GCE 2001 January Series

Transcription

1 GCE 2001 January Series Report on the Examination Physics Specification A ❶ Unit PA01 Particles, Radiation and Quantum Phenomena ❶ Unit PA02 Mechanics and Molecular Kinetic Theory Advanced Subsidiary - Subject Code (5451)

2 Further copies of this Report on the Examination are available from: Publications Department, Stag Hill House, Guildford, Surrey, GU2 7XJ Tel: or Aldon House, 39, Heald Grove, Rusholme, Manchester, M14 4NA Tel: or download from the AQA Website: Assessment and Qualifications Alliance 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 and a registered Charity Registered address Addleshaw Booth & Co., Sovereign House, PO Box 8, Sovereign Street, Leeds LS1 1HQ. The AQA was formed by the merger of the Associated Examining Board (AEB)/Southern Examining Group (SEG) and the Northern Examinations and Assessment Board (NEAB). Kathleen Tattersall, Director General

3 CONTENTS Specification A Unit PA01 Unit PA02 Page No. Particles, Radiation and Quantum Phenomena...5 Mechanics and Molecular Kinetic Theory...7 Ranges and Award of Grades...10

4

5 Report on the Examination Advanced Subsidiary Physics A Physics Specification A General Comments Two units were presented at this first examination of the new Specification A. These were: Unit 1: Particles, Radiation and Quantum Phenomena Unit 2: Mechanics and Molecular Kinetic Theory. It is pleasing to report that the examiners were well pleased with the response and attainment of candidates in both papers, even though the Specification has only been taught for a little over one term. A large number of candidates gained high marks with comparatively few being awarded very low marks. It may be that, at this stage, centres felt justified in only entering their best candidates. There was some concern that many candidates were not working to the correct number of significant figures in numerical answers. This was more apparent in Unit 2 than Unit 1. Many candidates lost marks by either omitting units in numerical answers or giving the wrong unit. It should be remembered that candidates could lose 1 mark per question for unit errors. In each paper, up to two additional marks were available for the Quality of Written Communication and, as indicated in the published Scheme, two sections in each paper were primarily considered for awarding these two marks. Although the quality of the written work was generally acceptable, it was noticeable that usually only the better candidates gained the two marks. Unit PA01: Particles, Radiation and Quantum Phenomena The majority of candidates were well prepared for this paper and it was encouraging to see how far many of them had progressed in one term. Most candidates scored marks on all the questions and there were a number of scripts where full marks were awarded. Use of appropriate significant figures was generally good but units were omitted in some numerical answers. This particular failing was not spread evenly across the whole entry and it was an issue that affected some centres more than others. Question One This straightforward, introductory question was answered very well on the whole. It was disappointing though to find that almost 25% of the candidates failed to give electrons as one of the constituents of the atom in part (i) and almost 50% selected the proton as having the largest charge-tomass ratio in part (ii). Question Two Many candidates produced good, precise answers to part (a). However, a basic error was in thinking that the photoelectron was removed from the atom rather than it being a free electron near the surface of the metal. Part (b)(i) was the only section that was not answered well by the majority of candidates. An answer of 2.0 µa for the current appeared as frequently as the correct answer of 4.0 če 5

6 Physics A Advanced Subsidiary Report on the Examination µa. The explanations offered were either vague and of the form it is obvious they are proportional or else were very confused in mixing up the properties of intensity and frequency. Units were sometimes omitted in the answers to part (b)(ii). Question Three This question performed extremely well. A number of candidates lost a mark in part (b)(ii) by not extending their answer beyond stating that the lepton was a fundamental particle. Several candidates penalised themselves by writing the symbol for the neutrino in such a flamboyant way that the symbol ν looked like a µ and since this could be regarded as an attempted answer, a mark was lost. Question Four This question proved to be a good discriminator. In part (a) the absorption spectrum was often confused with the emission spectrum. Part (b) was answered reasonably well but in part (iv) the weaker candidates assumed that the electron would either simply move up an energy level or not move from its current orbit. Part (c) involved a two-stage calculation but at least a quarter of the candidates stopped after calculating the frequency. Question Five This question also discriminated well. In part (a)(i) almost half the candidates failed to show the path of the ray changing direction due to refraction on entry into the core of the fibre and very few showed refraction on exit. The fact that they were not familiar with refraction was illustrated further by most of the candidates giving T.I.R. as their answer to part (a)(ii). Correct answers were often produced in part (a)(iii) but frequently the working was very sparse or confused. Part (b) worked well and allowed some candidates to show their clarity of thought. The answer to part (c) required only a statement, but even then some candidates were far too economical with their answers. For example, one word answers such as by surgeons, television or phone were not awarded a mark, but would probably have gained a mark if these words had been included in a simple sentence with a specific purpose. Question Six There was some evidence that quite a number of candidates were not prepared for this topic. Candidates who were conversant with it usually gave a completely correct answer, but others usually failed to score at all. In the calculation in part (b), a number of candidates started with ½mυ 2, showing that they were unfamiliar with the subject. Question Seven This question produced a good spread of responses, ranging from the vague to the highly articulate and technically correct. There were very few conceptual misunderstandings but some candidates found it very difficult to make simple correct statements. For example, candidates who wished to state as an answer to part (i) that the α particles hit air molecules would write the α particles interfere with the air or the α particles are aware of the air. Several candidates also slipped into using electrons rather than α particles as the projectiles. If this occurred it was penalised only once. 6 če

7 Report on the Examination Advanced Subsidiary Physics A Unit PA02: Mechanics and Molecular Kinetic Theory The response of candidates was, on the whole, very pleasing. Candidates seemed well prepared in most of the topics examined and many very good or excellent scripts were seen. The majority of candidates made a serious attempt at every question; no blank scripts were received and very few candidates achieved only single-figure marks. Almost all candidates completed the paper and there was little to suggest that shortage of time was a problem. Questions 1 and 5 produced many answers scoring full marks, whilst questions 4 and 6 proved difficult for many candidates; it was evident from the answers to question 6 that many candidates had only vague ideas about molecular kinetic theory. Many candidates lost marks by omitting the unit in numerical answers or by giving the wrong unit; some candidates lost three or four marks in this way. Only the best candidates avoided the one mark penalty for using inappropriate numbers of significant figures. Question One In part (a) only a small minority of candidates stated that the three forces were in equilibrium if their resultant was zero. Many candidates were awarded the mark for a clear and correct reference to a balance of the forces or to their vertical and horizontal components, or to a closed vector triangle. A sizeable minority of candidates stated incorrectly that either the forces themselves must all be zero or, more commonly, that the sum of the forces must equal zero. Vector sum is correct, but was almost never seen. Part (b) was almost invariably answered by calculation, most frequently by correctly resolving along the directions of the tensions. A number of candidates resolved incorrectly by transposing the sine and cosine terms and were awarded up to two of the four available marks. A small minority resolved forces vertically and horizontally and whilst the resolving was often correct, the subsequent calculation of tensions was usually wrong. Many candidates expressed the tension T 1 correctly, but to five significant figures and lost a mark. Answers attempted by scale drawing were extremely rare and almost always quite wrong or showed little more than a sketch without a stated scale, or direction arrows. Question Two This question was answered well by the majority of candidates. Most scored full marks in part (a)(i); perpendicular distance being usually stated but occasionally inferred from a correct diagram. Very few candidates did not know the correct unit of moment in part (a)(ii). In part (b)(i), the reaction force upwards at the pivot was often missing or placed elsewhere. The calculation of weight in part (b)(ii) was almost always correct but the answer often expressed to an inappropriate number of significant figures, usually too many but occasionally too few (the use of g = 10 N kg 1 was not appropriate here). The moments calculation in part (iii) was usually correct but many candidates used mass instead of weight and were awarded no marks. A significant number of candidates did not complete the calculation by converting the weight of the counterweight to its mass. Question Three In part (a) most candidates were able to interpret the graph correctly and almost all understood why the parachutist reached constant terminal speed in region C of the graph. Although many also understood and stated that the acceleration in region A was constant, few stated that this was because the drag on the parachutist was negligible or much smaller than his weight. Answers were generally well expressed and a mark of four or five was most common. A minority of candidates, however, was quite incapable of using physics terms accurately and subsequently scored few marks. če 7

8 Physics A Advanced Subsidiary Report on the Examination Many candidates understood that the acceleration in part (b) equalled the gradient of the line in region D of the graph and arrived at a correct answer (although the unit of acceleration was often given as m s 1 ). Candidates who used a = (υ u)/t very often chose points off the straight section of the graph and arrived at a value for a outside the acceptable range. In part (c) many candidates ignored the graph and attempted to use an equation of uniform acceleration to find the distance travelled. The majority, however, made some attempt to relate distance to the area under the graph and most of these answers fell within the acceptable range. Part (d) was most often correct, although in part (ii) many candidates inverted the tan function and found the angle to the horizontal rather than the vertical. Question Four Part (a) was answered correctly by most candidates, although F = 300 cos 20 was seen rather frequently. Almost no candidates used sin 20. The explanation in part (b)(i) was generally not very clear. Few candidates stated explicitly that the work done by a force is found by multiplying the force by the distance moved in the direction of the force or that F was not moving in its own direction. Many of those candidates who earned the marks for part (i) then multiplied their answer to part (a) by 8000 (or, very often, 8) and arrived at an incorrect answer. Most candidates knew that power = work done/time in part (iii), but a significant number did not convert the 5 hours to seconds. Most candidates found part (c) very difficult and only a very small number scored the maximum three marks available. An inability to express ideas clearly was seldom the problem: most made it adequately plain that they had misunderstood the physics of the situation. There were many answers in which energy transformations were not mentioned at all. Of those who did consider work and energy, most thought that all the work done by the force was converted into kinetic energy over level ground and that this kinetic energy was subsequently converted into potential energy on climbing the hill. A very few candidates successfully related the extra work needed in a climb to a longer time at constant power. Question Five This question was very well answered with many candidates scoring maximum or nearly maximum marks. Most candidates knew in part (a) that the momentum before a collision equalled the momentum after the collision, but rather few gave the condition that no external force must act on the system. Part (b) was almost always correct, although some candidates did not add the masses of the bullet and block. Parts (c)(i) and (c)(ii) were usually correct. In section (iii) some candidates failed to subtract the remaining kinetic energy of 5.0 J from the initial kinetic energy of 200 J, or alternatively used 5.0 J for the internal energy. Part (d) was most often correct, but a number of candidates used υ 2 = u 2 + 2as and scored no marks. Question Six A small minority of candidates clearly were not prepared for kinetic theory in any detail and produced nonsense answers in part (a). Most candidates remembered two of the assumptions and either left the final two unanswered or, quite commonly, were content to rephrase their first two assumptions. On the other hand many candidates did get all four assumptions correct. Very few candidates achieved all four marks for the explanation in part (b)(i). Many made no reference to molecules or to the kinetic theory but invoked the general gas law. Most candidates knew that gas molecules move faster on average when heat energy is transferred into the gas from the 8 če

9 Report on the Examination Advanced Subsidiary Physics A warmer surroundings and related this to an increased collision rate with the container wall. Few candidates made any mention of momentum exchange. Most candidates used the correct expression 3 / 2 kt for the mean kinetic energy in part (ii), but few used the absolute temperature in the calculation. A surprising number of candidates, having arrived at a correct answer, lost the mark by omitting the unit. če 9

10 Physics A Advanced Subsidiary Report on the Examination Ranges and Award of Grades Unit PA01: Particles, Radiation and Quantum Phenomena Grade A B C D E U UMS Boundary Component Maximum (Raw) Maximum Mean Standard Deviation Written Paper Unit PA02: Mechanics and Molecular Kinetic Theory Grade A B C D E U UMS Boundary Component Maximum (Raw) Maximum Mean Standard Deviation Written Paper Definitions Boundary : the minimum (scaled) mark required by a candidate to qualify for a given grade. Mean : the sum of all candidates marks divided by the number of candidates. The mean (or average) mark measures a central tendency of a mark distribution (provided that the distribution is not skewed). Standard Deviation: a measure of how widely candidates marks are spread about the mean mark. When expressed as a percentage of the Maximum mark (scaled), small standard deviations indicate that the marks are bunched and large standard deviations indicate a wide spread of marks. In general, the marks of approximately two-thirds of all candidates lie in a range of plus or minus one standard deviation about the mean mark. 10 če