Version 1.0
General Certificate of Education (A-level)
January 2012
Physics A
PHYA4
(Specification 2450)
Unit 4: Fields and further mechanics
Report on the Examination
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Report on the Examination – General Certificate of Education (A-level) Physics A – PHYA4 – January
2012
GCE Physics, Specification A, PHYA4, Fields and Further Mechanics
Section A
Keys to Objective Test Questions
1
2
3
4
5
6
7
8
9
10
11
12
13
A
D
C
C
B
D
A
A
D
C
B
B
A
14
15
16
17
18
19
20
21
22
23
24
25
D
C
D
C
D
D
B
B
A
B
C
A
The facility of a question is a measure of all students attempting a question who choose the correct
option. The mean facility of this paper was 68%. The facility for individual questions ranged from 89%
for question 23 to 39% for question 16. For the purpose of monitoring standards over time, objective
tests contain a proportion of questions that are re-banked after satisfactory use in an earlier
examination. This test contained six of these questions, with a mean facility of 57% when last used.
The nineteen new questions had all been pre-tested and had a mean pre-test facility of 46%.
Students invariably produce higher facilities for the questions in a real examination than in the
pre-testing situation. The improvement achieved on this paper for these new questions on average
was 21%. The mean facility of all of the re-banked questions improved by an average of 9%.
The point biserial index of a question is a measure of how well the question discriminates between the
most able and the least able students. The mean point biserial for this paper was 0.40. The new
questions had a mean pre-test point biserial of 0.38, whilst the value for the re-banked questions was
0.39. On average there was therefore a slight overall improvement in the discrimination of the
questions, but only half of the pre-banked questions actually showed better discrimination this time.
No fewer than 17 of the questions (questions 1, 4, 6, 8 – 14, 17, 19, 20 & 22 – 25) proved to be easy,
with facilities over 65%; none proved to be difficult (ie had a facility less than 35%).
Question 1 involved the calculation of an impulse from a force-time graph, and the consequent
velocity of a body at rest that was then subjected to the impulse. The question discriminated well.
Two-thirds of students arrived at the correct answer, whilst over a quarter of them selected distractor
B, which was double the expected velocity. This may be because they forgot a factor of a half when
calculating the area under a triangular graph.
An elastic collision between bodies of equal mass, one of which was stationary before the collision,
was the subject of Question 2. Students who had witnessed such a collision on an air track, for
example, should have had little difficulty in realising that the moving body stops whilst the second body
moves off with all of the first body’s momentum. Fewer than expected (56%) gave the correct
response. It is difficult to see why 36% of the students selected distractor B, in which neither
momentum nor kinetic energy would be conserved.
Question 3 had been used in a previous examination, when only half of the students gave the correct
response, this time 63% did so. Application of Newton’s second law in the form ‘resultant force
towards centre = mass × centripetal acceleration’ easily leads to a correct solution using the algebra.
Incorrect responses were fairly evenly spread amongst the other three distractors.
Circular motion was also tested in the next two questions. Question 4, solved by combining F = m ω r
and ω = 2πf, was found to be easy by the 84% of students who gave the correct answer.
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Question 5, a re-banked question, proved to be somewhat more demanding. Its facility was 56%, an
improvement of 11% over last time. Possibly it was the use of the diameter of the wheel, instead of its
radius, that caused almost a fifth of students to select distractor A, half the expected angular speed.
Questions 6 and 7 were also re-banked questions, were about features of simple harmonic motion.
Their facilities were 67% and 61% respectively, both significant improvements on the previous results.
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Report on the Examination – General Certificate of Education (A-level) Physics A – PHYA4 – January
2012
Those who chose distractor D in Question 7 (22% of students) clearly realised that the graph of kinetic
energy against distance should be curved, but they chose the wrong shape of curve.
Question 8 required students to choose an incorrect statement about a mechanical system oscillating
at resonance. The question had been used in a 2004 examination when it was found to be easy. It
proved to be slightly easier this time, with 72% of responses correct. None of the three distractors
attracted a response that was significantly higher than the others.
Question 9, involving statements about Newton’s law of gravitation, had a facility of 85%.
When pre-tested, this question had been found appreciably harder but was more discriminating than
on this occasion.
Data for the gravitational constant and the masses of the electron and proton had to be extracted from
the Data and Formulae Booklet for use in Question 9, where the topic was the gravitational force
between two particles. Over four-fifths of the students succeeded with this.
The algebra required to relate the density of a planet to its mass and gravitational field strength in
Question 11 did not prove to be an obstacle to most students because 79% of them gave the correct
combination from the table.
Appreciation that gravitational potential V is proportional to 1/r was all that was required to arrive at the
correct response in Question 12, which had a facility of 71%. The most common incorrect choice was
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distractor A, where the students may have thought V is proportional to 1/r .
In Question 13 three quarters of the students were successful when dealing with the algebra giving
the velocity motion of a satellite in stable orbit of radius (R + h). This question had appeared in a 2002
examination, when the students found it marginally harder and it was slightly less discriminating.
Question 14 was a fairly direct test of Coulomb’s law for charges under changing circumstances; 70%
of the students had the correct answer.
Question 15 was also on electrostatics but, with a facility of 48%, was much more demanding. At first
sight it appears necessary to solve a quadratic equation to answer the question, but this difficulty can
be overcome by taking the square root of the expression obtained. Incorrect distractor D was chosen
by 35% of the students and consequently the discrimination of the question was poor.
Question 16 was the most demanding question on the paper, with only 39% of the students giving the
correct answer. In order to identify the correct combinations of units to give V m −1, it was necessary to
remember that 1 V = 1 J C−1 and that 1 C = 1 A s. Distractor C was the choice of over a quarter of the
students.
The relationship defining capacitance, C = Q/V, was involved in both Question 17 (facility 79%) and
Question 18 (facility 54%). Distractor C in the latter is clearly a correct statement; no doubt it was
misreading of the question (an incorrect statement was required) that caused 30% of the students to
choose it.
Question 19 was a simple test of conservation of energy in the context of energy storage by a
capacitor. The question had a facility of 69% and was the most discriminating question in the test.
In Question 20 the correct answer could be found by equating the weight of a section of wire to the
magnetic force that acts on it when the wire carries a current at right angles to magnetic field. This
was answered correctly by 75% if the students. Distractor C, where the mass was approximately 9.8
times the expected mass, was the choice of almost one in five. This suggests that the students
involved equated the mass of the wire with the magnetic force, forgetting g.
Question 21 had the unusual outcome that one of the distractors (A) was slightly more popular than
the correct answer (B). This had not happened when the question was pre-tested. No doubt the
mistake made by students who chose distractor A was to forget that the charge of an α particle is +2e,
not +e. The question had a facility of 44%.
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Report on the Examination – General Certificate of Education (A-level) Physics A – PHYA4 – January
2012
Question 22 was a fairly standard test of the algebraic equations which govern the motion of a
charged particle as it moves through a magnetic field at right angles, but involving the frequency of
rotation around the circle. Almost 70% of the students selected the correct response.
The remaining three questions all tested aspects of electromagnetism. Question 23 which, with a
facility of 89% was the easiest in the test, was a straightforward calculation of flux linkage for a coil in
a magnetic field.
Questions 24 and 25, which were each correctly answered by just over three-quarters of the
students, respectively tested the rotating coil and energy losses in a transformer.
Section B
General Comments
Section B gave students good opportunities to show what they had learned and understood about
electric fields, capacitors, simple harmonic motion and electromagnetic induction. As PHYA4 includes
elements of synoptic knowledge, the question on simple harmonic motion included some work based
on the AS specification content about energy conservation and stress. Some very competent answers
were seen and most students knew enough to reach at least half marks on the paper.
Far more students now take care to include details of their working in calculations. The mark that
could be awarded for some of the written explanations, such as in question 3 (c), was sometimes
limited by a lack of clarity in the candidate’s expressions.
Two issues continue to be matters for general concern for students; how to deal with significant figures
in a final answer (question 1 (b) (i)) and how to produce a coherent piece of writing in which correct
and complete physical explanations are given using appropriate terminology (question 4 (b) (i)).
Question 1
The definition of electric potential in part (a) was generally well known. Where students did not score
all three marks this was down to oversight; typically either omitting to mention that the charge involved
in the definition is positive or that the definition involves the work done per unit charge.
In part (b) (i), most students successfully applied V = Q /4πε0r in order to determine the magnitude of
the charge (1.0 × 10−10 C) from the value of V when r = 0.30 m. As the data in the question is given to
two significant figures, an answer was expected to two significant figures. Some students need to
appreciate that the number of significant figures they should quote in an answer needs to be limited to
the least precise data they are working with, not the most (on the Data and Formulae Booklet ε0 is
given to three significant figures). At the same time, in these circumstances the answer should never
be abbreviated to one significant figure (1 × 10−10 C), as was the case in many answers.
The mark in part (b) (ii) was gained easily, usually by applying V = Q/4πε0r, although more perceptive
students saw that V ∝ 1/r could lead to a more concise answer. Part (b) (iii) caused a little more
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difficulty for some students. Application of E = Q/4πε0r with r = 0.60 m was the obvious route.
The pitfall for many was that, by first finding V at M (by the same method as before), they then had to
apply E = V/d to find the field strength. This last equation specifically applies to a uniform field and it
therefore cannot be used here. Surprisingly, there were many students who, having obtained an
incorrect charge in part (b) (i) as a result of an arithmetical slip, did not revisit part (i) when they could
not show either of the required values in parts (ii) and (iii).
Many good attempts to represent the electric field between two plates were seen in part (c) (i), but
careless sketching, such as field lines stopping short of the plates, often meant that it was not possible
to award both marks. Because this was the field between two plates at different positive potentials,
some students were thrown off course both when sketching the field and when the uniform field
strength had to be found in part (c) (ii). In part (c) (iii) the respective radial and uniform fields were
usually recognised but a precise statement that identified which was which was required to gain the
mark.
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Report on the Examination – General Certificate of Education (A-level) Physics A – PHYA4 – January
2012
Question 2
Good definitions of capacitance were usually seen in part (a), leading to an easily gained couple of
marks. Vague statements such as ‘capacitance is a measure of the ability to store charge’ went
unrewarded.
Part (b) caused very few problems, apart from those arising from careless arithmetic or misunderstood
powers of 10. In part (b) (iv) it was expected that students would identify the gradient with current;
‘rate of charging’ seemed too obvious for the mark to be given.
The maximum value of the current could be found directly in part (c) (i) by applying I = V/R, where V is
the emf of the battery and R is the resistance of the resistor. No doubt it was the previous part of this
question that directed so many students to base their response on the initial slope of the graph. This
was equally acceptable, and a wide tolerance was placed on answers arrived at by this technique.
The sketch graphs in part (c) (iii) were often too careless to deserve full credit. This exponential decay
curve should start at an intersection with the current axis (with Imax marked as required) and should be
asymptotic to the time axis. More able students realised that the gradient of this Q-t graph in part (b)
was practically zero at t = 60 ms, and that the current should therefore be very close to zero at this
time. There were many answers showing a current that increased with time, and many others that had
a constant negative gradient.
Question 3
This question, based on bungee jumping, tested simple harmonic motion in an unfamiliar context and
at the same time to provide a synoptic test of some AS content. Examiners were pleased to see that a
high proportion of the students were able to cope competently with this unfamiliar situation.
Application of energy conservation, or of the equations for uniformly accelerated motion under gravity,
led to a high proportion of correct answers in part (a) (i). The equation representing Hooke’s law was
well known in part (a) (ii) but a few students showed confusion between mass and weight.
Part (b) (i), which required the time for half of an oscillation, only caused problems for the small
number of students who misinterpreted the wording and determined the time for one-and-a-half
oscillations. Part (b) (ii) was much more challenging and turned out to be a question that many
students returned to answer on a supplementary sheet. The most direct solution came by applying
the equation 𝑣 = ± 2π𝑓√𝐴2 − 𝑥 2 , with careful choice of the earlier values obtained for v and x, and of
the derived value for f. Most students seemed to think a quick solution could be arrived at by applying
vmax = 2πfA, but this is incorrect. It is possible to reach a correct solution from energy considerations;
this needs particular care over the balance of gravitational pe lost, ke gained and elastic pe gained at
some consistent point in the motion. Nevertheless, a few correct solutions using this approach were
seen.
In part (c) most students realised that the bungee cord would cease to exert a force on the bungee
jumper once she was higher than point P. Few went on to mention that her motion was then purely
under gravity or that her acceleration became constant, although references to the fact that
acceleration would no longer be proportional to displacement were quite common.
Almost all students gave the correct answer – point R – in part (d) (i). The responses in part (d) (ii)
revealed a widespread misunderstanding of the significance of centre of mass, with statements such
as ‘the stress is a maximum at the centre of the cord because that is where the weight acts’ seen.
Acceptable answers included at the point where the cord is attached to the railing (where the greatest
weight is supported) and (because of possible thinning) half way along the cord. It was expected that
students would show that they understood what is meant by stress when formulating their reason,
whichever point in the cord they gave.
Question 4
The transformer turns ratio equation was familiar territory for most in part (a) (i), but correct application
of the efficiency formula proved to be a greater challenge in part (a) (ii). Many correct answers were
seen, and almost all students knew that the number of lamps has to be an integer. Most difficulties
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Report on the Examination – General Certificate of Education (A-level) Physics A – PHYA4 – January
2012
arose from mixing up data for the secondary coil with that for the primary (for example, multiplying the
primary current by the secondary voltage).
Parts (a) (iii) and (iv) proved to be an exacting test of whether students could think through to the real
reasons or had enough practical experience of transformers to know these reasons.
Many attempts at part (iii) were general answers about the reason for fitting a fuse in any circuit rather
than specifically in a transformer’s circuit. Very few students stated that transformer coils can
overheat and become damaged when they handle excessive currents and that they therefore need to
be protected.
Similarly, it was only a small minority of answers to part (iv) that were properly valid; that stopping the
primary current would isolate the whole transformer from the mains or that a failed fuse in the
secondary circuit would leave the primary live.
Most students find that electromagnetic induction is one of the most demanding topics in the
specification. In these circumstances perhaps it should not be surprising that many of the attempts to
answer part (b) (i) were very disappointing. Even when pointed at a logical and sequential structured
answer by three bullet points, many students could not construct a coherent, ordered response.
In assessing the Quality of Written Communication, one aspect that must be taken into consideration
is the appropriate use of technical terminology. This was often absent from the responses seen.
The term induction has a very special meaning in physics; magnetic induction involves magnetising a
material by applying a magnetic field, electrostatic induction involves charge separation by applying an
electric field, electromagnetic induction involves producing an emf by applying a changing magnetic
field. In all cases, direct contact is unnecessary. Many answers contained the word ‘induced’ used
much more carelessly than its technical meaning in physics; a current was stated to be induced in the
coil because it was connected to the ac supply, for example. This current was then said to induce a
magnetic field. Many students seemed obsessed by effects in the iron rod, rather than in the
aluminium ring. The aluminium ring was confused with the coil for example ‘the coil is pushed
upwards by the magnetic field’. It was evident that a large proportion of students were familiar with
statements of the laws of electromagnetic induction but could not apply them meaningfully to explain
what happens in this demonstration. The field produced was regularly referred to as an electric field.
The repulsion of the ring was sometimes attributed to Coulomb’s law and the repulsion between
charges. Fleming’s left hand rule was confused with his right hand rule, or with the right hand grasp
rule.
Broadly, an outline plan of an appropriate answer to this question would be along the following lines.
The ac current in the coil produces an alternating magnetic field, which is concentrated in the iron rod
and passes through the ring. This changing magnetic field induces an emf in the ring. Because the
ring is aluminium it is a good conductor and the emf causes a large current in it. A current-carrying
conductor in a magnetic field experiences a force so this current produces a magnetic field whose
direction opposes the applied field. Interaction between these fields gives a net upwards repulsion of
the ring. As the ring moves upwards the magnetic field becomes weaker and the force on the ring
decreases. The ring’s position becomes stable when the upwards magnetic force balances its weight.
Answers written in this fashion were rare but not difficult to identify and they were rewarded well.
Answers to part (b) (ii) suffered from the same lack of general understanding as the previous part.
It was often realised that the ring would move to a higher position, or be expelled upwards from the
rod. Reasons were less well presented. Reference to a larger induced current (or emf) in the ring was
considered a prerequisite for an acceptable explanation.
Please visit AQA’s Enhanced Results Analysis service. A free, online tool that gives you an
instant breakdown of your GCE Physics results.
Grade boundaries and cumulative percentage grades are available on the Results statistics page of
the AQA Website.
UMS conversion calculator www.aqa.org.uk/umsconversion.
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