phy315109 - physics - assessment report 2013 · 2019. 9. 28. · 2013assessment’report’...

Physics Course Code: PHY315109

2013 Assessment Report

Tasmanian Qualifications Authority of 8

The solutions to the 2013 paper are at the end of the report. Once again, they are written on the exam paper by the appropriate marking examiner as there was a lot of positive feedback when they were presented this way last year. The marking examiners saw little evidence that the paper was too long, which is excellent. They also considered the spread of difficulty was appropriate and there appeared to be relatively few questions that were ambiguous. The use of appropriate units and significant figures in the answers was good, with only the weakest candidates falling down in these areas. The only concern with the paper as presented was that the diagram used for Q16 b(ii) was not clear enough and the correct answer needed the detector to be placed right at the top of the page. Since most students mis-‐interpreted the diagram it was decided by the marking panel that the correct answer was to be awarded a bonus mark (this applied to 8 candidates) and the most common answer (arrived at by misinterpreting the diagram) was given full marks. The Assessment Panel set the following cut-‐offs, with each /40:

Criterion A B C

5 32 23 12

6 32 22 12

7 32 22 12

8 33 26 13 Following is a discussion of the exam paper question by question. PHYSICS EXAMINERS COMMENTS 2013 Question 1 a) Many candidates neglected to find the net force and simply used the resistance to determine the

acceleration. Of those who did take into account both the tension and the resistance, quite a few did not do a correct vector addition, so used an incorrect net force. Quite a few students misread 2.5 on the horizontal scale of the graph as being half way between 2 and 4 m s-‐1.

b) Satisfactory. c) i) Those who realised or derived P = Fv had no problems. Many tried to calculate the ship’s

kinetic energy and divide by some arbitrary time interval (usually one second). These answers scored zero.


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ii) A substantial number of candidates incorrectly said that doubling the power would double the force, and then either read the corresponding speed off the graph, or said that this would double the speed. In neither case did they acknowledge that the product of their two values gave nothing like the approximate value of 780 kW required. A large number of candidates simply ignored the graph. Perhaps 2% of candidates did this question correctly.

Question 2 Even in Year 12 Physics, people are mixing up vertical and horizontal components, as well as the hypotenuse, in projectile motion. Some students are simply performing arithmetic without quoting the equations or relationships they are using. Others are quoting the equations but then showing a final answer without showing what numbers they substituted into the equations. a) i) Generally satisfactory, although some students tried to do a trigonometric calculation

based on an initial velocity that was not given. a) ii) & b) Satisfactory c) Again, satisfactorily answered. Some students lost up to a mark if there was no indication (here

or in previous parts) of how they arrived at the required horizontal velocity before substituting it into the appropriate equation of motion.

d) Those who got this far generally had no problems in this section. Perhaps half of those who did

this part took a fairly inefficient approach, namely dividing the problem up into sections and considering the trajectory from the zenith onwards as a horizontal projectile motion problem, and then subtracting their answer from 15 m. The question can be done more quickly by simply substituting the flight time and vertical initial velocity into s = ut + ½ at2.

Question 3 a) Most students recognised that only two forces existed at the beginning of the rocket’s motion –

namely weight and the thrust of the engines. These were given strange names at times. b) Few students mentioned that Newton’s Second Law was relevant. Far too many students simply

used F = ma = 2.6 x 106 x 2.2 as their calculation, ignoring weight altogether. c) This was poorly done, with few students correctly identifying the necessary force as the thrust

and then realising mass per second = thrust/velocity of the gas. d) Poorly done with most students not including weight and many not converting tonnes to kg.


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Question 4 a) Most students calculated the very simple answer correctly though a few did not convert gram to

kilogram. Few put in direction. b) i) Extremely poorly done. Students had enormous difficulties with the directions and

subsequent momentum diagram. The fact that the first ball rebounded made the direction of the larger ball difficult to envisage. Many students failed to add the two final momenta to get the original, but added the final of the smaller to its original to give entirely the wrong momentum for the other ball.

ii) In lieu of the poor diagrams, this section was understandably badly done. The biggest

disappointment was the number of students who used 1D solutions rather than some form of 2D solution. The marker was generous.

c) The question was slightly ambiguous. It could be read that each ball individually had to have the

percentage loss calculated, rather than the system. Many students calculated the loss but did not move on to calculate the percentage as requested.

d) This answer depended on whether students knew the term ‘inelastic’. Some didn’t. Question 5 Overall this question was made more difficult by the use of many units rather than standard SI value, km mixed with metres, minutes rather than seconds. a) The first section was intended to be a straightforward use of a calculator, but many students

used the wrong radius (that of the planet) or added radii, or didn’t convert or…. But still somehow got the right answer! Amazing.

b) Poorly done, with students not really reading the question and sitting back to understand the

problem. Many students leapt to a formula and played with it incorrectly, interpreting the mass as that of the satellite rather than the planet.

c) Reasonably done but many students reverted to not using their own value of mass. Question 6 a) i) Field lines should start at the positive plate and end at the negative. Enough lines should

be drawn to show the field is uniform. ii) Well done, except for the conversion from cm to metres.

iii) Also well done, except the conversion from g to kg was often omitted.


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b) A range of answers was accepted, with credit given for good understanding. Many students inferred incorrectly that the force up had to be greater than force down in order for the sphere to move up. Alternative acceptable answers were:-‐

To move upwards at constant speed implies Fnet = 0. Since no magnitude of speed was mentioned, just a momentary increase in voltage would be enough to get the sphere moving up from rest and, if it were moving up slowly, it would continue to move up if V = 15 kV.

Or To move up with the same speed as it falls, with V = 0, would require a PD of 30 kV.

Question 7 a) Straight forward, but incorrect in about a third of the papers. b) Good understanding. Some students did not include the 50 turns. c) A commutator is needed to reverse the current each half turn so that the force is always acting

to turn the coil in the same direction. d) i & ii) Well done, except for some students ignoring the commutator and some thinking that V

should be constant if the rate of rotation is constant.

iii) The only problem was the emf was for a single wire, not 50 turns of the wire. Also units are volts.

Question 8 a) Magnetic field lines around the magnet should not meet or cross. More care needed in

sketching. b) Most students deduced that a south pole needed to be induced on the magnet side of the coil

but many then had the current in the wrong direction. c) The current needs to increase as the magnet approaches the coil, then go to zero then reverse

direction as the magnet moves away to the right. Symmetry was expected. Question 9 This was a fairly straightforward question but many students had difficulty completing it successfully. a) Forgetting to square ‘v’ and only calculating Ek were the most common mistakes. b) Many attempted to use E = V/d, which is not correct.


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c) Often well done, with excessive early rounding resulting in lost marks. Many students used the vertical distance of 2 cm instead of 1 cm to incorrectly test if the electron emerged.

Question 10 Few students received full marks for all sections. Part (d) was essentially a maths question that examined a student’s maths skills; partial attempts were given partial marks. The use of the term ‘null point’ in the question caused confusion and appeared to be a new term for many students which most interpreted to refer to the point “X”. a) Crossing field lines and lack of merged field lines were common errors. b) The wrong ‘k’ was often used. c) Few students answered the question as the examiner intended. Question 11 a) Well answered by the majority of students, but some just copied a inappropriate diagram

directly from the formula sheet and therefore got no marks. b) Well done by most candidates. c) Some found the length for the incorrect lower frequency, 858 Hz Question 12 a) Poorly answered. Only a minority of students could calculate the tension in the rope. b) Less than half the students recognised that the tension decreased on descending the rope. c) Descriptions were good but explanations were lacking. d) Most students focused on change to line density or tension but did not take both into account. Question 13 a) Polarisation was not well understood. Most answers were vague, describing light making an

angle to the polariser. b) There was considerable confusion about the orientation of the polarising filter and the partial

polarisation of the reflected light. c) This question was quite novel and led to a great variety of guesses.


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Question 14 a) Surprisingly poorly answered; far too many students believed the frequency had to change! b) i) Well done. ii) Very poorly answered. A common error was to confuse diffraction with refraction.

Huygens principle as applied to refraction was very poorly understood. c) This was a novel question that was surprisingly well answered. Question 15 All sections of this question were well answered, apart from the diagram showing the path of the light. Angles of incidence and refraction seemed poorly understood, as did how to decide if total internal reflection took place. Question 16 a) & b) i) Well done. b) ii) In placing the detector, candidates failed to realise that a path difference of 2.5 λ meant

the detector had to be on the 3rd nodal line above the central maximum. (In this case right at the top of the page.)

c) Part (i) was good but in Part (ii) very few candidates realised halving d would double W and so

there needed to be half as many fringes shown in graph 2 as in graph 1. Question 17 a) Most students named X as ‘stopping voltage’, but the explanation, in general, was poor. A

significant minority of students incorrectly thought the filter was relevant. b) Nearly all students recognised the gradient was h but a significant minority were unable to

correctly calculate that gradient. c) The threshold frequency was poorly identified. d) Poorly answered, with many students stating conclusions that were irrelevant to the PE

experiment. Any conclusion relevant to the PE experiment was accepted regardless of whether it was Einstein or not.


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Question 18 a) Both minimum excitation and ionisation energies were accepted as answers; done well. b) i) Correctly answered by nearly all students, although many thought their answer in metres

was incorrect because they were looking for an answer of 30.

ii) Answered correctly by just about everyone. (Both UV and X-‐ray was accepted) c) i) Only half the students answered this correctly with many students giving the Ek of the

electron after the excitation as the answer. Most students overlooked the n3 to n2 transition.

ii) Just about all students answered this correctly. Mentioning the original photon was not

mandatory in the answer as long as it was made clear that the entire 50 eV energy had to be transferred.

Question 19 a) Most students made reference to the high energy involved, but few referenced 1%. A lot of

students mentioned that the forces in tungsten must be strong in an irrelevant explanation as to why the melting point was high.

b) i) Poorly answered as most students had only a vague idea.

ii) Many students used the name – brehmstrahlung or braking (both words in a variety of spellings) – but not many gave correct explanations of what that was.

iii) Even more poorly answered than (ii)! The majority of students used the reference ‘threshold frequency’, revealing confusion with the photo-‐electric effect.

c) i) Quite a large minority of students correctly stated the spikes would remain at the

frequencies indicated, although many then sketched the spikes in different positions. Not many students indicated the intensities increased (again many stated it would increase but failed to show this on their sketch.) Few students recognised that fmax would increase.

ii) Quite a large minority of students correctly stated the spikes would change from the frequencies indicated, although many of those students then sketched the spikes in same position.

Question 20 a) Generally well done, with many students getting full marks. Common sources for error were

including the beta particle as an incoming particle, rather than one that was ejected, and failing to outline all three steps as asked.


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b) i) Generally well done. Most students recognised that they had to find the mass defect, and common errors were calculation mistakes, rounding off the masses, and failing to include two neutrons as products.

ii) Well done. c) i) Well done. Common errors were using a mass of 238.03 (from the table), rather than 233

for uranium, when working out the number of atoms, and students trying to use E=mc2. Some students used mole calculations successfully.

ii) Adequately done. Many students left out the 30% efficiency factor, or applied it wrongly. Question 21 a) i) Generally well done. Most candidates recognised the formula to use, but some did not

realise that the mass and years could be used directly. Determining the number of atoms was a common, but superfluous, approach.

ii) Poorly done. Many students did not realise that the mass from the previous question had to be subtracted from the total mass, or forgot to find the fraction required.

b) Generally well done. Common errors were mixing the time units of the decay constant and the

decay equation, and in solving the decay equation.

TASMANIAN QUALIFICATIONS AUTHORITY

ASSESSMENT PANEL REPORT

PHY315109 Physics

18% (50) 21% (59) 21% (59) 40% (110) 278

23% (56) 22% (54) 15% (37) 40% (100) 247

11 % 19 % 39 % 31 %

20 % 23 % 29 % 28 %

11 % 19 % 39 % 30 %

80% (223) 20% (55) 1% (3) 99% (275)

79% (196) 21% (51) 0% (1) 100% (245)

78% 22% 1% 99%

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EA HA CA SA Total

Previous 5 years (all examined subjects)

Last year (all examined subjects)

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phy315109 - physics - assessment report 2013 · 2019. 9. 28. · 2013assessment’report’...

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