the data will be used and it should include some indication of how

the aim, stated in the briefing, will be achieved. The intentions should be clear with few

grammatical or spelling errors and will benefit from subheadings that divide the text into clear

sections.

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).


12/87


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 appropriateinstrument to use. Different digital top pan balances have different ranges and different

precisions. Students 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. Experienceshows 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.


13/87



5

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 = y/x

y

x


14/87


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 theappropriate 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 three 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 valueswith 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). Itmay 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 two 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 andprecision ofmeasurements, for example although a stopwatch can read to highprecision (0.01 s) timingswill 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. Whenmeasuring the resistance of a length of wire the contact resistance can lead to asystematic 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 C thermometers can be read (by

interpolation) to aprecision of 0.5 C or better they are unlikely to be accurate (due to theirmanufacture) to within 1 C, or even worse. This has more effect when measuring a temperature

difference (eg determining the rise in temperature when a beaker of water is heated) and sostudents should still be trained to attempt readings to 0.5 C or better.


15/87



7

Students should recognise that even though an instrument is capable of highprecision (egdigital meter, electronic balance, digital stopwatch), its accuracy may well be in doubt(particularly if the student hasnt 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 / s

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 20Tand T), but therequired 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. The correct heading for a column of figures that is the

logarithm of another column is, for example, log (x/mm).

Analysing: Graphs

Plotting graphs is an important part of practical work that students must be familiar with before

they make routine use of software. It is also going to be difficult to ensure that candidates use

any computers solely for graph plotting when doing assessed practical work. All assessed

graphs should therefore be plotted by hand and they 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 T2 / s 2,ln (V/ cm 3), l /D2

/ 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 since theycan be useful in determining uncertainties. Students should be taught to draw the line of best fit,

whether it be a straight line or asmooth curve, preferably with a sharp pencil.

If a straight line graph is anticipated, it is appropriate initially to take six or more 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. It is therefore a good idea to plot the graph

before dismantling the apparatus and considering how the graph compares with the theory and

prediction.


16/87


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 A2students are expected to relate linear graphs to y = mx + c and to understand that astraight line graph mustpass through the origin to confirm a directly proportional relationship.They should, however, bear in mind that not all relationships in physics are linear! It is a

development from AS that 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 students graph may not pass through the origin, from which they might infer that there

could be a systematic error, eg there may be an additional constant term in the expression that

they are using.

Analysing: Limitation of results

In analysing their observations, students should be aware of the limitations of their experimental

measurements. They should understand that certain types of measurement are more reliable than

others. For example, finding the period of a mass oscillating on a spring from 20 oscillations

(say 20 s) should be a reliable, reproducible measurement, whereas the time for a ball to roll

down a slope is likely to be fairly unreliable for a number of reasons: human error in measuring

a time of about 2 s, the ball may not roll in a straight line and the ball might skid. Simple

electrical measurements using digital meters should be reliable, whilst thermal experiments may

be less so due to thermal energy losses and inaccurate and insensitive thermometers.

They should understand how repeat measurements and graphical methods can reduce random

and systematic errors and how such techniques can invariably improve the reliability of their

data.

Students should be aware of the precision of instruments as discussed previously. They should

recognise that if a measurement is the result of the difference of two readings (eg the depression

of a cantilever as measured by a metre rule), it would be unreasonable to quote an uncertainty of

better than 1 mm (ie 0.5 mm for each reading).

X


17/87


18/87


Advice for students

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 top pan balance to measure the mass. I will use a micrometer screw gauge to measure the diameter of the wire.Explain why you have chosen two of the measuring instruments that you have listed. For

example, you could write:

I will use vernier callipers to measure the internal diameter of the test tube as no otherinstrument has this facility.

I will use a multimeter to measure the resistance of the thermistor since it has a variety ofranges so I will be able to select the one that gives me the best precision.

Describe at least two measuring techniques that you have used to make your measurements

reliable. For example, you could write:

I will look horizontally across the wire with the metre rule behind in order to measure theposition of the node.

I will remove the Bunsen to slow the rate of heating as I measure the temperature of thethermistor. This will allow it to come to thermal equilibrium.

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 increased the pressure of the gas slowly so that the temperature stayed the same.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 warms up the thermistor so it will notbe able to repeat my readings. I will check each reading carefully before replacing the

bunsen.

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

you could write:

I will use a stand to make sure the beaker of boiling water is kept securely on the tripod andgauze.

Indicate how you intend to use the data that you collected. For example, in an experiment to

find out how the period, T, of a pendulum varies with its length, l, you could write:

I will plot the log of the time against the log of the length and find the gradient to give me thevalue of n in the equation T = kl

n.

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.


19/87



11

The sources of uncertainty and error should be commented on. For example you could write:

The uncertainty in my measurement of the period comes from the range of my repeatedreadings. It is caused by my judgement of when the pendulum actually stops.

The thermometer might introduce a systematic error since I am unable to check whether itreads 0

0C in melting ice. I will get an indication when the water boils and I can see if it

reads 1000C even though the water is not pure.

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 six readings and repeat these if

necessary. It is a good idea to draw a rough graph as you are taking the measurements so that

you can investigate anomalous readings or to take extra readings near any turning points in any

curves that you obtain.

Make sure that you take measurements over as wide a range as possible. For example, if you are

determining the distance between two nodes that are separated by a few centimetres then you

should not measure the distance between two nodes only. Instead, measure the distance

occupied by several nodes and then calculate the average distance between two of these nodes.

Think critically about your plan as you carry it out. Record any changes that you make to the

plan with a reason. Record any techniques that you use but might not have written in your plan.

Analysis

When you draw your graph, you should use more than half the graph paper in both thex andydirections. 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 log (T/s).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 mustpass through the originto 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 a value for the gradient. If the gradient is to

be used to calculate a value for a physical quantity then you must read the units carefully from

the axes.


20/87


You will need to discuss the sources of error and calculate the uncertainties that these contribute

to the result(s) of your experiment. At A2 you will need to compound your errors to estimate

their combined effect on the final result. You might use error bars on your graph to do this.

You should comment on the precision of your measurements and how these contributed to the

precision of your result. It might be that some of your readings were more precise than others in

which case the least precise determines the result. The likely accuracy of your result might be

commented on by reference to the uncertainties or by numerical comparison with the accepted

value of a quantity such as the acceleration due to gravity.

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 moretime I would have taken more readings will not score this mark. Vague suggestions such asIwould use a digital meterare only of use if they go on to describe how they improve theexperiment. Considering the precision of your readings is an appropriate way to do that.

Similarly you might consider using a more sensitive device. Certainly the accuracy of your

result merits comment.

You should suggest further work that will develop the investigation that this work started, often

it will involve changing different variables with the same apparatus. You should explain howthis work will add to your understanding of the investigation and what you might expect to find.

Conclusion

It is important to make a clear concise statement of your final conclusion. Make sure it is easy to

find the conclusion in your report. For example, draw a box round it, give it a prominent

heading, or underline it in a bright colour.

The conclusion should relate your results to the original aim of the experiment and should

include your final numerical result with its uncertainty. For example you could write:

From my measurements I found a value of 6.2 +/- 0.5 x 10-34 J s for the Planck constant.

or

The results from these experiments indicate that there is a power-law relationship between wave

speed v and tension T: v = kTa

where a = 0.48+/- 0.03. Theoretical analysis suggests that a = 0.5(ie v = kT), which is consistent with the data.

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

might involve algebraic manipulation of equations or a discussion of the phenomenon you have

been investigating. For example why the wire was resonating at all in an experiment to measure

resonant lengths.


21/87



13

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 themeasurement 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.

+-

+-

+-


22/87


Calculating percentage uncertainties

The percentage uncertainty in a measurement can be calculated using:

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

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

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

Compounding errors

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 total percentage uncertainty is calculated by adding together the percentage uncertaintiesfor each measurement if (1) all the measured quantities are independent of one another AND(2) they are multiplied together.

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 0.5mmShow 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 squareA = 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.

+-

+-

Uncertainty of measurementMeasurement taken

x 100%

Uncertainty of measurement

Measurement takenx 100% = 0.5

64x 100% = 1 %


23/87



15

Percentage uncertainty in the area of the square tile is:

A/A = 1% + 1% = 2%

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

Length of each side of the cube l= 24.0 0.5mm

Mass of cube m = 48.230 0.005g

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

material.

Solution 2:

(i) Density of alloy = mass/volume = 48.230 x 10-3

kg/ (24.0 x 10-3

)3

= 3500 kg m-3

.

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

l/l

Percentage uncertainty in mass of cube

m/m

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 36 mm with an uncertainty of

1% then the cross sectional areaA of the canister is:

A = r2

A = (36) 2

A = 4.1 x 103

mm2.

Notice that the result has been expressed using scientific notation so that we can write down just

two significant figures. The calculator answer (4071.5...) gives the impression of far greater

precision that is justified when the radius is only known to the nearest mm.

+-

+-

0.5

24x 100% = 2 %=

0.005

48.2x 100% = 0.1 %=


24/87


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

A/A = 1% + 1%

= 2%

Using error bars to estimate experimental uncertainties

The equation v = kTa

relates the speed of a wave, v in a string to its tension, T. In an experimentto verify this relationship, a graph of ln (v/ms

-1) against ln (T/N) is plotted and the gradient of

the straight line is the constant a. To determine the uncertainty in constant a, the uncertainties inv and Tcan be compounded by considering the difference between the best fit and worst fit linesthat can be plotted through the data using error bars.

To produce error bars in ln(T/N) you need the uncertainty in T. You then calculate the logarithmof your data point with the uncertainty applied and draw the error bar to this value. Suppose you

measure Tas T= 3.4N +/- 0.2N. Then the length of the error bar is [ln(3.6N)-ln(3.2N)]. Thisneed only be calculated for one data point and the same size error bar used for each value ofT.The uncertainty in ln (v/ms

-1) can be calculated in the same way and error bars drawn in that

direction to give, in effect, an error box around each plot. The best fit line is the line that passes

closest to all the plots. The worst fit line just passes through all the error boxes.

It is not intended that this should be a particularly lengthy procedure but it is one way of finding

an estimate of the uncertainty in an experiment.

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 then the teacher may decide to use the following session to complete the practical.

The unmarked plan 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.


25/87



17

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.

Providing guidance to students during the practicalsession

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 thecircuit 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 Candidate Record Sheet. It may be necessary to

tell a student how to connect up a circuit so that they can carry out the experiment and record

some measurements. In this situation, students will be penalised. If the teacher has to explain

how to use an instrument, eg micrometer, then the help given should be recorded and the studentshould lose the mark forP6: States how to measure a second quantity using the mostappropriate instrument. However, if the student provides a satisfactory reason for the choice ofthis measuring instrument they will not lose the mark forP7: Explains the choice of the second

measuring instrument with reference to the scale of the instrument as appropriate and/or thenumber 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 work that students produced for theexperiment. 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.


26/87


Returning work

Teachers must not return work to students to improve. However, students may do more than

practical assessment and some training exercises are highly recommended. Their best piece of

work should be submitted to Edexcel for assessment purposes.


27/87



19

Exemplar of assessed work: Interacting magneticfields

Briefing

Magnetic resonance imaging (MRI) is a medical tool that uses a magnetic field and the natural

resonance of nuclei in the body to obtain images of human tissues. The patient is placed in a

constant magnetic field and radiofrequency radiation is then applied to the system which causes

certain nuclei within the patient to resonate.

This can be modelled by using a small bar magnet as the nucleus and freely suspending it in an

external magnetic field. At first it lines up with the external field as a compass needle does.

When the magnet is rotated about its centre and released it experiences equal and opposite

forces at each end that rotate it back into line. The strength of these forces depends on thestrength of the external magnetic field. When released the magnet then oscillates about the

centre with a resonant frequency that depends on the strength of the magnetic field.

You can produce a magnetic field by passing electric current through a flat coil. This can be

made from a length of wire wrapped around a 250 ml beaker to form a flat coil with 10 turns

and held together with sticky tape. Your teacher will give you such an arrangement.

Make sure there is no current flowing in the flat coil and suspend the magnet so that it is at the

centre of the flat coil. Rotate the magnet about its centre and release it and you will observe

oscillations with a definite period T. This period Tis affected by the strength of the totalexternal field in which it is placed ie the strength of the Earths magnetic field and the strength

of the magnetic field in the coil depends on the currentIflowing in it.

Plan an experiment to determine how the period Tvaries with currentIin the coil. It issuggested that they are related by

1/T2

= kI

where k is a constant. You should plan to find out how well your data follows the suggestion.


28/87


Exemplar for an able student using own plan forinteracting magnetic fields

Apparatus

Coil of wire as advised by teacher

Small bar magnet

Thread

Stopclock

Ammeter

Power supply unit and leads

Retort stands

Method

1 Make the coil and hold it with a retort stand. Attach the power supply unit and ammeter

using crocodile clips. Use thread to suspend the magnet in the centre of the coil. See

diagram. I will rotate the coil so that it lies East West, this means the magnet will line up

with the Earths field.

2 Place a marker at the equilibrium position and rotate the magnet about 200

and release it.

Use the stopclock to time 10 oscillations to reduce the uncertainty in T. Record the time inthe table below; take repeat readings and find an average.

3 Turn on the power supply unit and adjust the current to read 0.5 A on the ammeter.

4 Rotate the magnet about 200

and release it. Time 10 oscillations and record the current andtime in a table like this.

I/ A 10 T/ s 10 T/ s 10 T/ s Mean T/s 1/ (T2) / s-2

0

5 Increase the current in steps of 0.5 A and repeat 4 for each current.

6 Plot a graph of 1/ T2againstI. The plots should be on a straight line and the intercept should

be very small.

A

n

s

N

X


29/87



21

Carrying out

I let the magnet swing freely so that it pointed North and South, I then placed the coil around it

using a second retort stand. I used wooden retort stands so that there was no influence on the

magnets swinging.

When I turned the current on I made sure the magnet did not turn round, I was then sure the coil

field reinforced the Earths field.

I recorded the following readings

I/ A 10 T/ s 10 T/ s 10 T/ s Mean T/s 1/ (T2) / s

-2

0 12.32 12.22 12.28 1.23 0.661

0.5 9.68 9.68 9.72 0.969 1.065

1.0 8.28 8.32 8.22 0.827 1.462

1.5 7.38 7.37 7.34 0.737 1.841

2.0 6.72 6.75 6.72 0.673 2.208

2.5 6.18 6.24 6.32 0.625 2.56

3.0 5.82 5.78 5.82 0.581 2.962

3.5 5.46 5.50 5.46 0.547 3.342

4.0 5.22 5.15 5.22 0.520 3.698

4.5 4.96 4.97 4.91 0.495 4.081

5.0 4.72 4.75 4.78 0.475 4.432


30/87


Analysis

I plotted a graph of 1/T2

against I as shown.

The current readings are to a precision of 0.01 A and the timings all agree very well suggesting

the uncertainty in I is a maximum of 2% - 0.01 in 0.5. Uncertainty in T taking the 4.5 A

readings 0.03/4.95 so 0.6% ( using half the range). So the uncertainty in 1/T2

is 1.2%. Thesewill be too small to plot as error bars on the graph.


31/87



23

Gradient (4.45 0.70) / 5.00 = 3.75/5.00 = 0.75

So the value for k is 0.75 A-1

s-2

with an uncertainty of about 3%.

The plots all lie very close to the Best Fit Line suggesting that this value for k is reliable and

that the measurements are accurate. It certainly supports strongly the equation describing the

motion especially since the uncertainty in the readings is so small. When the coil current

increases there is a stronger field and this will have a greater force on the poles of the oscillatingmagnet. There is a greater restoring force on the rotating magnet and so the period of oscillation

is smaller.

The actual value for k depends on the shape of the coil and the number of turns, amongst other

things. This could be developed by using coils of different shapes and radii. The field could also

be varied by moving the magnet out along the axis of the coil.


32/87


A2 Marking grid for interacting magnetic fields ablestudent

A: PlanningRef Criterion Mark Marking notes

P1 Identifies the most appropriate apparatus

required for the practical in advance.

1 List shown, assume that the

power supply is variable

and so a series adjustable

resistor is not necessary.

P2 Provides clear details of apparatus

required including approximate

dimensions and/or component values

(for example, dimensions of items such

as card or string, value of resistor).

0 Does not specify, eg, likely

ammeter range or typical

power supply value.

P3 Draws an appropriately labelled diagram

of the apparatus to be used.

1

P4 States how to measure one quantity using

the most appropriate instrument.

1 Ammeter is appropriate

P5 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 With no reason

P6 States how to measure a second quantity

using the most appropriate instrument.

1 Stopclock is fine

P7 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 Precision not mentioned

P8 Demonstrates knowledge of correct

measuring techniques.

1 Describes use of fiducial

mark.

P9 Identifies and states how to control all

other relevant quantities to make it a fair

test.

1 Takes account of the effect

of Earths magnetic field

and orientation of coil

P10 Comments on whether repeat readings

are appropriate for this experiment.

1 Plans to take repeats

P11 Comments on all relevant safety aspects

of the experiment.

0 None mentioned

P12 Discusses how the data collected will be

used.

1 Mentions how variables will

be used to plot graph.

P13 Identifies the main sources of

uncertainty and/or systematic error.

1 Explains use of repeats to

reduce uncertainty

P14 Plan contains few grammatical or

spelling errors.

1


33/87



25

Ref Criterion Mark Marking notes

P15 Plan is structured using appropriate

subheadings.

1 Structured plan

P16 Plan is clear on first reading. 1 Logical progression through

method

Mark for this section. 12/16

B: Implementation and measurements


M1 Records all measurements with

appropriate precision, using a table

where appropriate

1 All readings to 0.01 s

although final column is

optimistic

M2 Readings show appreciation of

uncertainty

1 Repeats taken, with small

variation, for mean

M3 Uses correct units throughout 1 Units good

M4 Refers to initial plan while working and

modifies if appropriate

1 Coil orientation described

M5 Obtains an appropriate number of

measurements

1 Plenty of readings, almost

too many, students need to

be careful that they dont

run out of time

M6 Obtains measurements over anappropriate range

1 Good range of currentalthough differences in 10T

are quite small at the end

Maximum marks for this section 6/6

C: Analysis

Ref Criterion Mark

A1 Produces a graph with appropriate axes

(including units)

1

A2 Produces a graph using appropriatescales 1 Plots fill page and arespread widely

A3 Plots points accurately 1 Six plots checked

A4 Draws line of best fit (either a straight

line or a smooth curve)

1 Easily drawn

A5 Derives relation between two variables

or determines constant

1 k determined


34/87


Ref Criterion Mark

A6 Processes and displays data

appropriately to obtain a straight line

where possible, for example, using a

log/log graph

1 Inverse square used

A7 Determines gradient using large triangle 1 Triangle stretches across

page

A8 Uses gradient with correct units 1 Unusual units

A9 Uses appropriate number of significant

figures throughout

0 Mostly fine not so for 1/T2

A10 Uses relevant physics principles

correctly

1 Motion discussed

A11 Uses the terms precision and eitheraccuracy or sensitivity appropriately

1 Precision and accuracymentioned

A12 Discusses more than one source of error

qualitatively

0 Mentions wooden retort

stands without saying why,

might have discussed the

position of the magnet.

A13 Calculates errors quantitatively 1 Considers uncertainty in

readings and combines these

to get an uncertainty for k.

A14 Compounds errors correctly 1 Doubles the uncertainty in Tfor 1/T2

A15 Discusses realistic modifications to

reduce error/improve experiment

0 Nothing suggested

A16 States a valid conclusion clearly 1 Conclusion valid and

A17 Discusses final conclusion in relation to

original aim of experiment

1 ..related to value for k

A18 Suggests relevant further work 1 Develops variables


Total marks for this unit 33/40


35/87



27

Exemplar for a less able student using own plan forinteracting magnetic fields

I will wrap the wire around the beaker and make a flat coil of wire. Which I will connect to the

labpack and ammeter to read the current in the wire. The labpack is safe for me to use in the lab.I will tie the magnet with thread to the top part of the coil so that it hangs in the middle. I will

then twist the magnet and time 10 swings and record them in a table.

I will turn on the labpack and measure the current. I will twist the magnet again and record 10

swings each time.

I will plot a graph ofTagainstIand see if it is a straight line.

Coil of wire

Small bar magnet

Stopclock

Ammeter

PSU and leads

I/ A 10 T/ s 10 T/ s Mean T/s

0 12.32 12.22 1.23

1.0 8.28 8.32 0.830

2.0 6.72 6.75 0.673

3.0 5.82 5.78 0.580

4.0 5.22 5.15 0.519

5.0 4.72 4.75 0.474


36/87


Analysis

The readings for time are pretty close to each other so I think these are accurate. The ammeter

was accurate. I plotted the graph as shown.

The plots all lie on a nice smooth curve which suggests that the theory is correct and that the

readings are accurate and with a small uncertainty. The graph shows that as the current increases

the period gets less.

It was quite difficult to ensure the magnet was in the centre of the coil and this might have

affected the readings. To improve the experiment I would take more readings and change theway I hung the magnet.


37/87



29

A2 Marking grid for interacting magnetic fields less able student

A: PlanningRef Criterion Mark Marking notes


required for the practical in advance

1 List shown, assume that the

power supply unit is variable

and so a series adjustable

resistor is not necessary,

ignore missing apparatus that

is standard and not specific to

this practical.


required including approximatedimensions and/or component values (for

example, dimensions of items such as

card or string, value of resistor)

0 Does not specify ammeter

range or stopclock precision


of the apparatus to be used

0 No diagram

P4 States how to measure one quantity using

the most appropriate instrument

1 Ammeter is appropriate


instrument with reference to the scale of

the instrument as appropriate and/or thenumber of measurements to be taken

0 With no reason


using the most appropriate instrument

1 Stopclock is fine



the scale of the instrument as appropriate

and/or the number of measurements to be

taken

0 No details of stopclock


measuring techniques

0 No mention of fiducial mark

or size of swing



test

0 No mention of the effect of

Earths magnetic field and

orientation of coil

P10 Comments on whether repeat readings are

appropriate for this experiment

1 Plans to take repeats


of the experiment

1 Shows awareness of labpack

as a potential hazard


38/87




used

0 Table produced with columns

but no awareness of need to

process data and so

inappropriate graph planned

P13 Identifies the main sources of uncertainty

and/or systematic error

0 Nothing mentioned

P14 Plan contains few grammatical or spelling

errors

1


subheadings

0 Any structure is not clear or

helpful

P16 Plan is clear on first reading 0 Plan shows path through

practical but ignores toomuch detail to be clear

Mark for this section 6/16


Ref Criterion Mark


appropriate precision, using a table whereappropriate

1 Ammeter readings to 0.1 A in

the table. To match the timemeasurement it would be

appropriate to have three

significant figures if they

were available. Since

candidate does not specify

give benefit of the doubt.


uncertainty

0 No mention of uncertainty,

possible confusion with

accuracy.

M3 Uses correct units throughout 1 Units ok



0 No mention of plan


measurements

1 Readings are sound in

number (just).

M6 Obtains measurements over an

appropriate range

1 and in range



39/87



31

C: Analysis

Ref Criterion Mark


(including units)

1 Graph displays data

adequately and has units

A2 Produces a graph using appropriate scales 1 Scales allow data to coverhalf the page just

A3 Plots points accurately 1 Six plots checked

A4 Draws line of best fit (either a straight line

or a smooth curve)

1 Smooth curve drawn although

line is thick

A5 Derives relation between two variables or

determines constant

0 Misses this aspect of the

experiment

A6 Processes and displays data appropriately

to obtain a straight line where possible,

for example, using a log/log graph

0

A7 Determines gradient using large triangle 0 Cannot draw gradient

A8 Uses gradient with correct units 0


figures throughout

1 SF ok

A10 Uses relevant physics principles correctly 0 No Physics mentioned

A11 Uses the terms precision and either

accuracy or sensitivity appropriately

0 Accuracy often used not

always correctly but no

mention of precision


qualitatively

0 No mention of errors

A13 Calculates errors quantitatively 0

A14 Compounds errors correctly 0



0 More readings by itself does

not merit the mark. Reference

is made to magnet position

but without any physics of the

reason for the change.

A16 States a valid conclusion clearly 1 Clear conclusion



0 ..but not based on proper

analysis

A18 Suggests relevant further work 0 Nothing suggested




40/87


Examiners comments for interacting magnetic fields

Help from the teacher is perfectly permissible where this concerns the setting up of complicated

apparatus that is unfamiliar to the candidate. In this case the teacher may provide help on the

manufacture of the coil and the way it is supported. However, the teacher should not help thecandidate to use the apparatus. In this case the teacher should tell candidates about the effect of

the Earths magnetic field and how the plane of the coil should be oriented. The briefing should

not tell the candidate about other apparatus as this will preclude the award ofP4, States how tomeasure one quantity using the most appropriate instrument.

Time is one of the resources the candidate should control and so the number of readings taken

should be considered carefully; teachers should not allow candidates an open ended time scale.

The final part of the briefing is open ended allowing the candidate freedom to test the data as

they see fit. From the assessment criteria they will be able to decide how to do that appropriately

and score marks but it is unlikely that the candidate will be able to compound errors. However,

error bars could be drawn on the graph to show the uncertainty in the gradient.


41/87



33

Exemplar of assessed work: Guitar strings

Briefing

When designing a guitar it is important that the structure is strong enough to hold the strings

tight without collapsing. To do this you must find out how much tension is required in a string

to make it resonate with a particular frequency at the length it will be on the guitar.

We can model this by using a metal wire as the string and placing it in a magnetic field. It is

made to vibrate by passing an alternating electric current though it, it then vibrates at the

frequency of the ac. In our model we shall use mains electricity from a lab power supply unit

(psu) so the frequency will be fixed at 50 Hz it is possible to vary this if you have access to a

signal generator. You should stretch the wire horizontally over two supports and by hanging

masses on the wire you can vary the tension T.

By varying the tension the speed c of the waves on the string varies

c = (T/)1/2

where is a constant.

Since c =fx variations in Tcause the wavelength to change. The first resonance (when thereis one antinode) occurs when l= /2 where l is the length between supports when there is oneantinode.

Plan an experiment to find how the first resonant length lvaries with T. You should use 0.27mm diameter (32 swg) constantan wire to give a wavelength of about a metre when you hang

100 g on the end.


42/87


Exemplar for an able student using own plan for guitarstrings

ApparatusBench mounted pulley

Bridge support for the wire

2 blocks of soft wood

G clamp

1.5 m length of 0.27 mm diameter (32 swg) constantan wire

Low voltage ac psu

2 magnadur magnets and yoke to produce magnetic field

Slotted masses and hanger- 100 g to 500 g in 50 g increments

Metre rule

Crocodile clips and connecting leads

Method

Safety - The low voltage supply will give no safety problems since it is too low to give a shock.

The hanging masses need to be secure on the wire but should not present a hazard. Use of a wire

under tension requires me to use safety goggles.

I will use a metre rule to measure the resonant length as it is large but under a metre.

I will connect up the apparatus as shown below and put the yoke with the magnets near the

middle for best effect. I will make sure the magnets have opposite poles facing.

I will start with 100 g on the end of the wire and connect up the electric circuit so that current

flows safely through the wire, the frequency cannot change since it is the mains. I turn on the

current and move the bridge back and forwards to find the position of maximum vibration. I will

check for this by placing my eye alongside the wire and looking horizontally.

When the vibrations are biggest I will measure the distance between nodes using the metre rule.

I will then remove the bridge and repeat my reading. I will increase the mass hanging on the

wire and take repeated readings and record my results in the table.

power supply unit

XX

magnets on yokebridgeclip clip

G-clamp

2 wood blockswire Gl

m

bench


43/87



35

Mass/g l1/cm l2/cm l3/cm Mean l/cm

100

150

200

250

300

etc

Analysis

From the briefing sheet c2

= f2

x 2 = T/

At the first resonance position = 2l

So f2 x 4 l2 = T/ so l2 = T/ 4 f2 cf y = m x + c

I will plot a graph of l2

against T which should give a straight line through the origin with a

gradient of 1/(4 f2). From this I will be able to read off the tension for the length of my guitar.

Carrying out

I found a piece of dark card behind the wire helped me to see the vibrations. I looked vertically

down on to the ruler when measuring the length to avoid parallax. I did not go over 500 g

because the length was over a metre and difficult to measure with a metre rule I had enough

readings for my graph anyway.

Mass/g l1/cm l2/cm l3/cm Mean l/cm

100 43.0 43.9 43.9 43.6

150 54.2 53.5 54.0 53.9

200 61.7 61.3 62.7 61.9

250 68.4 68.9 69.1 68.8

300 75.9 75.7 75.4 75.7

350 82.0 81.2 81.3 81.5

400 88.0 87.4 88.3 87.9

450 92.0 92.8 93.4 92.7

500 97.6 97.2 98.3 97.7

I decided that I would plot the length in centimetres squared against the mass in grammes, since

I was not concerned about the value of the gradient. Since the tension is just mass x g, the line

on the graph should still be straight.


44/87


I drew up the following table

m/g l2/cm

2

100 0.191

150 0.291

200 0.383

250 0.473

300 0.573

350 0.664

400 0.773

450 0.859

500 0.955


45/87



37


46/87


The gradient is 0.955 / 500 = 1.91 x 10-3

cm2

g-1

As the current alternates it produces an alternating force since the current is flowing through a

magnetic field. The alternating force is perpendicular to both field and current and so is up and

down. When the frequency of this force matches that of the natural frequency of the wire a large

oscillation is observed. By changing the tension the natural frequency changes and this needs

the length to change for resonance. We have found that as the tension increases so does the

length squared in proportion because the wavelength changes.

There is a large uncertainty in the length since it is possible to get the wire to resonate over quite

a range of distances. It is also difficult to measure the length once resonance is determined since

the rule is not along the wire. The uncertainty in lis at least 1 cm so the uncertainty in lwill be2% and in l2 will be 4%. The uncertainty in the mass is about 2% my teacher says.

To improve my experiment I would reduce the interval in the mass readings so that the graph

had more points. To develop the experiment I could use a signal generator to vary the frequency

and find how the resonant length varies with frequency or keep the length fixed and vary the

tension and frequency. This would tell me more about the way the guitar works.


47/87



39

A2 Marking grid for guitar strings able student

A: Planning




1 List shown



dimensions and/or component values (for



1 Although the safe value for

the voltage or current is not

specified the range of masses

and length of wire is.



1

P4 States how to measure one quantity usingthe most appropriate instrument

1 Metre rule is appropriate



the instrument as appropriate and/or the

number of measurements to be taken

1 With reason



0 Does not mention second

measurement



the scale of the instrument as appropriateand/or the number of measurements to be

taken

0 Might have checked

calibration of masses or

measured the frequency of themains using an oscilloscope



1 Eye level and card

background



test

1 Mentions frequency is mains



1

P11 Comments on all relevant safety aspectsof the experiment

0 Shows awareness of safetyaspects but fails to recognise

need for ammeter to monitor

current at safe levels


used

1 Plans to plot l2

against T



0 Although they plan to repeat

they do not mention the large

uncertainty in establishing the

resonant length


48/87




errors

1


subheadings

1

P16 Plan is clear on first reading1



Ref Criterion Mark

M1 Records all measurements withappropriate precision, using a table where

appropriate

1 Precision ok


uncertainty

1 Repeats show variation

M3 Uses correct units throughout1 Units ok



1 Changes plan in limiting

mass to 500 g to keep length

under 1 m


measurements

1 Almost too many

M6 Obtains measurements over an

appropriate range

1 Good range with reason


C: Analysis

Ref Criterion Mark


(including units)

1 Axes fine

A2 Produces a graph using appropriate scales 1 Scales have sensible numbers

and plots fill half the page in

both directions

A3 Plots points accurately 0 250 g plot is wrong

A4 Draws line of best fit (either a straight line

or a smooth curve)

1


49/87



41

Ref Criterion Mark

A5 Derives relation between two variables or

determines constant

1 Derives equation

A6 Processes and displays data appropriately

to obtain a straight line where possible,for example, using a log/log graph

1 Plots graph of length squared

A7 Determines gradient using large triangle 1 Whole page

A8 Uses gradient with correct units 1


figures throughout

1 3 SF ok throughout

A10 Uses relevant physics principles correctly 1 Uses wave equation and

explains phenomenon later

A11 Uses the terms precision and either

accuracy or sensitivity appropriately

0 Doesnt mention precision at

all


qualitatively

1 Considers percentage

uncertainty in both variables

A13 Calculates errors quantitatively 1 In lsquared

A14 Compounds errors correctly 0 Doesnt compound errors



0 More readings on graph is

unlikely to help the

conclusion

A16 States a valid conclusion clearly 1 Conclusion stated as thetension increases so does the

length squared in proportion

because the wavelength

changes



0 States proportionality but

does not relate findings back

to the aim of the experiment

the tension in the guitar

string.

A18 Suggests relevant further work 1 Discusses detail of the

experiment.




50/87


Exemplar for a less able student using own plan forguitar strings

ApparatusBench mounted pulley

Bridge support for the wire

2 blocks of soft wood

G clamp

32 swg constantan wire

Low voltage psu

2 magnets and holder

Metre rule

Method

I will connect up the apparatus as shown in the diagram and make sure that everything is safe. I

will hang 100 g on the end and I will connect up the power supply.

When everything is ready I will turn on the psu and move the bridge support until I can see

resonance. I will look carefully to see when the wire is vibrating at its maximum and then

measure the length of the wire.

I will increase the mass hanging on the end and repeat my readings in the table.

Mass/g l1/cm l2/cm Mean l/cm

100

200

300

400

500

Analysis

From the briefing sheet c2

= f2

x 2 = T/

At the first resonance position = 2l

So f2

x 4 l2

= T/ so l2 = T/ 4 f2

l2

is proportional to T and I will plot a graph of l2

against T.


51/87



43

Carrying out

I carried out the experiment as I said, it was difficult to find the resonance position. I got the

following readings

Mass/g l1/cm l2/cm Mean l/cm l2

100 43.3 43.9 43.6 1901

200 61.7 61.3 61.5 3782

300 75.9 75.7 75.8 5746

400 88.0 87.4 87.7 7691

500 97.6 97.2 97.4 9487


52/87


I plotted the graph and the gradient was 9.55/500 = .0191. The straight line shows that there is

a strong correlation between l2

and mass and that as the mass is increased the resonant length

gets longer.

The plots are all close to the line of best fit which means the experiment was a success.


53/87



45

A2 Marking grid for guitar strings less able student

A: Planning




1 List shown is adequate



dimensions and/or component values (for



0 Wire length not specified



0 No diagram

P4 States how to measure one quantity usingthe most appropriate instrument

1 Metre rule is appropriate



the instrument as appropriate and/or the

number of measurements to be taken

0 but not explained



0 Does not mention second

measurement



the scale of the instrument as appropriateand/or the number of measurements to be

taken

0 Might have checked

calibration of masses or

measured the frequency of themains using an oscilloscope



1 Looking carefully is not

enough, the actual method

must be specified



test

0 Doesnt realise that frequency

might be a variable



1 Text is unclear about what

will be repeated but table isclear


of the experiment

0 Very vague about safety

specific precautions are

needed.


used

1 Correct graph plotted



0 Although they plan to repeat

they do not mention the large

uncertainty in establishing the

resonant length


54/87




errors

1


subheadings

1

P16 Plan is clear on first reading 1



Ref Criterion Mark


appropriate precision, using a table where

appropriate

1 Table with units and precision

appropriate


uncertainty

0 Only one repeat is not really

enough in this experiment

M3 Uses correct units throughout 0 The units in the last column

have been omitted

M4

Documents

Guidance for the A2 Practical Assessment