GCE Physics PHA5C Nuclear and Thermal Physics & Applied Physics Report on the Examination 2450 June 13 Version: 1.0
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Section A Nuclear and Thermal Physics General Comments The exam produced a full range of marks from candidates. Compared to last year the paper seemed to give more opportunities for candidates to show their weaknesses. It was important for candidates to present their calculations in an organised manner in several questions. This was an issue in questions 1, 3 and 4. In answering some of the written questions candidates often did not explain their answers sufficiently well to gain all the marks available. Some candidates did tackle the paper very well but there seemed to be a marginal fall in attainment by the majority compared to last year. Question 1 Very few candidates knew what the atomic mass unit was. A surprising number thought it was simply another name for nucleon number. The next choice of candidates in giving the definition of the atomic mass unit was to give an energy or mass equivalence in J, MeV or kg. In part (a)(ii) a majority knew that the mass of the nucleus was less than its constituents but too many simply repeated back the question and said they were different. The explanation of why they are different was very vague by most candidates. The majority thought it was sufficient to simply state energy and mass are equivalent. Part (b) was very discriminating. Only the best candidates scored the 3 marks because for most there was a lack of appreciation that 3 marks usually equates to 3 points needing to be made in answering the question. Some of the vague statements made included, it takes a great deal of energy to get fusion to start and many also referred to the need to overcome the SNF rather than an electrostatic force before nuclei could get close enough to fuse. Completion of the first equation in 1(c)(i) was done incorrectly by most because they did not pick up the fact that the emitted particle must be an anti-lepton. The conservation laws covered in module PHYA1 were often flouted in answering this question. The second equation was done correctly by most. The main feature that came out in 1(c)(ii) was how disorganised a majority of candidates are in presenting their work. The consequence of this was that it became very difficult to award marks for incomplete answers when there was just a jumble of figures on the page. The calculation was difficult for many because they could not decide which units to work in. The more successful chose to work in atomic mass units. Even here many thought the mass of the hydrogen nucleus was 1u. In addition candidates answers often lacked precision leading to rounding errors by using, for example, 1.67 10-27 kg rather than 1.673 10-27 kg for the mass of the proton. Question 2 Most candidates were fully aware of the function of the control rods in absorbing excess neutrons and scored well in part (a). Some candidates said too much by explaining the role of the control rods to absorb neutrons and the moderator to slow neutrons down but then did not make it clear which reduced the power. The weaker candidates talked about control rods controlling the reactions without any further explanation. 3 of 8
Part (b) was answered well by most but it was common to give the answer fuel rods rather than spent fuel rods. In (c)(i) the most common answer was gamma rays but very few then went on to discuss energy levels. Some of those that did then spoilt their answer by referring to changing electron levels. Part (c)(ii) was a very good question to distinguish between the weak and strong candidates. The weaker candidates focussed on the wording in question concerning elastic collisions. They interpreted this to mean the neutrons maintain their kinetic energy or momentum during all subsequent collisions. Question 3 Candidates found 3(i) quite difficult for a number of reasons. Some started correctly by equating heat supplied to glass equals heat gained by cola but then they could not make the final temperature the subject of the resulting equation. Others substituted the temperature the wrong way round and used (3-T f ), which was negative and fudged the arithmetic. As in a previous question candidates did not explain their approach which made it difficult to award partial marks. It was interesting to see some candidates who jumped in too quickly and made an initial mess of the calculation fared better on additional pages when they thought more carefully over the problem. Part 3(ii) was also very discriminating. Only the best candidates scored full marks. Good candidates who just missed full marks usually forgot about the 3 degree rise in temperature of the ice after it had melted. Most other candidates were aware of the mc T and ml equations but then made all manner of different errors. Question 4 A majority of candidates referred to obeying a gas law in answer to part (a). A second marking point was often missed out, wrong or vague. This is illustrated in the two answers that follow: It has properties of a gas such as Brownian Motion, and The gas obeys the assumptions of the kinetic theory. Parts (b)(i)+(ii) were done well by most. Only a few did not convert the temperature to Kelvin before performing a calculation. Again very few did not know the unit for density. In part (b)(iii) more than half the candidates could perform the calculation but a significant number of those did not quote the answer to 2 significant figures. Of those missing out on the calculation many did score the first mark but then went wrong by using the wrong density or by not finding the proportion of the gas still in the container. 4 of 8
Question 5 On the whole most candidates knew what approach to take and attempted to explain a suitable experiment. Weak candidates had issues over the language used to answer this question. Often they would state that the intensity of radiation needs to be calculated rather than a count rate needs to be recorded. Also they often stated facts instead of describing an experiment. For example, alpha particles can be stopped by a sheet of paper is a poor substitute for explaining what data to take and how to interpret the data to arrive at the conclusion that alpha rays are emitted from the source. Slightly better candidates started to discuss the background radiation but they did not always carry on to explain how this would be used in the analysis. Safety in the experiment was usually given but a majority of candidates tended to overstate the precautions necessary. It was common to see references to remote handling, lead gowns, and keeping metres away from the source. Only the better candidates could adequately determine that gamma rays were given out by the source. These either talked about count-rate falling with the inverse square of distance or they discussed an absorber, which would have eliminated any beta radiation but still allows some radiation to pass through. The only way to know radiation passes through is to compare the count-rate with the background radiation. It was this last point that many candidates missed. Overall candidates seemed to lack planning. They often missed important considerations and bolted them on at the end. The standard of English still leaves a lot to be desired. The writing in several cases was virtually illegible and keywords were often misspelled. Fortunately there were candidates in contrast to this description who performed the writing task exceedingly well. 5 of 8
Section B Applied Physics General Comments Most candidates were able to make a good attempt at the paper, especially the first three questions. The examiners saw many high-scoring confident answers from candidates who were well prepared for the examination. As is usually the case with this option, candidates generally showed greater confidence with calculations than with written explanations or descriptions in fact more marks were allocated to qualitative questions in this year s paper than in previous years. It was, however, pleasing to see many well-written and thorough answers to Q3 part (c), where candidates were tested on quality of written communication in a question centred on the differences between theoretical and real diesel engine indicator diagrams. They were better prepared for this than in the previous year, when the corresponding question was set on the use of a flywheel for smoothing out variations of torque. Despite questions on heat pumps and refrigerators in recent papers, the answers to Q4 showed that candidates still struggle with this part of the specification. Question 1 Candidates were asked to apply their knowledge of rotational dynamics to the motion of a turntable in a microwave oven. The calculations were generally performed well, with those who fell at the first hurdle (not converting revolutions to radians) being penalised only once, with a carried error given in subsequent parts provided the rest of their working was correct. In part (a) (i), nearly all candidates gained the mark for the correct unit. In parts (a) (ii) and (b) (i), most candidates had a good understanding of the part played by frictional torque. In part (b) (ii), some candidates made the mistake of dividing power by work. Question 2 This question on moment of inertia and angular momentum was set in the context of a diver somersaulting during a dive. In part (a) (i) most candidates quoted I = Σmr 2 but fewer were able to explain that by bringing his legs closer to the chest, the diver altered the way the mass was distributed about the axis of rotation. It was not enough to say that because the radius (without saying what they meant by radius here) had been reduced, the moment of inertia must decrease. Part (b) (i) was generally answered well. The examiners were looking for: conservation of angular momentum, angular momentum = Iω (or in words) and that if I varies ω must vary to keep Iω constant. In part (b) (ii) the question asked for a description of how the angular velocity varied throughout the dive, so some evidence that candidates had studied the graph in Figure 1 was needed. The most common way of gaining the mark was to write that the angular velocity increased, decreased, increased slightly then decreased. A significant number of candidates failed to get the mark because they referred only to the angular velocity in positions 1, 2 and 3. 6 of 8
The angular momentum calculation in part (c) was correctly carried out by the majority of candidates. One error, made by relatively few candidates, was to read the lowest moment of inertia from the graph as 6.2 kg m 2 instead of 6.4 kg m 2, and this cost them a mark. A very small number of candidates calculated an answer by mistakenly thinking that conservation of rotational kinetic energy applied here. Question 3 In part (a) most candidates were aware that in an adiabatic compression no heat transfer occurs, strictly from the gas, but as long as they had the idea of zero heat transfer the mark was given. The second mark was harder to get: candidates had to state or imply clearly that there was no time for heat transfer it was not enough simply to say the compression stroke occurred quickly. Candidates are well practised in calculations involving pv γ = c and pv/t= c and parts (b) (i) and (b) (ii) were tackled well by the majority of candidates, though some failed to achieve the significant figure mark in part (b) (i). Part (b) (iii) proved more difficult all too many candidates thought that all the fuel had to be injected into the cylinder before the piston reached the top of its stroke, or before it was ignited. Some thought there would not be enough space in the cylinder for the fuel when the piston was at the top of its stroke. There were some very confident answers to part (c), with a pleasing proportion of candidates showing a good understanding of both theoretical and real diesel cycles as represented on a p V diagram. Candidates did not find it difficult to describe the most important differences, but many failed to give reasons, some described the differences but gave the wrong reasons for the differences. A very common misunderstanding was to think that friction accounted for a difference in the area enclosed by the loops in the diagrams, not realizing that an indicator diagram is taken before frictional losses are accounted for. The better answers to the efficiency part of the question mentioned where friction occurred, usually by giving an example (e.g. at the bearings), and also mentioned the losses due to oil viscosity and/or the work needed to circulate cooling water. Lower scoring answers simply mentioned friction, or missed out this part of the question altogether. Another fairly common misconception was to think that the theoretical engine cycle must be 100% efficient. Examiners were wary of vague statements about heat losses as there has to be heat loss (i.e. cooling) in the theoretical engine cycle as well as the real. Question 4 In part (a) candidates were asked to state what is meant by a reversed heat engine. Most of those who scored any marks were able to say that it is an engine in which energy (or heat) is transferred from a cold space (or reservoir/sink/place) to a hot space but for the second mark it was considered too vague to add by doing work. It had to be clear that there was a work or power input or that work was done on the system. Part (b) could be answered by quoting COP hp = Q in /W and COP ref = Q out /W from the Data and Formulae Booklet and then writing that because Q in is always greater then Q out, the COP hp must be greater than COP ref. This scored zero marks, because there was no evidence that candidates had done anything other than use the formulae booklet. To score any marks at all it was essential to give the reason why Q in is greater than Q out. i.e. to state that Q in = Q out + W. Of those who scored the full two marks a significant proportion were able to correctly manipulate the formulae for the coefficients of performance and Q in = Q out + W to arrive at COP hp =COP ref + 1. 7 of 8
A common mathematical error was to write Q in /(Q in - Q out ) = 1 Q in /Q out. Mark Ranges and Award of Grades Grade boundaries and cumulative percentage grades are available on the Results Statistics page of the AQA Website. Converting Marks into UMS marks Convert raw marks into Uniform Mark Scale (UMS) marks by using the link below. UMS conversion calculator www.aqa.org.uk/umsconversion 8 of 8