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Tutor support materials
Edexcel GCE in Physics
Guidance for the A2 practical assessment
February 2009
GCE
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Edexcel, a Pearson company, is the UKs largest awarding body, offering academic and
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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
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This document should be read in conjunction with the GCEPhysics specification Issue 3 (publications code UA018902)
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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
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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
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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
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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.
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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.
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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).
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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.
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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 = y/x
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 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.
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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.
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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
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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.
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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.
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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.
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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 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.
+-
+-
+-
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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%.
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 %
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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 %=
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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.
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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.
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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.
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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.
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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
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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
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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.
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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.
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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
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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
Ref Criterion Mark Marking notes
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
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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
Maximum marks for this section 15/18
Total marks for this unit 33/40
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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
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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.
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A2 Marking grid for interacting magnetic fields less able student
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 unit is variable
and so a series adjustable
resistor is not necessary,
ignore missing apparatus that
is standard and not specific to
this practical.
P2 Provides clear details of apparatus
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
P3 Draws an appropriately labelled diagram
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
P5 Explains the choice of the measuring
instrument with reference to the scale of
the instrument as appropriate and/or thenumber 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 No details of stopclock
P8 Demonstrates knowledge of correct
measuring techniques
0 No mention of fiducial mark
or size of swing
P9 Identifies and states how to control all
other relevant quantities to make it a fair
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
P11 Comments on all relevant safety aspects
of the experiment
1 Shows awareness of labpack
as a potential hazard
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Ref Criterion Mark Marking notes
P12 Discusses how the data collected will be
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
P15 Plan is structured using appropriate
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
B: Implementation and measurements
Ref Criterion Mark
M1 Records all measurements with
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.
M2 Readings show appreciation of
uncertainty
0 No mention of uncertainty,
possible confusion with
accuracy.
M3 Uses correct units throughout 1 Units ok
M4 Refers to initial plan while working and
modifies if appropriate
0 No mention of plan
M5 Obtains an appropriate number of
measurements
1 Readings are sound in
number (just).
M6 Obtains measurements over an
appropriate range
1 and in range
Maximum marks for this section 4/6
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C: Analysis
Ref Criterion Mark
A1 Produces a graph with appropriate axes
(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
A9 Uses appropriate number of significant
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
A12 Discusses more than one source of error
qualitatively
0 No mention of errors
A13 Calculates errors quantitatively 0
A14 Compounds errors correctly 0
A15 Discusses realistic modifications to
reduce error/improve experiment
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
A17 Discusses final conclusion in relation to
original aim of experiment
0 ..but not based on proper
analysis
A18 Suggests relevant further work 0 Nothing suggested
Maximum marks for this section 6/18
Total marks for this unit 16/40
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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.
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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.
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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
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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.
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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
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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.
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A2 Marking grid for guitar strings able student
A: Planning
Ref Criterion Mark Marking notes
P1 Identifies the most appropriate apparatus
required for the practical in advance
1 List shown
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)
1 Although the safe value for
the voltage or current is not
specified the range of masses
and length of wire is.
P3 Draws an appropriately labelled diagram
of the apparatus to be used
1
P4 States how to measure one quantity usingthe most appropriate instrument
1 Metre rule 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
1 With reason
P6 States how to measure a second quantity
using the most appropriate instrument
0 Does not mention second
measurement
P7 Explains the choice of the second
measuring instrument with reference to
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
P8 Demonstrates knowledge of correct
measuring techniques
1 Eye level and card
background
P9 Identifies and states how to control all
other relevant quantities to make it a fair
test
1 Mentions frequency is mains
P10 Comments on whether repeat readings are
appropriate for this experiment
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
P12 Discusses how the data collected will be
used
1 Plans to plot l2
against T
P13 Identifies the main sources of uncertainty
and/or systematic error
0 Although they plan to repeat
they do not mention the large
uncertainty in establishing the
resonant length
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Ref Criterion Mark Marking notes
P14 Plan contains few grammatical or spelling
errors
1
P15 Plan is structured using appropriate
subheadings
1
P16 Plan is clear on first reading1
Mark for this section 12/16
B: Implementation and measurements
Ref Criterion Mark
M1 Records all measurements withappropriate precision, using a table where
appropriate
1 Precision ok
M2 Readings show appreciation of
uncertainty
1 Repeats show variation
M3 Uses correct units throughout1 Units ok
M4 Refers to initial plan while working and
modifies if appropriate
1 Changes plan in limiting
mass to 500 g to keep length
under 1 m
M5 Obtains an appropriate number of
measurements
1 Almost too many
M6 Obtains measurements over an
appropriate range
1 Good range with reason
Maximum marks for this section 6/6
C: Analysis
Ref Criterion Mark
A1 Produces a graph with appropriate axes
(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
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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
A9 Uses appropriate number of significant
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
A12 Discusses more than one source of error
qualitatively
1 Considers percentage
uncertainty in both variables
A13 Calculates errors quantitatively 1 In lsquared
A14 Compounds errors correctly 0 Doesnt compound errors
A15 Discusses realistic modifications to
reduce error/improve experiment
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
A17 Discusses final conclusion in relation to
original aim of experiment
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.
Maximum marks for this section 13/18
Total marks for this unit 31/40
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42 Tutor support materials Edexcel GCE in Physics Guidance for the A2 practical assessment Issue 1 February 2009 Edexcel Limited 2009
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.
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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
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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.
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A2 Marking grid for guitar strings less able student
A: Planning
Ref Criterion Mark Marking notes
P1 Identifies the most appropriate apparatus
required for the practical in advance
1 List shown is adequate
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 Wire length not specified
P3 Draws an appropriately labelled diagram
of the apparatus to be used
0 No diagram
P4 States how to measure one quantity usingthe most appropriate instrument
1 Metre rule 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 but not explained
P6 States how to measure a second quantity
using the most appropriate instrument
0 Does not mention second
measurement
P7 Explains the choice of the second
measuring instrument with reference to
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
P8 Demonstrates knowledge of correct
measuring techniques
1 Looking carefully is not
enough, the actual method
must be specified
P9 Identifies and states how to control all
other relevant quantities to make it a fair
test
0 Doesnt realise that frequency
might be a variable
P10 Comments on whether repeat readings are
appropriate for this experiment
1 Text is unclear about what
will be repeated but table isclear
P11 Comments on all relevant safety aspects
of the experiment
0 Very vague about safety
specific precautions are
needed.
P12 Discusses how the data collected will be
used
1 Correct graph plotted
P13 Identifies the main sources of uncertainty
and/or systematic error
0 Although they plan to repeat
they do not mention the large
uncertainty in establishing the
resonant length
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Ref Criterion Mark Marking notes
P14 Plan contains few grammatical or spelling
errors
1
P15 Plan is structured using appropriate
subheadings
1
P16 Plan is clear on first reading 1
Mark for this section 8/16
B: Implementation and measurements
Ref Criterion Mark
M1 Records all measurements with
appropriate precision, using a table where
appropriate
1 Table with units and precision
appropriate
M2 Readings show appreciation of
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