The task
The Quality of Measurement task builds on the practical work done throughout the AS course. It will be done after you have had some experience of practical work, and in particular will exploit the following skills involved in making good measurements, and knowing how good they are:
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identifying and estimating the largest source of uncertainty in measurements with sensors and other instruments
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taking account of properties of sensors and instruments: resolution, sensitivity, stability, response time, systematic and zero error and calibration
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using dot-plots or histograms of repeated measurements to estimate mean and range of values (
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spread), and identify possible outliers
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plotting graphs including uncertainty bars, using them to estimate uncertainty in gradient or intercept, and to analyse and check theoretical relationships between the variables
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suggesting ways to reduce the largest source of uncertainty in an experiment
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identifying possible sources of systematic and zero error in measurements, and suggesting ways to eliminate or reduce them
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being critical about measurements, looking for ways of cross-checking results
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analysing data carefully and thoroughly, using graphs and other plots to detect and display their important features
There are five different types of activity that you might use for this piece of coursework:
1. A careful measurement of a physical quantity
In the Advancing Physics course, you often make a quick measurement of some property in order to understand the relationship between the variables, and the magnitude of the value obtained. Examples include the Young Modulus of copper, the conductivity of constantan alloy wire, the wavelength of light, or the Planck constant. If you wished to use one of these for your coursework, you would be aiming to identify the shortcomings of your original experiment, and to improve it according to the list above. This does mean that you should keep a detailed log of all your practical work during the course: this is standard practice in research labs!
2. A careful quantitative study of the relationship between two or more variables, where there are some indications from theory of what to expect
Much experimental work in the course is done to illustrate and explore relationships between variables. Theory indicates that resistance of metals should increase with temperature, while that of semiconductors falls, and also tells us that frictional forces on moving objects will increase with velocity, and that the speed of waves along a stretched string will increase with tension. A systematic investigation of any one of these, or of a similar relationship, will involve use of the ideas in the list above.
3. A careful calibration of a sensor or instrument
In Chapter 2 you meet a number of sensor circuits which can be used to measure temperature, or light intensity, or rotation, or linear displacement. Careful calibration of one of these would involve critical analysis of the resolution and sensitivity of the circuit concerned as well as checking on uncertainties and systematic errors in the measurements made.
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4. A careful study of one or more of the properties of a sensor or instrument
Some sensors met in chapter 2 have properties that limit or enhance their performance. If you wish to time an event, for example, the very different response times of LDRs and phototransistors may determine which is the more useful, and the same is true of a thermistor and a thermocouple as temperature sensors. In both cases, the sensitivity of the sensor will depend on its circuit, as well as the actual sensor used, and this is also a feature that needs consideration when it is to be used. A careful study of one or both of these properties for a sensor would allow you to demonstrate the skills required in this coursework task.
5. A comparison of methods of measuring the same thing
Sometimes there are more than one way of making a measurement: the acceleration due to gravity, g , can be measured in a number of different ways, including timing accurately the fall of an object over a known distance, and measuring the gain in velocity of a falling object during a measurable time. Deciding which is better will involve identifying and minimising and quantifying the uncertainties and systematic errors involved in each case. Other examples here could be the comparison of a thermistor and a thermocouple as calibrated thermometers, or comparing the measurement of the depth of liquid in a container with a float attached to a rotary potentiometer with the use of an LDR at the bottom detecting attenuation from a light source at the top.
Questions to ask yourself before deciding on your Quality of Measurement task
1. Is there an experiment that you have done in the course that you would like to do again, and do with greater precision, and with more confidence in your final result? One in which you are already aware which of the measurements you have made was the one with the greatest uncertainty, and where you have some idea of possible systematic errors? This could indicate choosing ‘A careful measurement of a physical quantity’.
2. Have you done an experiment that reveals a relationship between the variables concerned? One in which physics theory makes predictions about this relationship? One where you could test this relationship, providing that you had confidence in the accuracy of your measurements? This could indicate choosing ‘A careful quantitative study of the relationship between two or more variables, where there are some indications from theory of what t o expect’.
3. In chapter 2, did you use a sensor which could give a useful measurement, but which would need careful calibration to be reliable? One where there was a clear variation of the measured output with the input variable, but where more work would be needed to establish the best values of components, and careful consideration of the optimum sensitivity and resolution of the circuits? This could indicate choosing ‘A careful calibration of a sensor or instrument’.
4. Some of the sensors met in chapter 2 perform the same tasks, but do so in different ways, with different response times, or resolutions, for example. To make a proper comparison would require a systematic study of the critical factor involved, and could indicate choosing ‘A careful study of one or more of the properties of a sensor or instrument’.
5. Some physical quantities can be measured in quite different ways, using quite different aspects of physics. If you wished to compare these different approaches in terms of the quality of the measuremen t, you might choose ‘A comparison of methods of measuring the same thing’.
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In choosing the measurement task you wish to work on, and the techniques you intend to use, you will need to discuss it with your teacher
Possible starters
1. A careful measurement of a physical quantity
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Breaking stress of steel
Electrical conductivity of constantan wire
Emf and/or internal resistance of a cell
Gravitational field strength near the Earth’s surface
Planck constant
Power and/or magnification of a lens
Speed of sound in air
Wavelength of light using Young’s slits
Wavelength of a spectral line using a diffraction grating
Young modulus of copper
2. A careful quantitative study of the relationship between two or more variables, where there are some indications from theory of what to expect
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Absorption of light with thickness of materials, or concentration of a solution
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Frequency of a standing wave on a string with length
Resistance of a metal wire with temperature
Response of a solar cell to varying light intensity
Speed of a trolley down a slope with angle
Speed of falling paper cones with area
Speed of wave on a string and tension
3. A careful calibration of a sensor or instrument
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LDR
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Linear sprung potentiometer
Rotary potentiometer
Thermistor
Thermocouple
4. A careful study of one or more of the properties of a sensor or instrument
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Resolution of a thermocouple
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Response time of a photodiode and/or phototransistor
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Response time of a thermistor and/or thermocouple
Sensitivity of a thermistor
5. A comparison of methods of measuring the same thing
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Airflow from a fan using a hinged flap and angular variable resistor or with a hot wire anemometer
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Liquid level based on flotation and a rotary potentiometer or using an LDR and light source
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Measuring ‘ g ’ in more than one way
Proximity using an ultrasonic sensor or a photodiode
Temperature using a thermocouple, thermistor or using change of resistance of a metal
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Assessment
This forms one of two coursework tasks which make up assessment unit 3 of the AS course.
There are four strands to the assessment of this piece of work:
1. Quality of the practical work in the laboratory
2. Quality of thought about uncertainty and systematic error, and attempts to improve the measurements
3. Quality of communication of physics in the report
4. Quality of handling and analysis of data
The criteria for assessment are given in Appendix D of the specification http://www.ocr.org.uk/Data/publications/key_documents/AS_ALevel_GCE_Physics_B_Advancing_Physics_Specification.pdf
and further details, interpretation and advice in the Coursework Handbook http://www.ocr.org.uk/Data/publications/support_materials/A_Level_Physics_B_SM_Coursework_Guidance.pdf
This task provides an opportunity for you to show that you can do practical work with both
‘hands-on’ (making skillful measurements and observations) and ‘mind-on’ (analyzing the problems and data thoughtfully), demonstrating an ability to make good measurements and to know how good they are. Your teacher will assess you using the criteria below. The maximum in each of the four strands is 5, so that this task is marked out of 20. In each strand, a level of performance between those described as meriting 1, 3 or 5 marks would be awarded 2 or 4 marks, as appropriate.
Your teacher will be able to give you a copy of the marking grid that uses these criteria
Strands of assessment in the Quality of Measurement task
Strand 1: Quality of practical work in the laboratory
You should work systematically and carefully, showing skill in handling and using apparatus, and making sufficient observations and measurements to deal with the problem. Your results should be tabulated carefully in a well-thought-out structure, recorded as you took them. You should make measurements so as to minimise systematic error, and to reduce uncertainty to the limits allowed by the apparatus.
1 mark:
3 marks:
Methods and approach have limitations, with shortcomings in the measurements and observations and limited attention to practical detail.
Methods and approach are adequate, with relevant measurements and observations and competent attention to practical detail.
5 marks: Methods and approach are well chosen, with sufficient measurements and observations made to deal with the problem and considerable skill used to obtain them, avoiding unnecessary systematic error.
Strand 2: Quality of thought about uncertainty and systematic error, and attempts to improve the measurements
You should study and assess the relevant properties of sensors and measuring instruments systematically and carefully, and consider the calibration of instruments, and attempt this where possible. You should identify and estimate the largest source of uncertainty. You will need to
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consider possible systematic errors, and identify whether they will lead to a value which is too large, or too small. You should suggest and try out possible improvements to the experimental method and apparatus used.
1 mark: Measuring instruments are used directly without consideration of their properties; little understanding of the nature of systematic error or uncertainty is shown, and data is limited to a simple set of measurements.
3 marks: Some efforts are made to use measuring instruments to their best advantage; systematic error and uncertainty are considered, possibly with some flaws in approach, and some improvements to experimental method are considered.
5 marks: The relevant properties of measuring instruments are assessed systematically; errors and uncertainty are identified and, where possible, reduced and improvements to the experiment to reduce error and uncertainty are tried out.
Strand 3: Quality of communication of physics in the report
Your report should be clear, well-ordered, and concise, with enough detail to allow someone else to repeat the experiment and obtain similar measurements. Make sure that you explain clearly the physics of the experiment, and use available ICT – word processing, spreadsheets and graphical programs – well. Your graphs and tables should be well-chosen, and presented so as to communicate the findings as well as possible. Take care that your use of English is clear and effective for the purpose.
1 mark: Recording and presentation of data lacks clarity; graphical plots may be inappropriate or incorrect and the report is poorly structured and presented.
3 marks: Data are presented clearly, with some possible inconsistencies in headings, units or significant figures; graphical plots are clear and the report covers most details needed to repeat the experiment.
5 marks: Data are presented clearly and effectively with correct headings, units, tolerances and significant figures; graphical plots are well chosen to display the data to good effect and the report is clear, concise, well-structured and gives all details needed for someone else to repeat the work.
Strand 4: Quality of handling and analysis of data
Make sure that you analyse your data with care and attention, looking for anomalies or unexpected features, and extracting as much information as possible. The analysis needs to demonstrate a clear and correct understanding of the physics involved. You should have crosschecked your results, either through alternative ways of looking at the data or by going back to the apparatus. Ensure that you make a clear claim about the outcome, qualified with statements of uncertainty and possible systematic error.
1 mark:
3 marks:
Analysis is limited to direct calculations or plots of measured data with possible major flaws in physics, or no attempt to explain the outcomes in terms of physical ideas, and limited attempt to discuss shortcomings in procedures.
Some correct calculations of relevant quantities are made with an attempt to discuss the outcomes in terms of physical explanations (including discussion of errors and uncertainties) with minor errors of physics in the analysis. Obvious anomalies are noted.
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5 marks: Data are analysed carefully, extracting as much information as possible, leading to a clear claim about the outcome, well-founded in the data and analysis, demonstrating understanding of the physics and relating uncertainties in the conclusions to limitations of the procedures and the measurements made.
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