Report on the Examination - Skinners' School Science 12,13/A level Physics past papers... · General Certificate of Education Physics ... The number of candidates who aggregated in

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abcGeneral Certificate of Education

Physics 5451/6451 Specification A

Report on the Examination January examination - 2006 series

! Advanced Subsidiary 5451 ! Advanced 6451

Further copies of this Report on the Examination are available to download from the AQA Website: www.aqa.org.uk Copyright © 2006 AQA and its licensors. All rights reserved. COPYRIGHT AQA retains the copyright on all its publications. However, registered centres for AQA are permitted to copy material from this booklet for their own internal use, with the following important exception: AQA cannot give permission to centres to photocopy any material that is acknowledged to a third party even for internal use within the centre. Set and published by the Assessment and Qualifications Alliance. The Assessment and Qualifications Alliance (AQA) is a company limited by guarantee registered in England and Wales 3644723 and a registered charity number 1073334. Registered address AQA, Devas Street, Manchester. M15 6EX. Dr Michael Cresswell Director General.

Contents

AS Units

PA01 Particles, Radiation and Quantum Phenomena .............................................................. 5

PA02 Mechanics and Molecular Kinetic Theory..................................................................... 8

PHA3/C Coursework.............................................................................................................. 10

PHA3/P Practical Examination .............................................................................................. 12

PHA3/W Current Electricity and Elastic Properties of Solids............................................... 16

A2 Unit

PA04 Waves, Fields and Nuclear Energy Section A ............................................................. 18

PA04 Waves, Fields and Nuclear Energy Section B ............................................................. 21

Mark Ranges and Award of Grades ....................................................................................... 24

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AQA GCE Report on the Examination, 2006 January series � Physics A

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Advanced Subsidiary Examination General Comments The number of candidates who aggregated in January 2006 for GCE increased by about 9% over the number who aggregated in January 2005. The number of unit entries increased significantly for the PA02 examination, (almost 18%), but in the other examinations, PA01 showed a slight decease of 3% in entries with PHA3/W showing an increase of about 9%, with almost two thirds of these candidates opting for practical rather than coursework. In general, candidates found the two examinations, PA01 and PA02, more difficult than last year, with a resulting slight decrease in the number attaining A-grade. Details are given in the individual reports. The other paper, together with the practical, was more to the candidates� liking and high marks were obtained in both examinations. For the second year running, examiners have commented adversely on the number of significant figure errors which occurred. In certain questions, such errors were commonplace, with candidates in many instances rounding down to a single figure. It was quite common in paper PHA3/W to find all the candidates from a given centre making the same error. Unit errors were no better or worse than last year. In general, the Quality of the Written Communication was adequate, although there seemed to be more candidates than usual who gained zero marks for this exercise. PA01 Particles, Radiation and Quantum Phenomena General Comments As already mentioned, compared to recent examinations in this unit, candidates found this examination difficult. There was a long tail of less able candidates who made errors rarely seen in previous papers, and a very large number of candidates who made unit errors or calculation errors in almost every question. Even the more able candidates had misconceptions that were expressed in their answers. In particular Question 2 (b), set on the photoelectric effect, prompted more incorrect answers than correct answers for the reason that individual atoms were discussed, rather than the whole metal effect. The other area in which the more able candidates failed to produce good answers occurred in Question 6 (b); very few candidates considered electrons in the higher energy levels cascading down to fill the vacant ground state. As usual, the optics question discriminated well and the more able candidates achieved high marks in this question. Question 1 Answers to this question generated many errors. Less than 50% of the candidates gained full marks for what was really an easy starter question. In part (b), the incorrect answer of +4e appeared very frequently and likewise, in part (c), the incorrect inclusion of the electron mass was a very common occurrence. Additionally, calculation errors, failure to give the correct units and significant figure errors all contributed to an overall poor performance. Question 2 Part (a), in general, performed well, even though the question asked for the definition of the work function in an indirect way. In the early years of the current specification, candidates often interchanged ideas about photoelectric emission with ionisation. In latter years candidates have been much better at distinguishing between these two phenomena, but unfortunately these misconceptions returned in part (b) this year.

Physics A � AQA GCE Report on the Examination, 2006 January series

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Previous questions have focused mainly on the distinction between the energy and intensity of the incident light. The current question asked about the range of kinetic energy of the photoelectrons, which was a new approach. In their search for an answer, the majority of candidates either thought that the electrons must be in the excited states of the metal atoms or that they absorbed only a fraction of the photon energy. Although part (c) was a straightforward calculation, it did discriminate, simply because of the errors generated. Question 3 Normally the question concerning fundamental forces and particles is answered well, but this time very few candidates scored full marks. Part (a) (i) gave rise to very few problems to the prepared candidate, but in part (a) (ii), the usual answer gave only one role played by the exchange particles in the interaction, thereby losing a mark by omitting to give a second role. Another common error was to suggest that the exchange particle somehow gave energy or momentum to the interaction, rather than transferred energy or momentum. More able candidates had no trouble with part (b), but the less able candidates failed badly by not identifying all the examples given. The π0 particle was accepted as a possibility for an antiparticle, being its own antiparticle, but it does not appear as a required answer. Question 4 This question discriminated very well, again because of the errors incurred. In addition to getting the basic physics wrong, all the following errors were seen; using the wrong numerical data when substituting for symbols, incorrectly rearranging the equations, calculation errors, failing to give the correct units and giving the answer to the wrong number of significant figures. Some of the more able candidates made more work for themselves in part (ii), by trying to convert the numerator and denominator to Kg, rather than cancelling out the units at an early stage in the calculation. On the positive side, nearly all the candidates had very little difficulty in finding the necessary information about the particles from the Rest Energy column in the Data Sheet. Question 5 Answers to this question were relatively good. The typical candidate knew the appropriate equations to use in part (a) and obtained good marks. The less able candidates, however, failed to come up with the correct equations when there were two media involved, or failed to choose the correct data to substitute in the equation. In part (b), the explanation given by many less able candidates was insufficient to gain the allocated mark. Simply stating �it is less dense� was quite a common but unsatisfactory answer. The diagram in part (c) was completed quite well by most candidates, but many showed the final ray emerging at 90º to the surface. Question 6 Part (a), on the whole, was answered quite well. The common error which occurred in part (a) (ii) was to multiply rather than divide by the electron charge. In part (b), the more able candidates considered the electron emitted from the atom in an ionisation process, the electron taking with it the remaining energy as kinetic energy. Most candidates did discuss ionisation and obtained a mark, but many thought an electron was only promoted up a few energy levels.


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In part (b) (ii), only the most able candidates considered electrons in higher orbits cascading down energy levels, emitting photons as they eventually occupied the ground state. The incorrect answer of D → C and C → B in part (c) was very common, probably because the gaps between the lines in the diagram looked approximately the same.


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PA02 Mechanics and Molecular Kinetic Theory General Comments The performance of candidates in this examination was a little patchy, and many found the paper difficult. Questions 2 and 3 were particularly discriminating and a significant proportion of candidates scored less than half marks in these questions. Presentation was generally good and candidates set out calculations so that there was a logical structure to their answers. Significant figure errors were common and this was most noticeable in Question 5. Quality of Written Communication was good and most candidates structured their answers to the relevant questions well. Question 1 For the most part this question was answered well. In part (a), a significant minority of candidates did choose inappropriate scales for the graph, but plotting errors were rare. The calculation for the average acceleration in part (b) was done well but that for the distance travelled, in part (c) was less so. Most candidates appreciated that they needed to estimate the area under the graph, but many failed to do this within the allowed range. Parts (c) and (d) caused problems to a number of candidates because they did not understand the term resultant force. This was often confused with air resistance which, candidates correctly assumed, increased with speed. The mark scheme did allow candidates to score marks in part (d), even if they had misunderstood resultant force, provided their description of the effect of an increasing resistive force was correct. Question 2 Candidates found this question far less accessible than expected. Most were able to answer part (a) well, but found it difficult to describe the energy changes that took place in the described experiment. There was much confusion as to exactly how energy conservation might be demonstrated and a common mistake was to assume that gravitational potential energy was converted to kinetic energy in either the mass or the trolley alone. There was also a tendency for candidates to write about momentum conservation and elastic and inelastic collisions. Question 3 Although this question was similar to Question 6 in the January 2005 examination, candidates found it far less accessible this time. This was probably due to the trolley having two sets of wheels in contact with the ground as opposed to only one set in the previous paper. This meant that unless candidates were familiar with this type of problem, there was no obvious pivot point about which to take moments. This unfamiliarity was noticeable in the calculations for part (b), which caused difficulties for large numbers of candidates, and it was clear that not all centres had given candidates experience of this type of question on moments. Part (c) proved to be even more difficult and only the more able candidates were able to calculate the force necessary to lift the front wheels off the ground. Part (d) was answered consistently better, with even less able candidates explaining why the required force would be less than that in part (c). Question 4 Many candidates scored full marks in this question. Less able candidates created the usual confusion between the Kelvin and Celsius scale, but this was less of an issue than has been the case in the past. Candidates are clearly benefiting from practice with thermal energy questions. Question 5


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Again many candidates scored high marks in this question. There was a tendency to incur a significant figure penalty for either quoting an answer to too many significant figures or rounding down to one significant figure. Part (b) caused problems for less able candidates who, although correctly determining the change in number of moles, were then either unable to calculate 23% of the number or else did not multiply the change in number of moles by Avogadro�s number. Question 6 Responses to this question were extremely mixed with the term resultant force once again confusing a significant proportion of candidates. The tension was resolved correctly very frequently, but the next step of calculating the resultant of the two horizontal and the two vertical components was often omitted. Part (b) produced better responses, although a significant minority failed to answer this part at all. There were also a number of candidates who thought the weight of the block must be 30 N. The written explanation in part (b) (ii) generated better responses than had been found in answers to questions of this type in previous papers, but there is still much confusion about when it is appropriate to apply Newton�s Third Law. Comments such as �the tension is the action on the block, so the weight is the reaction on the block� are still quite common. Candidates who used Newton�s First Law were far less confused and were able to explain correctly how the weight was determined. PHA3/C Coursework


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In line with past January examinations, only a relatively small number of centres submitted new work for this examination. Although most centres completed the administration procedures correctly, moderators did report several cases where samples were either late or incomplete, with unsigned or missing cover sheets. Whilst in general the quality of annotation was good, a small but significant proportion of centres failed to adequately annotate the work submitted. It should be noted that every marking point must be annotated at the precise point where the mark was awarded. The annotation should be written in the form of �A4b� or �B6a� etc. referring to the appropriate marking point. Written comments are also helpful, particularly in clarifying why a particular �marginal� point has been awarded. The use of a suitable marking grid to record the individual marking points is also strongly recommended since this makes it much easier to interpret the hierarchical scheme and determine the total mark for each skill. Unfortunately, this was omitted in some cases. Overall, a significant number of adjustments were made to marks submitted by centres. It was also disappointing to find that a small number of centres had incorrectly applied the hierarchical scheme, apparently using a �best fit� approach. Mark adjustments were mainly due to misinterpretation of specific points in the assessment criteria, and, as in previous years, these often involved skill A4c (fully labelled diagrams), B6a (significant figures) and C4c (unsuitable graph scales). Due to the hierarchical nature of the scheme, an error in interpretation of the criteria can cause a significant adjustment to the overall mark, for example, a candidate who, in the opinion of the moderator, has failed to achieve A4c will be limited to a maximum mark of three for planning. If this mark had been awarded by the centre it could result in a mark change from eight to three. In most cases the investigations used were appropriate, allowing candidates access to the full range of assessment criteria. Experiments on measurement of resistivity and emf/internal resistance were again very popular and were successful in allowing a full range of marks to be achieved by candidates. Unfortunately, there are still a small number of centres who presented investigations which were too simple. This can limit the total mark which can be achieved in some skill areas, for example, refractive index of glass by tracing a ray of white light, Hooke�s law for a simple spring. As in previous examinations, a small proportion of candidates made use of ICT. Whilst appropriate use of ICT is to be encouraged as part of investigative science, many candidates were penalised through presenting graphs and results tables which did not meet the assessment criteria. Where spreadsheets were used for tabulated data, inappropriate numbers of significant figures were often quoted. This was mainly due to �dropping� of the last zero. Candidates should be aware of this and make suitable corrections. Only a few ICT graphs were presented, and again a significant number did not meet the relevant criteria. Candidates should produce a graph which covers a full side of A4 paper, together with a suitable title and fully labelled axes. Points should be plotted as points or crosses and not shapes, such as large squares or diamonds, which make precise location of the plotted point more difficult. The line of best fit should be drawn taking account of any anomalous points. The graph should have suitably labelled gridlines so that accurate readings for gradients or intercepts can be recorded. A suitably large triangle for measuring the gradient must also be shown.


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The advice which follows addresses issues raised by moderators on the marking of specific skills. Many of these points were also discussed in the recent series of Teacher Support Meetings held last autumn. In skill A, there were still a few cases of candidates failing to consider safety issues, thereby effectively limiting their mark to a maximum of one. To achieve A4c, diagrams must be two dimensional, fully labelled and the dimensions being measured must be clearly indicated. As mentioned previously, this point was often misinterpreted by centres, and frequently caused a significant adjustment in the marks awarded. This was usually because a length being measured was not indicated on the diagram. To achieve A6d, full instrument specification is required and for electric meters this requires both range and sensitivity. In skill B, some candidates failed to take sufficient readings with appropriate repeats to achieve B4c. In an experiment to investigate the variation of resistance with the length of wire, it would be expected that candidates would take at least seven or more different lengths of wire, with repeat readings for resistance at each length. Some centres awarded this mark incorrectly when candidates had only taken five or six different readings. Quoting results to an inappropriate number of significant figures was the main cause for concern in skill B. This usually occurred where a length measured to the nearest mm was quoted only to the nearest cm, for example, 0.20 m rather than 0.200 m. This failure often resulted in a mark adjustment from eight to five on this skill. In B6d, candidates must clearly identify the significant source(s) of error which occurred in their experiment. Although they might have suggested in the planning stage particular sources of error, a further statement would be required after the results have been taken to confirm whether or not this was still considered to be the most significant error. In skill C, to achieve C4c an appropriate scale must be used so that the plotted points occupy more than half the length of each axis. If this makes it impossible to read a particular intercept directly, a suitable calculation should be done instead. Inappropriate graph scales was the most common reason for adjustment of marks in this examination series, and a mark adjustment from eight to three on this skill was not uncommon. C4c was also awarded for graphs with incorrectly plotted points, missing titles and missing units on scales. In skill D, the majority of candidates scored less than in the other skill areas. In many cases the comments made by candidates were again very superficial or vague. Many candidates failed to achieve all four marking points in D2, effectively limiting their mark to a maximum of one for this skill. In particular, for D2b a simple statement concerning discrepancies or anomalous results is required. For D2c, candidates must state whether there is much variation in their repeated results, indicating the level of uncertainty in the data. In D4b, candidates frequently calculated errors based on instrument sensitivity only, often giving unrealistic error estimates. Where possible, the error estimate should be based on the spread of repeated results, for example, in an experiment to investigate the variation of resistance with the length of wire, the error in length might reasonably be based on the accuracy of the rule (+ 1mm). The error in resistance, however, should be taken from the spread of repeated readings, and not from the sensitivity of the meters used. In D6a, a large proportion of candidates were unsure of the difference between random and systematic errors. In D6b/c, some reference to a quantitative estimate of error or uncertainty is usually required.


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PHA3/P Practical General Comments A 21% increase in the number of candidates in January 2006 followed the 17% increase in 2005. Although more than 50% of centres entered only three candidates or less, more centres than previously had large entries. The paper performed in a similar fashion to that set in 2004 with a mean mark of 2.7, higher than that in 2005. There were accessible marks throughout the paper for candidates at both A and E grade boundaries, and very few scripts were seen in which candidates were unable to make some progress in either question. The general trend of improvement in the candidates� work at AO3a continues. Candidates at the A and E grade boundaries scored typically one or two marks higher in Question 1 than last year. The technique for success with this style of question centres on structuring the answer in response to the bulleted points which are given at the end of the printed question. There was plenty of evidence in the scripts of careful planning and the use of clear sketches in support of the text. At least in AO3a, more candidates than previously seemed to be writing in a clearer and more focused way, although for others there is still an urge to fill every bit of the available space with repetitive material. The one aspect lacking in most candidates� answers is a clear explanation as to how suggested procedures overcome difficulties in the proposed experiment. Question 2, dealing with optics, was simpler in terms of manipulation, processing and analysis than either of the questions set in 2004 and 2005 on statics, but yet the mean mark for all candidates in AO3b was slightly reduced compared with the 2005 mean mark. This was mostly due to candidates giving the initial measurements, made with a ruler, to the nearest centimetre rather than to the nearest millimetre or half millimetre, as appropriate. Other candidates lost marks because of poor tabulation techniques, such as the omission of a separator between quantity and unit in the table headings. The same candidates almost invariably did not repeat the error in marking the axes of their graph. Although a very large proportion of candidates plotted graphs using only five of the six sets of data recorded, the quality of the graphical work was generally good and led to a gradient value that was nearly always creditworthy. The mean mark in AO3c was higher than that in 2005. As usual, most candidates found AO3d the most difficult skill and 80% failed to get more than 3 out of the available 6 marks. Although E grade candidates did slightly better in AO3d than in 2005, the performance of A grade candidates was unchanged. Significant numbers of the candidates stated that the light ray was diffracted as it passed out of the semicircular block. Despite the wording in the question, many candidates attempted to explain (e) (iii) by stating that the student had used a rectangular block rather than a semicircular block. Question 1 Candidates were required to design a procedure to determine how the deviation of a vertical stream of water, flowing over a table tennis ball, varied as the angle made by the thread supporting the ball was increased. Many candidates used the space below the question on page seven to draw a diagram and/or plan out the answer they would give on pages eight and nine. Details as to the use of measuring instruments or that a plumb line could provide a reference were evident, and ensured that credit would be given. Many took the hint given in the question (that the largest angle to be measured should not exceed 15°) to suggest that trigonometry, based on two linear measurements, could be employed to find the angle made by the thread. Once again, diagrams and suitable algebra made interpretation of the candidates� intentions clear. In other instances, diagrams revealed misconceptions such as the incorrect assumption that the distance w


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between the stream and the vertical would be the same as the horizontal distance moved by the point of suspension, S, of the thread. The means of taking measurements were rarely omitted from the plan, although steps to provide a reference, against which the deviation of the stream or the angle made by the thread could be measured, were frequently overlooked. Of those candidates opting for a trigonometric solution, hardly any explained how to establish some reference point or position against which the linear measurements could be made. In establishing a reference for w, candidates were expected to find and mark the position of the undeviated flow at the base of the sink. Some candidates drew sketches based on the middle diagram in Figure 1, but with the ball hanging vertically, thus failing to appreciate that any force applied to the water must produce an equal and opposite force on the ball. Nearly all candidates explained that they would move point S to obtain further values of α and w. Further credit could then be gained by stating they would plot a graph, provided some clear justification was given for this (for example, to see which of the predictions made about the outcome was correct). Credit was given for identifying that the flow rate of the water stream should be controlled, although many candidates simply stated that the �flow should be constant�, which did not convey the correct idea. A mark could also be gained for stating that the vertical height of the ball relative to the tap (or relative to the sink) should remain constant. The fact that very few candidates gained the maximum mark of eight was mainly to do with their limited success in identifying good procedures to overcome difficulties in making accurate measurements. Candidates tend to make generalised comments about repeating measurements and averaging; but this is such an intrinsic and fundamental idea in practical work that the examiners are looking for something extra before credit is given. Few candidates realised the benefit of increasing the flow rate or of using a large radius protractor. Of those who chose the use of trigonometry to find α, hardly any explained that this would reduce the uncertainty in calculating the small angles involved, particularly if the thread attached to the ball was lengthened. Candidates typically used all the space available to them on pages eight and nine of the answer booklet, but thankfully, few felt the need to use supplementary sheets. Question 2 Candidates were required to trace the path of a light ray through a semicircular transparent block. In comparison with some recently set experiments, this was a straightforward task to perform, and the processing presented the majority of candidates with only few problems. Marks were lost by a surprising number of candidates in recording the diameter of the circle on the printed insert and/or in recording the initial data set. All these and subsequent measurements, were made with a ruler capable of reading to the nearest millimetre, yet a majority of candidates felt that the values of D and x in part (a) could be given as 19 cm and 6 cm respectively, rather than 19.0 cm/190 mm and 6.0 cm/60 mm. Such errors incurred the significant figure penalty. Some candidates have been taught that rulers can resolve to the nearest half millimetre; this was acceptable provided all the measurements were recorded as .0 mm and/or .5 mm. Candidates should understand that raw readings should reflect the resolution of the instrument with which they are made. Derived data should be given to the number of significant figures determined by the processing that generated it. In the case of both raw and derived measurements, the examiners expect consistency throughout the tabulated data. It was for this reason that candidates who gave D as 19.0 cm and the initial x as 6.00 cm lost a mark.


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Marks were deducted for poor tabulation. Candidates should understand that examiners insist on seeing a variable (usually a symbol) followed by a separator (solidus) and unit, e.g. z/mm, before awarding marks. Units cannot be allocated to a separate row in the table below the variable, nor can the order of symbol/unit be reversed. It was also puzzling to find that the initial data set of x, y and z was largely ignored, many candidates only considering the five additional sets when plotting their graph. This error lost one mark from the graph and often led to the loss of the quality mark, awarded here for plotting five points to no more than 2 mm from the best-fit line. Very few candidates failed to carry out the procedure described in the question and it was rare to find the tabulated data covering too small a range. The graph provided a useful measure of discrimination in favour of the more able candidates. The majority of A grade candidates scored seven or eight marks out of the maximum eight in AO3c. E grade candidates scored three or four marks out of the same maximum and almost invariably plotted five rather than six points. Relatively few candidates used difficult scales although some compressed one or the other of the scales with no apparent logic, as the inclusion of an origin had no impact on the decision. It was noted that some candidates were forced into a compressed horizontal scale because they drew the axes too far from the left-hand margin of the grid. In some cases these candidates had to plot the sixth point off the grid in the right-hand margin for which credit could not be given. Gradient calculations were generally successful and the result obtained usually fell inside the range for which credit could be given. When explaining the procedure used to position the semicircular block, candidates were expected to state that they located and then marked the centre of the diameter printed on the loose insert. They were then expected to use the set-square to align the flat edge of the block with the diameter and then apply some logical test of their own making to align the centre of the flat edge with the marked centre of the circle. The examiners expected that the need to mark the centre (and the perpendicular through it, on which to align the flat edge of the block) would have been fairly obvious to candidates, because finding the centre repeatedly before repositioning the block would be time consuming and could also lead to inconsistency and error. Surprisingly few A-level candidates knew the correct term for a set-square and many felt that its principal use was as a measuring instrument. While some candidates gave simple concise answers that were almost identical with the mark scheme, others displayed poor vocabulary and muddled thinking, confusing �parallel� with �perpendicular� or �level� with �aligned�. A few of the candidates saved themselves by providing a sketch of the steps in their procedure. Many of the candidates who omitted to state that they had marked the page to show the position of the block, were quick to point out that the student in (e) (ii) was remiss in not doing so. Others thought that the student�s mistake was not marking arrows on the diagram to show the direction of the light rays. A more common view was that the student had used a rectangular block in error, although careful inspection of the diagram would show that the incident and emergent rays were not parallel. A more imaginative (but still unsuccessful) idea was that the block had been positioned so that light entered through the curved face; the wording in the question told the candidate that this was not what had happened. A good number of candidates did realise however, that the light had not entered the block at the centre of the flat edge because had it done so it could not refract as it passed out of the block. Why the light should not be refracted had the apparatus been properly arranged was a point that eluded most candidates. Of those that correctly deduced what was going on, roughly half argued (mostly correctly) what should have happened while the remainder explained the situation it terms of what did happen. Providing that the physics was correct both groups could gain full credit, although sadly for some they overlooked the need to mention what mistake the student had made.


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AO3d remains the hardest skill area in which to gain marks. These questions are set in the context of the work that the candidates have just performed. A good practical physicist should review the procedures carried out while the work is underway, asking �am I doing this in the best possible way?� or �is there a simple test I can apply to tell whether I�ve done this correctly?�. By acquiring such habits candidates will become much better placed to anticipate and respond to the types of question raised in AO3d.


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PHA3/W Current Electricity and Elastic Properties of Solids General Comments All five questions on this paper proved to be very accessible to the candidates, with many scoring high marks, and very few failing to score less than ten. All the marking points were awarded. The examiners expressed concern at the large increase in the number of significant figure errors. In fact, in many instances nearly all the candidates from a given centre were failing to gain these marks. On the other hand, errors involving units were few and far between. Most of the answers to sections where marks for the Quality of Written Communication were being awarded appeared to have been well written, but the examiners were conscious that more candidates than usual gained no marks for the written quality of their answer. Question 1 This proved to be a comparatively easy question, with many candidates gaining full marks. Most of the errors in part (a) were due to the equation relating charge, time and current being incorrect. The necessary conversion of the time involved into seconds seemed to be well understood, but the same point in part (b), namely the conversion of hours into seconds, proved to be more open to errors. There were very few arithmetical errors. A consequential error in part (b) (ii) ensured that even if the incorrect answer was obtained in part (b) (i), candidates could still gain the allocated mark for the power of the bulb. Question 2 Although the type of question set in part (a) has appeared in previous examinations, many candidates lost marks unnecessarily due to the use of incorrect significant figures in part (i), the usual answer being given as 0.04 A or as a fraction or as a repetitive number. None of these are acceptable. The type of answer required in part (a) (ii) should, by now, be familiar to candidates. The question requires a candidate to reason through the problem without the aid of a calculation. It was pleasing to see how many candidates realised that not only did the resistance in the parallel resistance section decrease, but that this also produced a decrease in the total resistance of the circuit. This step in the answer enabled many candidates to gain full marks. A surprising number of candidates did carry out a calculation and were duly penalised. The calculation in part (b) was usually correct, but the majority of candidates either ignored the request in the question to state the assumption made or were not aware that a voltmeter has a very large, or infinite resistance. The ideas of short circuiting a section in part (c), is now well understood and the large majority of candidates obtained the correct answer. Question 3 Questions requiring a description of an experiment invariably perform well and part (a) was no exception. Many candidates produced clear, concise and logical answers. This could be partly due to the fact that measuring the resistivity of a wire is a popular experiment for coursework. In the circuit diagrams submitted, the only point that attention needs to be drawn to is when candidates use a potentiometer rather than a variable resistance. It is important that the ammeter should be placed in the correct position to measure the current through the wire, and not measure the current through the battery. Many of the circuit diagrams did not include the resistance wire, the candidates seemingly being confused between the resistance wire and the variable resistance. Unfortunately, candidates who produced such a circuit invariably penalised themselves not only in part (i) but also in part (ii).


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When listing, in part (ii), the measurements which needed to be taken, a mark was usually lost by candidates who stated that the area of cross section, rather than the diameter, had to measured. Several candidates referred to the diameter as the thickness of the wire, a term which was not acceptable. The other point which was missed quite frequently, was the need to make a series of measurements of voltage and current by altering the variable resistor. Another point which needs to be stressed is that many candidates stated that the length of the wire was changed, contradicting the evidence of their own circuit diagram, which showed a fixed length resistance wire. In part (iii), the majority of candidates gave the correct equation relating resistivity to the other parameters and stated how to calculate A from the diameter and how to calculate the resistance R of the wire. Most candidates used a graph to obtain R, with those who used a variable length using a graph of length against R, calculating ρ from the gradient. Very few candidates failed to obtain the correct value for the length of wire in part (b). Question 4 Explaining what is meant by emf in part (a) is still beyond the capabilities of a very large number of candidates. The most popular acceptable definition was that of the voltage across the terminals when no current flowed. Many candidates attempted to define emf in terms of the energy produced in the battery, but either forgot, or did not know, that it was the energy per unit charge. In part (ii), the majority of candidates were aware that the reason involved the internal resistance, but merely quoting internal resistance on its own was not sufficient to gain a mark. There must be some reference to the voltage or pd across this resistance when a current flows. The graph section in part (b) was answered well by the large majority of candidates, who drew excellent graphs. Some candidates missed out by using almost impossible scales in their bid to use the full page of the exam paper. Again, most candidates, having produced the correct equation in (i), knew how to obtain ∈ and r from the graph. Some candidates, having produced an acceptable value for∈, proceeded to insert values in their equation. This was not acceptable. Another incorrect method was to obtain ∈ from the area under the graph. Question 5 The definitions of tensile stress and tensile strain in part (a) were usually correct, the most notable omission being not defining A as the area of cross-section rather than just the �area of the wire�. There were some very good descriptions given in part (b) (i), but many candidates used the question as an opportunity to write everything they knew about stretching a wire, with elastic limit, yield point, break point etc. thrown around at random. For some reason, many candidates assumed the wire broke at the point C, but then went on quite happily to describe what happened as the masses were unloaded. The remainder of part (b) met with mixed success. The fact that the wire was ductile was usually well known and also what AD represented, although phrases such as �wire was deformed� were not accepted, neither were the shape or size of the wire. It is quite obvious, and candidates should be made to realise it also, that the shape of the wire does not change when it is permanently extended. Parts (iv) and (v) in (b), suffered due to candidates not being precise, for example, the gradient of the graph was not sufficient; the line AB had to be specified. Likewise the area under the graph in (v) had to be the area under the graph ABC. The calculation in part (c) was usually correctly performed.


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Advanced Examination Since the introduction of this specification the largest entry in the January examinations at A-level has always been for Unit 4, followed by the Synoptic paper and then the option units. The current series of examinations differed from previous series in that only Unit 4 was offered for examination. This resulted in a 23% increase in the number of candidates taking this unit. The additional candidates were quite proficient at answering the objective test questions, but answers to the longer questions in section B showed that a large number of candidates were inadequately prepared. Full particulars are given in the detailed reports which follow. PA04 Waves, Fields and Nuclear Energy The reduced availability of tests in the optional components of A2 (Units 5-9) in 2006 brought about a shift in the entry pattern for Unit 4. Centres entered almost 3800 candidates for the January 2006 examination, an overall increase of 23% from just under 3100 candidates in January 2005. The results showed that many of the additional candidates had developed the necessary skills to give a proficient performance when tackling the objective questions of Section A, but were still short of practice when assembling their answers to the longer questions in Section B. Section A Objective Test Questions The keys to the objective test questions were:

1C

2C

3B

4C

5A

6D

7B

8B

9A

10D

11C

12A

13B

14B

15D

The facility of a question is a measure of all candidates attempting a question who choose the correct option. The mean facility of this paper was 66%, compared to 62% and 63% respectively in January 2005 and June 2005. The facility for individual questions ranged from 79% for Questions 7 and 14 to 48% for Question 11. The point biserial index of a question is a measure of how well the question discriminates between the most able and the least able candidates. The mean point biserial for this paper was 0.39, rather lower than the value of 0.44 produced by each of the 2005 tests. Candidates' responses showed that as many as eight of the questions (Questions 1, 2, 3, 4, 6, 7, 14, and 15) proved to be easy, with facilities over 65%, whilst no question was found to be difficult. Two of the questions had been used in earlier Unit 4 tests, and candidates' performance on both questions this time was appreciably better than that on the previous occasion. One question had been used in a previous Advanced Level examination and the candidates again performed better in January 2006 than previously. Data from the pre-testing of the questions has shown this test to be very similar in its demands to the 2005 tests. The mean facility of the questions increased by 12% over the pre-test values, which represents a significant advance on the 9% achieved in January 2005. Question 1 involved deciding when a particle oscillating in simple harmonic motion experiences its greatest negative acceleration. Almost four-fifths of the candidates chose the correct answer, at maximum positive displacement. The most common distractor was D, representing maximum negative velocity, which was chosen by 11%.


19

The outwardly more demanding Question 2, which had appeared in a previous PA04 test, was also on shm. It required an algebraic expression for the magnitude of the acceleration of a body when at maximum displacement. This time 75% of the candidates gave the correct response, but perhaps it was the same 11% of them as in Question 1 that were tempted by distractor A (zero acceleration). This was the most discriminating question on the paper. Question 3 required familiarity with the relative wavelength of audible sound waves and light waves. Almost three-quarters of the responses were correct, with the most popular distractor, C, again chosen by 11% of the candidates. The discrimination of this question was the least satisfactory of any on the paper; in fact the question was far more discriminating when it was pre-tested. Question 4, dealing with the formation of stationary waves, had been used previously in an Advanced level examination. The facility of the question improved from 61% last time to 73% in 2006. The 19% of candidates who took the inter-nodal separation value to be λ , instead of λ / 2 , went for distractor A, but the question produced good discrimination between candidates� abilities. Quite a lot of calculation was needed to arrive at the right answer in Question 5, where the separation of double-source interference fringes was the topic. 56% of the candidates made the correct choice. Almost a quarter of responses were for distractor C, no doubt because these candidates had seen interference fringes in red and blue light produced by slits having the same separation; in this question the source separations for these wavelengths were different. Question 6 involved the charging of a capacitor by a constant current. The two-stage calculation, using Q = CV and I = Q / t , caused no difficulty for 69% of the candidates. Incorrect responses were almost all divided between distractors A and B, with very few selecting distractor C. Question 7 required the angular velocity of the Earth�s surface. This proved to be one of the easiest questions, with a facility of 79%. The remaining candidates split their responses almost equally between distractors A, C and D. The question gave good discrimination. The angular velocity of the Earth was also to be considered in Question 8, but for points at different heights and therefore at different gravitational potentials. This was more demanding than Question 7, as is shown by its facility of only 51%. The common value of ω for the whole Earth was not always appreciated. Over one-fifth of the candidates chose distractor A, where the higher point was supposed to have a greater value of ω, whilst almost as many selected distractor C (a smaller ω at the higher point). The direction of forces in gravitational, electric and magnetic fields continues to be an area of misunderstanding, as illustrated by the responses in Question 9, which had a facility of 55%. Despite the fact that this question was about gravitational fields, just over a quarter of the candidates selected distractor C, where the force is supposed to be at right angles to the field. This confusion with a magnetic field is no more understandable than that of the 11% who chose distractor B, where the force would be in the opposite direction to the field. Perhaps this latter group were thinking of electrons in an electric field. Such incorrect responses suggest that candidates were not always reading the questions with sufficient care. Question 10 was a test of understanding of the speed, acceleration and kinetic and potential energies of an α particle during an encounter with a nucleus. Unusually, the question�s facility of 62% was no improvement over the value obtained when it was pre-tested. The most common incorrect choice was distractor B (17%), which would imply that the α particle would travel fastest when closest to the nucleus.


20

Question 11 turned out to be the most demanding question on the paper, with only 48% of the candidates able to find correctly the magnitude of the force on a charge in a uniform electric field. The main

difficulty appears to have been that of deciding that the field strength is dV rather than

)2/(dV , because

35% of the candidates chose the incorrect distractor A. The discrimination of this question was quite weak. Question 12 was correctly answered by 61% of the candidates. It involved the topic, familiar from previous tests, of the forces acting on the sides of a current-carrying coil when in a uniform magnetic field. This is another question that had been re-banked after use in an earlier PA04 test, when the facility had been only 53%. Evidently, a greater majority were able this time to spot that sides PQ and RS would experience no forces when I and B are parallel, but distractors C and B were chosen by 18% and 15% respectively. Difficulties over rearrangement of the algebra no doubt caused 20% of the candidates to choose distractor A and 13% distractor C in Question 13, but the facility was still 55%. This was another two-stage calculation, where Bev, mv2/r and 2πr/v were all to be combined to find the time for one orbit of a proton in a magnetic field. Question 14, despite its apparent complexity, was one of the two easiest questions on this paper. The facility was 79%. Failure to subtract the mass equivalent of the energy released, Q, from the total mass of the particles undergoing fusion caused 12% of the candidates to select the incorrect distractor C. Question 15 gave a spectacular advance in facility from 47% when pre-tested to 75% under full examination conditions. Clearly candidates make more effort to learn the facts of nuclear reactors when revising for real examinations than when preparing for a pre-testing exercise. The most common incorrect choice was distractor A (selected by 12%); this involved an understandable preference for lead over concrete as the shielding material, but also the incorrect use of carbon for controlling, rather than for moderating, the reactions.


21

Section B Much of the work presented in the scripts showed, once again, how difficult it can be to cover all the material in Unit 4 adequately in time for the January examinations. There were many examples in which candidates� answers to different questions showed complete competence in one area of the specification, but a fairly total absence of knowledge in another. No pattern seemed to emerge overall about which parts had been overlooked or omitted. In general, Section B was found to be demanding, but marks of over 40 out of 45 were still attained by the most able candidates and the test produced a good spread of marks. A majority of the candidates seemed short of ideas when attempting to answer the specific requirements of part (b) in Question 1, whilst part (b) of Question 5 often caused candidates to give irrelevant wordy descriptions that were devoid of mathematical content. Quite often candidates did not attempt the whole of Question 5, whilst sometimes it was just part (b) that had been omitted. Examiners were not sure whether this was due to a lack of time, or due to a lack of familiarity with the mechanics of charged particles in an electric field. Carelessness over missing or incorrect units in final answers was more prevalent in this test than has been the case recently. Significant figure penalties were also imposed more often than has usually been necessary in the last two years. Candidates again need to be reminded to retain more significant figures in intermediate stages of a calculation than are required in the final answer. When (as is usual) two significant figures are appropriate in the final answer, at least three should be retained in the earlier working. Failure to do this often causes the final numerical answer to be outside the acceptable limits. Many more candidates now take care over the Quality of their Written Communication, especially in the sections where it is to be assessed. In this test, written communication was assessed primarily in Question 3 (b). Question 1 Good progress was generally made in part (a), but the unit of the spring constant was not always correct and often omitted. Clear and concise answers were common, usually allowing all four marks to be awarded. The most common difficulty occurred where the candidate thought that k = (m/e) instead of (mg/e); these candidates were then unable to show that the frequency was 1.5 Hz. Part (b) caused great difficulty for a majority of candidates, many of whom seemed to have little or no detailed knowledge concerning forced vibrations and resonance. Phase relationships proved to be particularly demanding, although the mark scheme was adopted and made it possible to score all six marks without referring to phase at all. Responses were often confusing, making it difficult for examiners to decide whether the frequencies and amplitudes referred to were those of the support rod, the spring, or (as the question intended) the masses. More candidates ought to have realised that phase could only be correctly described by comparing the oscillation of the masses (the driven system) with that of the support rod (the driver). They should also know that phase is measured by an angle, not a wavelength. There were many references to the frequencies and amplitudes of waves, and even to interference. Perhaps the rather simple demonstration that formed the basis of this question should receive greater prominence when teaching the characteristics of vibrations. Question 2 Candidates with a sound knowledge of capacitors and capacitor discharge had little difficulty in gaining all six marks. However, it did seem that some centres had not been able to cover these areas fully (if at all) in time for the January examination; candidates from such centres were frequently unable to make anything of the complete question.


22

Almost inevitably, misunderstanding of E = ½ Q V in part (a) led many candidates to believe that the pd at 25 s would be 3 V . These candidates were then unable to arrive at a time of 36 s for the time constant in part (b), but could still access both marks in part (b) (ii). Many excellent responses were seen in part (b) (i), where familiarity with logarithmic solutions to exponential relationships was almost essential. Examiners gave no credit in part (a) to those candidates who attempted an exponential solution by using the 36 s given in part (b); a successful solution had to come from the energy information. Similarly, only one of the two marks in part (b) (i) was available for those who turned the question on its head by showing that V would be 6 V after 25 s, if the time constant were 36 s. Question 3 Many answers to part (a), which for full credit required explanations rather than statements, would have benefited from a more precise use of terminology. In part (a) (i) for example, �the fuel runs out� is much inferior to �the amount of fissionable uranium remaining decreases�. In part (a) (ii), a mark was awarded for neutron bombardment causing radioactive instability, but the real reason is that the fission fragments themselves are unstable neutron-rich β and γ emitters. Furthermore, since some of them have very short half-lives, their activities can be very high. In part (b), the treatment and handling of spent fuel rods was often understood very well. However, the need to use remote control, and to take precautions over the transport of used material, was commonly overlooked. Credit was given equally to procedures involving reprocessing and to those where the spent rods are simply buried deep underground in stable rock formations. Question 4 In part (a), the principal features of geo-synchronous satellites were known very well, as was their ability to provide uninterrupted communication between devices that do not need to be steerable. Less able candidates sometimes strayed into extensive discussions of weather and surveillance satellites in this part. In part (b), the examiners were surprised to find how few candidates could give a correct answer for the force equation in terms of G, M, m, ω, R and (especially) h. This was particularly disappointing when the candidates had shown, by the algebra they produced, a good appreciation of the underlying principles. Attempts to answer part (b) (ii) often revealed a determination to arrive at the given equation, irrespective of whether the algebra could be made to hang together. Many very thorough and complete answers were given in part (b) (iii). The anticipated route was by substituting h = 0 in the earlier equation, to show that a satellite in a grazing orbit has a period of 85 minutes. Equally convincing were those solutions which used a period of 85 minutes (5100 s ) to show that h = 12 km, which would be within the Earth�s atmosphere. The answer to what happens to the speed of a descending satellite in part (c) seemed to be an arbitrary choice for some candidates. Many were misled into thinking that the satellite could still be geo-synchronous when at the lower height, causing the false conclusion that v would decrease because ω has

to be constant. Those candidates who appreciated that v2 ∝ r1 were invariably home and dry, but those

who stated that v increases because the centripetal force increases, had greater difficulty in obtaining the second mark. When a satellite descends, F, v and r all change; making explanation difficult.


23

Question 5 Application of E = V/d and t = d/v brought two straightforward marks for most candidates in the first two parts of (a). Part (iii) caused greater difficulty, often because F = EQ was not known. One incorrect approach, adopted in several scripts, involved assuming a vertical displacement of 7.5 mm, corresponding to half the vertical separation of the deflecting plates. Using t = 9.4 × 10−10 s from part (ii), a = 2s/t2 was then applied, giving a vertical acceleration of 1.7 × 1016 m s−2. The question required candidates to give the direction of the acceleration as well as its magnitude, but this requirement was often overlooked. A few candidates wrote more than they need have done, and in doing so condemned their own answer; �the acceleration is upwards in a parabola�. Confusion between the trajectory and the directions of acceleration and velocity are understandable, but cannot be tolerated in examination answers. Part (b) was either omitted or answered in a descriptive, non-mathematical way in more than half of the scripts. Those who understood the principles of projectile motion usually had little difficulty in gaining all three marks. A very common mistake however, was attempting to find the new velocity by use of v = u + at with u taken to be 3.2 × 107 m s−2. In the work of the more able candidates, whether the direction of the calculated angle (about 26°) was �up� or �down� was often clarified by a diagram.


24

Mark Range and Award of Grades Unit/Component

Maximum Mark (Raw)

Maximum Mark

(Scaled)

Mean Mark

(Scaled)

Standard Deviation (Scaled)

PA01 50 50 28.6 10.7 PA02 50 50 27.0 9.6 PHA3/W - Written 50 50 31.5 9.4

PHA3/C - Coursework 30 30 22.5 5.1

PA3C 80 80 54.0 12.1 PHA3/W - Written 50 50 32.9 9.8

PHA3/P - Practical 30 30 18.8 4.2

PA3P 80 80 51.7 12.0 PA04 75 90 39.9 13.9

For units which contain only one component, scaled marks are the same as raw marks.

PA01 Particles, Radiation and Quantum Phenomena

(5052 candidates)

Max mark A B C D E

Scaled Boundary Mark 50 36 31 26 22 18

Uniform Boundary Mark 90 72 63 54 45 36

PA02 Mechanics and Molecular Kinetic Theory

(4026 candidates)

Max mark A B C D E




25

PA3C Current Elasticity and Elastic Properties of Solids Coursework

(890 candidates)

Max mark A B C D E

raw 50 40 36 32 29 26 PHA3/W Boundary Mark

scaled 50 40 36 32 29 26

raw 30 25 22 19 16 13 PHA3/C Boundary Mark

scaled 30 25 22 19 16 13

PA3C Scaled Boundary Mark 80 65 58 51 45 39

PA3C Uniform Boundary Mark 120 96 84 72 60 48

PA3P Current Electricity and Elastic Properties of Solids Practical

(476 candidates)

Max mark A B C D E

raw 50 40 36 32 29 26 PHA3/W Boundary Mark

scaled 50 40 36 32 29 26

raw 30 22 20 18 16 15 PHA3/P Boundary Mark

scaled 30 22 20 18 16 15

PA3P Scaled Boundary Mark 80 62 56 50 45 41

PA3P Uniform Boundary Mark 120 96 84 72 60 48

PA04 Waves, Fields and Nuclear Energy

(3790 candidates)

Max mark A B C D E




26

Advanced Subsidiary Award Provisional statistics for the award (384 candidates)

A B C D E

Cumulative % 27.1 43.2 62.5 79.4 91.7 Advanced Award Provisional statistics for the award (22 candidates)

A B C D E

Cumulative % 27.3 45.5 72.7 86.4 100.0

Definitions

Boundary Mark: the minimum (scaled) mark required by a candidate to qualify for a given grade.

Mean Mark: is the sum of all candidates� marks divided by the number of candidates. In order to compare mean marks for different components, the mean mark (scaled) should be expressed as a percentage of the maximum mark (scaled).

Standard Deviation: a measure of the spread of candidates� marks. In most components, approximately two-thirds of all candidates lie in a range of plus or minus one standard deviation from the mean, and approximately 95% of all candidates lie in a range of plus or minus two standard deviations from the mean. In order to compare the standard deviations for different components, the standard deviation (scaled) should be expressed as a percentage of the maximum mark (scaled).

Uniform Mark: a score on a standard scale which indicates a candidate�s performance. The lowest uniform mark for grade A is always 80% of the maximum uniform mark for the unit, similarly grade B is 70%, grade C is 60%, grade D is 50% and grade E is 40%. A candidate�s total scaled mark for each unit is converted to a uniform mark and the uniform marks for the units which count towards the AS or A-level qualification are added in order to determine the candidate�s overall grade.

Documents

Report on the Examination - Skinners' School Science 12,13/A level Physics past papers... · General Certificate of Education Physics ... The number of candidates who aggregated in