Physics Examiner Report June Essay

Further copies of the Introduction In the theory papers (Units PHI III AAA Knowledge with understanding PHI) there are two skills being assessed: ; Application of knowledge and understanding, synthesis and evaluation Further elucidation of these is given on pages 5 – 6 of the Specification. In formulating their answers to particular questions candidates should bear these skills in mind. In certain responses, direct quotation of theory, however correct, will only gain full credit when it is applied to the situation described in the question. Some questions expect interpretation of the information given in the stem.

Again, full credit will only be gained when the candidate’s response specifically addresses the stem – he general quotation of ‘bookwork is not what is expected. 6731 unit -rest PHI Maximum mark Mark range Mean mark Standard deviation 0-59 32. 7 11. 9 When reading through and checking their answers, candidates should ensure that these are sufficiently precise, are not ambiguously worded and answer the question posed. Qua. L A surprisingly large number of candidates, having drawn correct vector diagrams, calculated the force rather than using their diagrams to measure it (the number of significant fugues being the ‘give-away’).

Many candidates used the wrong diagonal, giving the force as (about) 11 N. Candidates’ success with identifying vectors was noticeably centre specific. Qua. 2 Those who selected u = u + as were generally successful. Those who tried to apply other equations of motion generally did not realism that two formulae would be needed and stalled half-way through the calculation. Weaker candidates used acceleration = velocity/distance. The common error in calculating tension was to use an incorrect value for mass. A surprisingly large number of candidates omitted the assumption, thereby not gaining this mark.

In explaining why the tension must be less than the eight many candidates tried to explain what would happen had the tension been greater, with the ‘string will snap’ being a frequent answer. Being more accurate. A wide range of experimental methods was seen. Generally speaking, those who described the electromagnet and trapdoor or lichgates methods were successful, although many gave themselves a time penalty by unnecessarily explaining how to calculate g from the measurements. A surprisingly large number of responses used a hand-held stopwatch for the timing.

Many candidates identified g as ‘gravity or ‘gravitational force’ – such answers were too oily. 6731 Examiner Report Summer 2003 Qua. 4 The most common error in adding arrows to the diagram was to show Y acting at the left foot. In explaining “how this situation illustrates the principle of moments” a common fault was to quote formula(e) from the given list without adapting or relating it (or them) to the situation posed. Most candidates ignored force Z completely rather than attempt to explain why its moment about P was negligible. Weaker candidates spoke of (anti)clockwise forces rather than moments. Qua. Many candidates applied MGM – h mum here. Even though the stem of the question states the cyclist travels at constant speed, candidates wrote that the lost g. P. E. Is transformed into kinetic energy. Few correctly estimated the rate of doing work climbing the hill, most candidates failing to recognize the connection with the first part of the question. Qua. 6 The most common slip amongst even the best candidates was not to state that the momentum of the alpha particle is equal to that of the radium nucleus. Some went no further than to state ‘momentum before equals momentum after’ which did not Justify their answer.

Qua. 7 The nuclear equation was incorrectly completed in a surprisingly large number of scripts. The decay constant and activity were calculated by a pleasingly large number of candidates. However, there were two very common slips. First, few candidates quoted a unit for the decay constant and, secondly, even fewer converted the value of TTL/2 from years to seconds. Qua. 8 A large number of candidates quoted correct facts about a, b and g radiations, but unfortunately these were not relevant to which test identifies what radiation as asked in the question. Qua. All parts of this question were answered well, but individual candidates did not answer all parts equally well. On several scripts responses were ambiguously order. 2 6732 unit -rest PHI 31 . 6 10. 9 The paper proved quite accessible with all but the weakest candidates scoring well on the calculation questions. Questions that require the use of algebra and or symbols, prove to be a challenge for most candidates. It is worth noting that where candidates are taught beyond the Specification it is useful to them to know exactly what is required for the actual Specification and consequently the examination.

This is particularly relevant to Qua. 7. Qua. L The calculation was generally well done. Not all candidates understood the implications of the graphs, assuming that the graphs eave a visual impression of resistance values. Some candidates failed to read the question properly and did not refer to current in their answers while others had the right idea but omitted key words leading to an ambiguous answer. The reason why Lamp A was brighter was rarely explained properly with too many candidates failing to realism that in a series circuit the current is the same through both lamps.

Far too many said that the current reaches A first and so it used up most of the energy. Qua. 2 Virtually no candidates scored full marks for this question. The value of the internal resistance of an e. . T. Supply was rarely known despite it being included in the Specification. There was a general lack of awareness of magnitude and often a complete disregard of units. Despite the question saying that values could be used more than once there was a reluctance by candidates to do so. Qua. 3 The first part of this question had a similar success rate to the first part of Qua. . Candidates who identified I = Nava as the key to the problem were often successful in the second part of the question but a relationship between two variables can only be established if the other terms remain constant. Candidates who did not refer individually to the other quantities that remain constant lost marks. A significant number of candidates gave answers based on resistance and ease of travel. Such answers scored no physics marks but many candidates scored the written communication mark. Qua. Since this was a ‘show that’ style of question it was important that candidates started with R = Pl/A. Those that did not lost the first two marks. Some candidates showed that it was dimensionally possible but this also did not score the marks. The calculation in this question was more difficult than the earlier ones and this was monstrance in candidates’ work with a significant number failing to manage the powers of ten. In the last part of the question some candidates gave themselves a time challenge by completing two arithmetic calculations instead of one algebraic one but could score full marks.

Difficulty arose here with candidates who could not 6732 Examiner Report Summer 2003 3 Qua. 5 A sizeable minority of candidates confused specific heat capacity with latent heat but in general the definition was known. Acceptable units of temperature were co or K but not o K. The question asked for a circuit diagram with the circuit symbol for either a heater r a resistor. However, many candidates either drew a circuit with nothing between the sockets or a diagram of a combination of circuit and apparatus. Neither of these could score full marks.

Because this experiment involves heating the metal for a significant amount of time, it was expected that a means of adjusting the circuit to maintain constant current and potential difference would be included. This was very centre dependent. For “other apparatus” some candidates listed everything including crocodile clips and wires but sometimes omitted the balance and/or timing device. The last part on experimental techniques was well answered by most candidates but quite a few candidates think that digital means more accurate. Qua. 6 This question was often well answered with a significant number of candidates scoring full marks.

Common errors were to forget to convert k to J or to subtract the temperatures and then add 273. Quite a few candidates having made one of these errors then equated two obviously different values without comment. Most candidates referred to the heating of water but less than half referred to the latent heat and considered the heating of the trough itself. Qua. Many candidates could not express the Pascal in base units and many thought that as n is a number it has no units. Showing that average kinetic energy of a molecule is proportional to temperature was a challenge and proved a good discriminator.

Candidates need to know that for this module they do not require knowledge of the Avogadro number or the Balletomane constant and therefore there are no marks for using these values. Some were so determined to use Abolition’s constant that they missed out the early stages of the work or did not define the constant. The main reason that many candidates scored badly was cause of a lack of understanding of the macroscopic and microscopic properties of a gas and so they quite happily used m as the mass of the gas and the mass of a molecule. The graph produced some interesting responses.

The challenge of reading the question correctly weeded out the most careless and the rest went on to usually give the correct straight -line relationship. Failure to label the axes properly cost many the easy first mark but many managed to get the mark for the intercept on the temperature axis. Of the best candidates, only a few correctly linked R to the gradient. A good number came close, emphasizing the linear relationship between iv and temperature T but failing to make the explicit link. Qua. 8 Very poor definitions were given, many Just elaborating on the abbreviation or referring to forces pushing charges round.

A good number realized that they were looking for a statement about energy but very few produced a fully correct answer worthy of two marks. The circuit diagrams often scored no marks because candidates and candidates who showed e. M. F. Varying with current demonstrated a complete lack of understanding. Even when candidates proposed a graph of V against I many loud not over-ride the instinct to associate increasing current with rising potential difference. 4 6733/01 unit PHI 32 0-32 17. 5 6. 4 Each Topic started with a general prefix question which illustrated some confusion in this area.

Whilst most scored well here, Pico- and cent- often appeared, as did kill, kill and killer, none of which were accepted. A frequently seen error was kilo with an ‘a’ and ‘o’ superimposed; candidates should appreciate that any doubt over what they have written could lose marks and simply re-writing their answer may have avoided this. Topic A (Astrophysics) Red giant formation was generally well described with sufficient detail to secure full marks; references to where the processes occurred (e. G. In the core/in outer shells) was key.

Some knowledge of the electromagnetic spectrum is a part of Wine’s law and candidates needed to refer to an increase in Imax as being related to a shift towards the red end of the visible spectrum. Very few candidates appreciated the importance of the decrease in T and the effect on TO in Stefan law. An understanding of pulsars was fairly centre-dependent; the concept of radio beams sweeping through space as often missed; references to radio waves being emitted as pulses were often made. Calculations were generally well done; many candidates wrote down the correct equation but then neglected to square D in the pad term.

There was a general misunderstanding about Scads, which were often said to have better resolution than photographic emulsion. Topic B (Solid materials) It was a shame to see many candidates losing a mark by not using a ruler when adding a line to the stress-strain graph, although some free-hand lines were deemed straight enough here. Relating strength and toughness to the graph proved to be ended many candidates. The instruction ‘use your graph’ for the hysterics question was often ignored; candidates need to ensure that they do what is asked. Slip planes also rarely mentioned, as many candidates simply repeated the question.

Calculations were mostly done fairly well, although ‘energy stored = force was often used rather than ‘average force ‘ distance’. 6733 Examiner Report Summer 2003 5 Topic C (Nuclear and Particle Physics) distance’ Binding energy was a somewhat vague concept to most candidates. Responses included ; energy to bind a nucleus together energy to bind an atom into molecules energy required to make a nucleus Accuracy proved a problem for candidates when sketching the binding energy per nucleon graph; peaks at nucleon number 56 with a slight subsequent fall were needed.

Relating this to stability proved centre-dependent. Charge and baryon number conservation was done well, with zero values being shown most of the time. Worryingly, balancing a nuclear equation 1 proved difficult for a large number of candidates, with O n eluding many. The half-life question was often over-complicated by the use of exponential equations which are not part of the Specification at this level.

Topic D (Medical Physics) Explanations of uptake studies were, on the whole, of a good standard, although arteriosclerosis’s were infrequently mentioned and the concept of requiring several readings for a function test was very rarely seen. Reasons for technetium were often far too vague; repetition of the question was insufficient to earn marks here. It would appear that diagnosis with X-rays is studied more than therapy, since the proton number dependence on X-ray attenuation was rarely given by candidates.

Multiple-beam explanations were unusually good. Precautions during therapy reduced an interesting range of answers, sometimes highlighting a confusion with diagnosis: wear a lead Jacket (or even lead suit) wear a white coat and a badge use safety glass occurs at the skin because of a large change in specific acoustic impedance’ is the sort of detail expected in a specialist Topic. Translating 4 ‘ 10-5 s to 40 ms in order to add two pulses to the grid was done only by the best candidates. 6 6733/02 practical -rest PHI 1-48 26. 8 8. Dispatch of Confidential Instructions For this examination we trialed releasing a simple list of apparatus in advance of the Confidential Instructions. This was not found to be helpful and will not be repeated. Dispatch of the Confidential Instructions will continue to be about 2 months before the examination. For January examinations the dispatch is sent to centers who have made actual entries; for June it is sent to centers on the basis of estimated entries, since the deadline for entries is within 2 months of the summer practical tests.

If the Confidential Instructions are not received in your centre please check the status of your entries with your examinations officer and with Diesel’s Entries and Certification Department. In general, virtually all candidates showed competent skills n obtaining adequate data of suitable precision and accuracy. The better candidates improved the quality of their observations through using good techniques and repeat readings where appropriate. Candidates were considerably less skilful in the planning and analysis aspects of the paper. This was particularly so in the second question of both papers.

Candidates need to be taught that discussion of results must be based on quantitative analysis and that the description of how they would plan an experiment should clearly state all the measurements that they would take and how they would process them. GROUP 1 (AAA) Question IA The majority of candidates used ten spheres in a channel which were shown as close packed. Often set squares were shown at the end of the line of spheres but they were rarely shown with one of the perpendicular edges of the set square along one of the horizontal edges of one of the rules.

The scale reading that would need to be taken from the rule in order to determine the length of the ten spheres were rarely exactly to O mm. Good candidates obtained a value of d within the expected range of the Supervisor’s value. In order to gain full credit for the percentage uncertainty examiners expected antedates to show clearly that the uncertainty was in the measurement of I rather than d. For this reason a unit was required with the uncertainty so that this could be checked. For example 0. 1 CM in 15. CM giving a percentage uncertainty of 1. 3% gained 2 marks, but 0. 01/1. 52 giving 1. 3% only gained one mark. Most candidates used 10 marbles when finding the mass and obtained a value which was in the expected range of the Supervisor’s value. Good candidates obtained a density value which was in the correct range and was quoted to an appropriate number of significant figures. Weaker candidates often attempted to convert to keg m-3 (which was not necessary) and obtained incorrect answers because of incorrect conversion of the volume to mm.

Weaker candidates often quoted values of density to four or more significant figures. The experimental arrangement in part (b) discriminated well. Good candidates showed two 7 meter rules on the diagram checking the height of the string above the bench at two different places. Set squares were also shown at the bases of the rules to show that the rules were perpendicular to the bench which was assumed to be horizontal). Weaker candidates often attempted to check that the string ABA was horizontal by using a set square against the vertical stand.

This was given 1 mark but is not regarded as a very good technique. Some attempted to check that there was a right angle between the vertical string connected to the mass hanger and the string ABA. This was not given any credit because it was felt that the strings would move when the set square was positioned. The main fault on the free-body force diagram was that many candidates failed to distinguish between the tensions in the horizontal and the inclined string. Most candidates gained reasonable marks for the calculation.

Typical mistakes included: ; not putting their data on to the diagram using a vertical force of 0. 3 g = 2. 94 N, rather than 3. 92 N using too many significant figures for the value of the tension in the horizontal string using 400 g = 3920 N for the weight of the mass in some cases obtaining a negative value for the horizontal tension (by taking the square root of a negative number) because their measured tension was less than 3. 92 N The majority of candidates obtained full marks for setting up the circuit and taking he appropriate reading.

Minor errors at this stage included: the omission of units using units of A for current rather than ma Good candidates had no difficulty with parts (b) and (c) and often the total of 10 marks was scored. The mark scheme allowed a number of compensation marks at each stage so that the full range of marks between O and 10 was scored in these two sections. Compensation marks included the following: In part (b) if candidates chose the wrong set of results they could still gain the calculation mark, e. G. 6. 11 V/80. 5 ma = 75. 9 W gained 1 out of 4.

In (c)(I) if candidates divided either VI or IV or the average of the two values by their answer to part (b) they gained 1 out of the 2 marks. As an alternative to this they could also state their answer to (a)(ii). Correct answer should have been (a)(I) value – (c)(I) answer, but (a)(I) value – value was allowed. Candidates should have subtracted 0. 7 V from VI in order to obtain the p. D. Across the resistor RI . However, examiners allowed 0. 7 V to be subtracted from either VI or IV or the average of the two values. An alternative approach to parts (c) and (c) (iii) was successfully applied by some antedates.

The total resistance of the arrangement was found by using VI Ill . The resistor RE was in parallel with the series arrangement of the diode and the resistor RI, hence the resistance of the series combination could be found. The current in the series combination 8 could then be found by using VI ‘(series combination resistance). The resistance of RI could then be found by using series combination resistance – resistance of diode The diode resistance was found using 0. 7 (answer to Despite the impressive approach used by some candidates in parts (b) and (c), the answers in part (d) were

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