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Cambridge International Advanced Subsidiary Level and Advanced Level 9702 Physics June 2013 Principal Examiner Report for Teachers PHYSICS Paper 9702/11 Multiple…
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Cambridge International Advanced Subsidiary Level and Advanced Level 9702 Physics June 2013 Principal Examiner Report for Teachers PHYSICS Paper 9702/11 Multiple Choice Question Question Key Key Number Number 1 D 21 D 2 C 22 A 3 C 23 A 4 D 24 C 5 D 25 A 6 D 26 C 7 B 27 A 8 D 28 C 9 A 29 B 10 D 30 B 11 D 31 D 12 A 32 A 13 C 33 A 14 B 34 A 15 A 35 B 16 C 36 D 17 B 37 D 18 B 38 C 19 B 39 A 20 B 40 B General Comments This paper demands quick and accurate working by candidates, but the time of 90 seconds per answer is made possible by having many questions that can be answered after a quick read through the question. Candidates should be advised never to spend a disproportionately long time on any one question. Candidates must be encouraged to work through many questions. There is plenty of space on the paper for writing. Care with units is essential. Prefix errors are a cause of many wrong answers as are corresponding power of 10 errors. Candidates must be certain to look critically at any answer they give to see if it makes basic sense. Comments on Specific Questions Question 7 Answer C was slightly more popular than the correct answer B. There is no way in which the body can have a constant acceleration. It is the acceleration that decreases as the body falls. 1 © 2013 Cambridge International Advanced Subsidiary Level and Advanced Level 9702 Physics June 2013 Principal Examiner Report for Teachers Question 8 Many candidates gave the time at which the two trains are travelling at the same velocity, but the express train is not overtaking the goods train until it is travelling at twice this velocity, as is shown clearly on a velocity-time sketch graph. This is the sort of question where a graph should be quickly sketched by a candidate on the question paper. It is important to write out the working and not to try to answer this type of question by thinking only. Question 14 This question was found to be problematic. A significant number of candidates chose C, implying that you needed a greater force as the distance x is increased. About half of the candidates chose A: although if you double x you will only need half the value of F, if you double it again you cannot possibly need zero force. The graph cannot be a straight line. Question 16 There is plenty of space on the paper for a triangle of forces to be drawn, and if the angles are reasonably accurate it will be seen that the order is W, H, T. (A less accurately drawn diagram might get H, W, T, but that is not an option.) Question 17 This was tricky. It is necessary to deal with the bottom of the ball at 72 cm at the start and 37 cm at the end. This gives (37/72) × 0.75 J = 0.3854 J. Question 18 Candidates work out the mass of air passing the blades in one second and use its speed in this calculation. 2 Many then do not realise that the speed must be used again in ½mv . Question 19 The force exerted by the wheel on the rope is 80 N. The distance moved against this force in one second is 50 × 0.30 = 15 m. The power provided by the motor is therefore 80 N × 15 m s–1 = 1200 W = 1.2 kW. Question 24 Many candidates forget to include the intensity of the light wave. Twice the amplitude implies four times the intensity, and then focussing increases the power per unit area by a further factor of 3. Question 28 Answer B was more popular than the correct answer C, but B cannot in any way keep the same wave pattern. Question 33 Many candidates thought that the terminal p.d. would be unchanged. An increase in internal resistance will decrease the terminal p.d. and also decrease the output power. Question 35 This question was difficult and the statistics suggested that many candidates tried to guess the correct answer. Obtaining the correct answer requires careful determination of the resistance on each ‘half’ of the circuit. Question 36 Many candidates thought that the voltmeter would read zero. The resistance of the ammeter and resistor in parallel is close to 0.1 Ω, so the potential difference of 2 V must be mostly across the much larger resistance of the voltmeter. 2 © 2013 Cambridge International Advanced Subsidiary Level and Advanced Level 9702 Physics June 2013 Principal Examiner Report for Teachers Question 37 This was another electricity question that candidates found difficult. If the variable resistance is zero the current will be large and the voltmeter reading will be zero. When the variable resistance is 10 Ω the current will be reduced, but not zero, and the voltmeter reading will be high. 3 © 2013 Cambridge International Advanced Subsidiary Level and Advanced Level 9702 Physics June 2013 Principal Examiner Report for Teachers PHYSICS Paper 9702/12 Multiple Choice Question Question Key Key Number Number 1 B 21 C 2 B 22 B 3 A 23 B 4 D 24 D 5 C 25 D 6 B 26 B 7 D 27 C 8 B 28 B 9 D 29 C 10 B 30 D 11 D 31 A 12 A 32 C 13 B 33 A 14 D 34 A 15 B 35 D 16 B 36 D 17 C 37 C 18 A 38 C 19 C 39 B 20 D 40 B General Comments This paper demands quick and accurate working by candidates, but the time of 90 seconds per answer is made possible by having many questions that can be answered after a quick read through the question. Candidates should be advised never to spend a disproportionately long time on any one question. Candidates must be encouraged to work through many questions. There is plenty of space on the paper for writing. Care with units is essential. Prefix errors are a cause of many wrong answers as are corresponding power of 10 errors. Candidates must be certain to look critically at any answer they give to see if it makes basic sense. Comments on Specific Questions Question 1 A significant number of candidates could not pick out B as containing the required one vector and one scalar quantity. This was a straightforward recall question. 4 © 2013 Cambridge International Advanced Subsidiary Level and Advanced Level 9702 Physics June 2013 Principal Examiner Report for Teachers Question 3 Many candidates chose C here, where the resultant is clearly at 45° to the horizontal instead of the direction shown in the correct answer, A. Question 9 Candidates found this question difficult. There is a pitfall here that many of the able candidates fell into. In calculating the mass m of air that hits the wall in one second, the speed of the air must be involved: 2 –3 m = 12 m × 33 m × 1.2 kg m . The rate of change of momentum is therefore this expression multiplied by its –1 velocity, 33 m s , giving an answer of 16 000 N. Keeping units in a calculation like this will result in an answer with the units of momentum. Missing out the 33 m will give incorrect units for force. Question 13 About a third of candidates chose the correct answer for this question. The key to the answer is in the word ‘must’. At first sight both B and C seem possible, but C is not correct if the mass of the body is changing, whereas B is always correct under any circumstances. Question 14 Many candidates did not include the weight of the beam here. Question 19 Temperature depends only on the average speed of molecules. Molecules in ice at 0 °C and molecules in water at 0 °C have the same average speed and therefore the same average kinetic energy. Question 20 The answer to this question cannot reliably be obtained by guesswork. Eliminating the pressure at P gives (h1–h2)ρg = 8000 Pa, so (h1–h2) = 0.060 m. D is the only answer that fits. Question 23 The model has a load that is 1/1000 of the full-size load and its cable will have an area of cross-section that is 1/100 that of the crane’s. This gives the ratio of the stresses to be 10. Question 24 Taking the wavelength of violet light to be 4 × 10–7 m gives the frequency to be 7.5 × 1014 Hz or 1015 when rounded to the nearest power of 10. Question 32 Many candidates correctly found the resistance of one strand of wire, but forgot to divide by 12, the number of strands. This gave D as a very popular incorrect answer. Question 39 Roughly the alpha particle has a mass of 4 u, so 4 × 1.66 × 10–27 kg. This gives 6.6 × 10–27 kg or 10–26 when rounded up. A number of candidates chose A, and these candidates may have incorrectly rounded. 5 © 2013 Cambridge International Advanced Subsidiary Level and Advanced Level 9702 Physics June 2013 Principal Examiner Report for Teachers PHYSICS Paper 9702/13 Multiple Choice Question Question Key Key Number Number 1 B 21 D 2 A 22 A 3 C 23 C 4 A 24 C 5 D 25 C 6 B 26 B 7 B 27 D 8 C 28 D 9 B 29 C 10 B 30 D 11 A 31 D 12 B 32 C 13 D 33 C 14 A 34 B 15 A 35 C 16 A 36 B 17 C 37 C 18 A 38 C 19 C 39 D 20 A 40 B General Comments This paper demands quick and accurate working by candidates, but the time of 90 seconds per answer is made possible by having many questions that can be answered after a quick read through the question. Candidates should be advised never to spend a disproportionately long time on any one question. Candidates must be encouraged to work through many questions. There is plenty of space on the paper for writing. Care with units is essential. Prefix errors are a cause of many wrong answers as are corresponding power of 10 errors. Candidates must be certain to look critically at any answer they give to see if it makes basic sense. Comments on Specific Questions Question 8 For an object starting from rest, s = ½at2 so s/t2 = ½a. On the graph s/t2 is the gradient so, to find the acceleration, twice the gradient is needed. Some candidates incorrectly chose A but a significant number chose B, forgetting about the ½ term completely. 6 © 2013 Cambridge International Advanced Subsidiary Level and Advanced Level 9702 Physics June 2013 Principal Examiner Report for Teachers Question 9 The unbalanced force, 392 N, has to accelerate all the mass, 1240 kg, and not just the mass of the lift and passenger. Question 14 Answers A and D were almost equally popular. The correct answer is A, because the total work done depends only on the difference in height (which is zero). Question 19 The full-size load has 1000 times the weight of the model; the area of cross-section of the cable itself is 100 times that of the model; the length of the cable is 10 times that of the model. The ratio of the two extensions using force × length / (Y × area) = 1000 × 10 / (1 × 100) = 100, answer C. Question 21 The problem here was the units, as so often happens when making measurements. Using cm and mJ meant factors of 100 and 1000 had to be dealt with. Question 27 There are three maxima on each side and one undeflected, giving 7 in total. Many candidates simply gave 3. Question 32 This question needs careful thinking. The diameter of the wire is falling linearly so the area of cross-section is not falling linearly. There is a greater percentage fall per unit length at the narrow end than at the wide end. This means that the potential difference per unit length is less at the wide end than at the narrow end. This makes the correct answer C. More candidates thought that B was the correct answer, but B describes a wire of uniform cross-section. Question 36 When the variable resistor has value zero, there is 12 V across the 2 Ω resistor and the current is a maximum. When the variable resistor is at 10 Ω, the current is a minimum and the voltmeter will read 2 V. This gives B as the answer. 7 © 2013 Cambridge International Advanced Subsidiary Level and Advanced Level 9702 Physics June 2013 Principal Examiner Report for Teachers PHYSICS Paper 9702/21 AS Structured Questions Key Messages ã Candidates should be encouraged to treat the number of marks for each part of a question as a guide to the amount of detail required. ã Questions that ask for a specific reference to details given on graphs or diagrams should be answered with specific reference to these details, and should not be answered merely in general terms. ã In answering “show that” questions, candidates should show all the steps in their working. If the final answer is numerical, it should be calculated and not simply assumed to be the given value. ã Physics is a precise science. Candidates at this level should choose key words with care when writing any explanation. The distinction between terms such as mass and weight, stress, strain and force, and size and shape should be appreciated. Definitions and principles should be learnt in the detail stated in the learning outcomes in the syllabus. General Comments Many candidates lost credit because they were not able to recall standard definitions or principles. The statements given were not satisfactory at AS Level for many of the questions. Candidates should be encouraged to learn definitions thoroughly. It should be remembered that a proportion of the marks are dedicated to application and extension of the basic content of the syllabus. In order to score highly, candidates do need to have a thorough understanding of the subject matter. Comments on Specific Questions Question 1 (a) A common answer was based on extension being proportional to force. Reference should have been made to the wire returning to its original length on removal of the load. Candidates often used imprecise or unsuitable terms such as “size”, “shape”, “position” and “state”. (b) The final expression for the SI units of energy per unit volume was given. Many candidates did derive the units of energy. Others lost credit by just giving an expression for the units of energy, without any explanation. In “show that” questions candidates should be advised that full details of the procedure are vital. (c) This proof required that candidates should derive the units for the Young modulus and also state that strain has no unit. A common error was to quite correctly state that strain is the ratio of extension and original length but then to go on to give the unit of strain as ‘0’ or ‘1’. Question 2 (a) Candidates should be advised to learn the meanings of such terms. Many merely mentioned scalar and vector quantities. Others referred to “gravity” rather than gravitational force. 8 © 2013 Cambridge International Advanced Subsidiary Level and Advanced Level 9702 Physics June 2013 Principal Examiner Report for Teachers (b) (i) This was generally answered correctly. A minority did not show the 69 N force acting along the rope. Rather, they showed vertical and horizontal components, many without explanation, and consequently did not answer the question. (ii) The majority of answers were correct, although some confused sine with cosine. Candidates should be advised to use the data provided on page 2 of the question paper. The use of the approximation g = 10 m s–2 should be avoided unless the question states specifically that this value is to be used. (iii) This was generally answered well. Most used 69 cos θ, but a minority correctly used Pythagoras’ theorem. Question 3 (a) There were very few complete answers. Many candidates merely stated that, as the object falls, potential energy is converted into kinetic energy. At this level, it was expected that reduction in gravitational potential energy would be related to decrease in height, kinetic energy would be related to increase in speed and increase in thermal energy would be related to work done against air resistance. (b) (i) There were many correct answers. The usual errors were either associated with powers of ten or a failure to square the speed. (ii) 1. A significant number of candidates gave the change in energy as being equal to the change in potential energy. (ii) 2. A majority of answers were based on the energy loss being equated to the product of the frictional force and the distance moved. A significant number of candidates gave their answer as the weight of the object. Question 4 (a) When defining pressure, a statement of the ratio is essential. The use of the unclear term “force over area” should be discouraged. (b) Many answers lost credit by being too simplistic, being based on a statement that the molecules collide with the walls, producing a force/pressure. It was expected that reference would be made to the random nature of the motion of the gas molecules – not a random motion of the gas. Any discussion of force/impulse should have been preceded by a reference to momentum change. Finally, the averaging of the force/impulse from many collisions to give pressure should have been included. (c) Most answers were confined to a statement as to what is meant by elastic collisions. Some described elastic collisions as where there is energy conservation instead of kinetic energy conservation. Very few realised that the consequence of elastic collisions is that the temperature of the gas does not change. Question 5 (a) The necessary conditions (i.e. coherence, superposition and phase/path difference) for maxima were stated in many answers, but frequently there was a lack of clarity. When referring to phase, it was not made clear whether the sources or the waves were being considered. Some candidates did, quite correctly, consider polarisation. (b) This was a simple derivation. A small minority of answers involved the speed of sound. Others gave an incorrect power of ten for GHz. (c) There were very few comprehensive answers where the number of maxima or minima between O and P were considered. The most common answer was to assume a minimum half-way between O and P and then to have the first maximum at P. (d) Many answers did include narrower slits placed closer together. It was common to find that the question had not been read carefully and, consequently, changes other than those to the slits were 9 © 2013 Cambridge International Advanced Subsidiary Level and Advanced Level 9702 Physics June 2013 Principal Examiner Report for Teachers suggested. Candidates do need to be encouraged to be precise. Vague answers such as “make the slits smaller” do not make it clear as to what aspect of the slits is being considered. Question 6 (a) Generally, correct answers were given. Some candidates did not read the question carefully and, as a result, did not consider these specific examples but instead referred to “other forms of energy”. (b) Weaker candidates found difficulty with this question. There was confusion between power and energy. Frequently, general expressions were quoted that did not include the terms E, R1 and R2. (c) Candidates should be encouraged to quote relevant formulae at the start of any determination. Many answers involved a jumble of numbers and letters without any explanation. A significant number of candidates gave their answer as the ratio of the resistances without any consideration of the same current in the resistors. (d) There were some answers that were explained adequately but it was common to find that it was not appreciated that the p.d. across the resistors would be the same, with different currents. Question 7 (a) Many answers involved α-particles having no deviation, rather than small deviations. It is important that candidates realise that the great majority were subject to small deviations of less than 10° and that very few had deviations greater than 90°. A reference to the relative numbers and to the sizes of the angles was expected. (b) A common statement was that the nucleus has a large mass. This is insufficient. It should be stated that the mass of the atom is concentrated in the charged nucleus. Likewise, stating that the nucleus is small has little meaning until a comparison is made with the size of the atom. Few candidates linked the observations in (a) with the conclusions in (b). 10 © 2013 Cambridge International Advanced Subsidiary Level and Advanced Level 9702 Physics June 2013 Principal Examiner Report for Teachers PHYSICS Paper 9702/22 AS Structured Questions Key Messages ã Candidates should be encouraged to treat the number of marks for each part of a question as a guide to the amount of detail required. ã Questions that ask for a specific reference to details given on graphs or diagrams should be answered with specific reference to these details, and should not be answered merely in general terms. ã In answe
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