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Physics: Post your doubts here!

ruby_fern

ruby_fern

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found some really good tips for paper 5 on the phy ref site:

Paper 5 Planning, Analysis and Evaluation
Planning question
  • Do not panic if the context of the question appears unfamiliar to you. During your A Level studies you will have used or learnt about suitable apparatus for completing the task. If you are asked to ‘use’ any unfamiliar apparatus the question will supply you with all the details that you need to know about.
  • Read the question very carefully – it may give you guidance on those aspects of your plan to which you need to pay particular attention. It will also help you to identify the independent and the dependent variables.
  • When writing your answer you will need to consider some or all of the following:
– what apparatus you will use
– what experimental arrangement will be used
– what procedure will be followed
– the independent and dependent variables
– the means of keeping other variables constant – use the word ‘constant’ when identifying these variables, saying you will ‘control’ them is insufficient
– how the raw data readings will be processed to give the desired result, e.g. what derived quantities you might calculate or what graph you might plot
– what relevant safety precautions should be in place
  • The relationship to be tested, given to you in the introduction to the task, will suggest the type of graph to be expected. You will need to describe it as precisely as possible. For example, is it linear, does it pass through the origin? If you choose a logarithmic graph, you will be expected to predict its slope from the given expression.
  • When writing your answer you must write down all the information clearly and explicitly – the examiner cannot give you marks for things that are vaguely implied.
  • Many of the marks can often be scored by having a good working diagram (even if the accompanying explanation is weak) and so you should spend time making sure that your diagram shows all the relevant details and is fully labelled. For example, make clear the exact points between which measurements, such as distance, are to be made.
  • The equipment and procedures that you describe in your answer should be realistic and workable.
  • One mark is available for describing safe working. This must relate specifically to the apparatus being used. It is not sufficient to write, for example, ‘keep all bags and coats out of the way’.
  • Additional marks are available for detailed descriptions of apparatus/techniques. There are always more possible answers than marks available, so if you write your plan carefully, then some these marks should be gained as you go along. It is not expected that you write a separate section solely for the detail marks.
  • As part of your preparation for this question you should plan some of your own experiments, but this should be done under the close supervision of your teacher. Also practise answering past papers.
  • A sketch graph is not necessary, but if drawn it should be consistent with your description of the graph.

Analysis and Evaluation question
  • See advice for Paper 3: the comments regarding significant figures, plotting graphs and calculating gradients and intercepts apply equally for this paper.
  • It is particularly important that the rules previously given for significant figures are followed.
  • You will be expected to use the uncertainty given in the raw data to find the uncertainty in calculated data. The latter will often involve a function such as a logarithm. This requires plenty of practice, if you are to be able do it with confidence in the examination.
  • You will need to be able to translate the calculated uncertainties into error bars on your graph and then to draw the worst acceptable line. Again, this requires plenty of practice.
  • Once the graph has been drawn, you will be expected to find uncertainties in both the gradient and the intercept – using your line of best fit and your worst acceptable line. A lot of marks depend on your being able to calculate the uncertainties in the calculated data.
  • Every candidate is provided with the same data and so the final values calculated should be very similar.

Paper 3 Practical skills

  • Do not panic if the context of the practical experiment appears unfamiliar. Where appropriate the question paper will tell you exactly what to do and how to do it.
  • If you find yourself in real difficulty setting up your practical equipment you may ask your supervisor for help. You will only lose one or two marks for this.
  • There are a number of things that you can do to save time: Draw a single table for your results in advance of taking any readings and enter your readings in the table as you take them (so that you do not waste time having to copy them up later). This is also important because you must record all your raw readings before you calculate and record any average readings. If the number of readings that you need to take is indicated in the question paper, do not waste time by exceeding this number. Repeat your readings, but remember that it is only necessary to repeat them once (so that you have two sets of values) – do not waste time repeating them more than once.
  • All the raw readings of a particular quantity should be recorded to the same number of decimal places which should in turn be consistent with the precision of the measuring instrument.
  • The uncertainty in a measurement can sometimes be larger than the smallest interval that can be measured by the measuring equipment. For example, a stopwatch can measure time to a hundredth of a second, but human reaction times will mean that the uncertainty in the reading given by a stopwatch is (typically) 0.1 s to 0.4 s.
  • Each column heading in your table must contain both a quantity and its unit. For instance if you have measured time t in seconds, your column heading would be written as “t/s” (“t in s” or “t(s)” would also be acceptable). The quantity or unit or both may also be written in words rather than symbols.
  • The number of significant figures used in a derived quantity that you calculate from your raw readings should be equal in number to (or possibly one more than) the number of significant figures in the raw readings. For example, if you measure potential difference and current to 2 and 3 significant figures respectively, then the corresponding value of resistance calculated from them should be given to 2 or 3 significant figures, but not 1 or 4. If both were measured to 3 significant figures, then the resistance could be given to 3 (or 4) significant figures.
  • When drawing your graph, do not forget to label each axis with the appropriate quantity and unit, using the same format for expressing column headings in a table. Choose a scale such that the plotted points occupy at least half the graph grid in both the x and y directions. The x-axis scale should increase positively to the right and the y-axis scale should increase positively upwards. Use a convenient scale such as 1, 2 or 5 units to a 2 cm square as you will then be less likely to make a mistake with the position of your plotted points and it will be easier for you to read off points from your graph if you are calculating the gradient or finding an intercept. Similarly, it is good practice to mark values on at least every other 2 cm square.
  • All your plotted points should be on the grid; points in the white margin area will be ignored. Plot all your observations and ensure that they are accurate to half a small square. A fine cross (or an encircled dot) drawn with a sharp pencil is acceptable, but be careful not to obscure the position of your points by your line of best fit or other working.
  • When drawing your line of best fit, ensure you have an even balance of points about the line along its whole length. If it is a straight line, use a clear plastic ruler so that you can see points on both sides of the line as it is being drawn.
  • Show all your working when calculating a gradient. It is helpful to draw the triangle used to calculate the gradient on the graph and to clearly label the coordinates of the vertices (accurate to half a small square). These values can then be used in the gradient calculation. The length of the hypotenuse of the triangle should be greater than half the length of the graph line.
  • If you are required to give a value for the y-intercept, it may be possible to directly read it off from your graph from an axis where x=0. If this is not possible you can instead calculate the y-intercept by using the equation of a straight line. In this case you should substitute into this equation a pair of x and y values from your line of best fit along with your calculated value of gradient.
 
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