A-LEVEL
PHYSICS A
PHA5D – Turning Points in Physics
Report on the Examination
2450
June 2014
Version: 1.0
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REPORT ON THE EXAMINATION – A-LEVEL PHYSICS A – PHA5D – JUNE 2014
Section A – Nuclear and Thermal Physics
General Comments
The examination as a whole produced a good spread of marks from candidates. It showed up
strengths, such as the understanding of nuclear binding energies and skills in calculations over a
range of topics from half-lives to heat and kinetic theory. It also highlighted a gulf in many
candidates knowledge. Very few had a clear picture of how nuclear waste is treated.
Question 1
Question 1(a) was very straightforward for many candidates. Less able candidates did not
elaborate on what the nucleus split up into or they referred to some other splitting such as fission.
Another common error was to talk about the separation of an atom into protons, neutrons and
electrons. Very few candidates got the direction of energy flow wrong.
In part 1(b)(i) very few candidates referred to particles other than neutrons but a significant number
failed to write down two neutrons in the correct manner.
and a single
were extremely
common.
The follow on part 1(b)(ii) turned out to be an extremely good discriminator. It highlighted the types
of errors candidates were making. There was a group who did not appreciate the data was given
per nucleon and used the figures without multiplying up by the respective nucleon numbers.
Another significant group added the mass energy of mostly one but sometimes two nucleons into
the proceedings. The last major group got into difficulties because they changed units
unnecessarily or only changed the units of some of the terms. Overall there was a good
percentage of correct answers.
A good proportion of candidates could accomplish the conversion of units required in part 1(b)(iii)
with relative ease, sometimes from an error carried forward from the previous part. With the
conversion needing two stages using the datasheet there was plenty of opportunity for errors by
dividing instead of multiplying or using an incorrect conversion factor. A separate but common error
was to ignore the answer to 1(b)(ii) and simply use the difference in mass of the nucleons involved,
ignoring the binding energy per nucleon completely. Less able candidates did not really help
themselves in this question because very few put words or units to intermediate stages of the
calculation. If they had done so fewer would have lost their way.
A majority of candidates did not score marks in part 1(c)(i). Many knew the general shape but very
few remembered any details so they had no idea which coordinates to draw their line or band
through. Some gave no real thought to the problem and drew graphs that did not make sense. For
example some graphs went vertical or turned back on themselves.
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REPORT ON THE EXAMINATION – A-LEVEL PHYSICS A – PHA5D – JUNE 2014
Part 1(c)(ii) was very discriminating. Less able candidates simply referred to any radiation that
came to mind and forwarded very little explanation. The bulk of the candidates knew beta minus
radiation was emitted but they were not careful how they expressed their reasons. For example,
stating that there are more neutrons than protons is not sufficient to imply the nuclei are neutron
rich. A majority in this group of candidates made no reference to the graph at all. Many that did had
flawed reasoning. They thought the isotopes were neutron rich because the large number of free
neutrons in the core. Good candidates also got into some difficulty by not reading the question
carefully. Typically these candidates would start their answer with, 'If there are a lot more neutrons
than protons then... but if the neutron to proton ratio is small then...'. These candidates obviously
knew the subject matter but did not score many marks as the question clearly asks for a choice to
be made.
Question 2
The introductory part 2(a)(i) was done extremely well with over 90% gaining the mark. A few got
the equation incorrect and fewer still lost their mark by calculating the half-life in s-1 units.
Less able candidates failed at the first step in part 2(a)(ii) by not using the exponential decay
equation. In other cases it was almost universal that mistakes were made at the substitution stage.
Some common errors were to swap round the abundance of C-12 atoms initially and at a later time
in the equation. Others sometimes added 1 to the proportion 0.375 and used N0 = 1.375 and Nt =
.375. Many candidates understood how to process the mathematics and there were a high
percentage of full marks.
Although students appeared to have a good idea of the difficulties in carbon dating young and old
objects in part 2(b)(i + ii) many could not express their ideas well enough to score marks. It was
common to see, 'In less than 200 years none of the C-14 would have decayed'. Very few referred
to the small difference in activity when comparing a new sample with the 200 year old sample.
Candidates were much more successful in the second part when referring to the much older
sample. Most knew that the activity would be very small and it would be difficult to get statistically
significant data. Several did not gain this last mark because of lack of care in what they put down
on paper. It was common to see variations of, 'There has been so much decay there is no C-14
atoms left'.
Question 3
A majority of candidates obtained the mark for 3(a) by stating it was equal to the number of atoms
in one mole of substance. Very few gave the full definition which relates to carbon-12. Many
candidates must have been aware of the definition because they tried to incorporate it into what
they put down. For example, 'The number of atoms in one mole of carbon-12'. Sometimes the
halfway approach went wrong and we saw, 'The number of particles in 1 atom of carbon-12', or
similar. There were also a few candidates who took a kinetic theory equation, which had Avogadro
constant, which they then rearranged to make the constant the subject. This was not regarded as a
definition.
Part 3(b)(i) was an easy substitution into an easy equation and most candidates scored the mark.
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REPORT ON THE EXAMINATION – A-LEVEL PHYSICS A – PHA5D – JUNE 2014
By contrast in 3(b)(ii) it was only the very best candidates who completed the whole of the
question. The main problem was that the majority did not appreciate that the mass of an individual
krypton atom was required in the equation for mean kinetic energy. The other surprising difficulty
was the unit of mean square speed. Only about a quarter of candidates got this correct.
In 3(c) most candidates scored the mark for krypton's mean square speed being less. As expected
the most common error in the explanation was to suggest the kinetic energies of both gases are
the same rather than having the same means for their kinetic energies.
Question 4
A majority of candidates only scored one mark in part 4(a). These candidates either forgot to
indicate a unit mass or, as in a majority of cases, they omitted the phrase, 'without a change in
temperature', or equivalent. A few had problems in appreciating whether energy was required or
whether energy was given out. It was very noticeable that at the lower ability end candidates have
a poor vocabulary associated with this area of physics. Phrases like, 'to change water to a gas
without changing state', or 'condense water into steam', and others showed a lack of distinction
between boiling and evaporation.
The calculation of 4(b)(i) did not hold many difficulties for the bulk of the candidates but the
significant figure issue did. In part 4(b)(ii) most candidates were relatively clear how to tackle this
question. It was in the detail that errors were made. The most significant was to forget about the
copper can, which also gained energy to reach the final temperature. Also at the lower ability end
there were many opportunities to make arithmetic errors.
The scores were much better for part 4(b)(iii) albeit from an error carried forward from 4(b)(ii) in
many cases. So the use of the latent heat equation is not difficult to grasp for a majority of
candidates. The main error was from rounding off incorrectly or making errors in powers of 10
when converting to SI units.
Question 5
In part 5(a) the idea that the containment vessel will stop various forms of radiation was well
understood. However the knowledge that these radiations include neutrons was not appreciated by
most.
Many candidates had problems with question 5(b) mainly because of lack of knowledge. Students
often made up scenarios that fitted their ideas about how waste might be treated. Now and again
they struck lucky with their ideas. Very few knew exactly how waste was treated and in what order.
In many cases candidates spent most of their time answering the question that they wanted it to be
and it was common to see more than 50% of a script dealing with why radiation is bad for the body.
So scripts were very polarised. Some scored full marks very easily. The others had an uphill
struggle. These candidates used a huge number of space filler sentences that could easily have
been given credit if they had contained a few more specific facts.
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REPORT ON THE EXAMINATION – A-LEVEL PHYSICS A – PHA5D – JUNE 2014
A few examples are: 'X has a short half-life so it will quickly become safe.' or 'Y is easy to screen
so it does not need to go in anything special.' or 'When waste is taken out of the reactor it is
dangerous so you must take care.' or 'With nuclear waste you can never be too careful.'. The
candidates who made a serious attempt at this question still had a few misconceptions. It was
thought by most that a cooling pond was used to quench the heat left over from the waste being in
the reactor. They failed to realise it was still generating heat from the continuing decays. It was
also thought the waste could be used, apart from reprocessing plutonium and uranium. Many
thought individual isotopes were extracted from the waste for use in industry. So they obviously do
not understand how individual isotopes are generated. Overall the question was interesting in that
it highlighted a large void in students’ knowledge.
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REPORT ON THE EXAMINATION – A-LEVEL PHYSICS A – PHA5D – JUNE 2014
Section B – Turning Points in Physics
Question 1
(a)(i)
Most candidates were able to describe one way in which positive ions are produced
although few recognised that positive ions and electrons are responsible for conduction in
the tube.
(a)(ii) Many candidates successfully explained how excitation by collision caused photons to be
released although answers that referred to the gas being excited rather than gas atoms or
molecules or particles were not uncommon. Few candidates did recognise recombination
as a cause of light emission and those who did usually explained light emission due to
excitation by collision as well. Very few candidates gained full credit for their explanation of
why the gas needed to be at low pressure although some did recognise that at high
pressure the particles would not be accelerated sufficiently to cause excitation. Few
candidates related the pressure to the (mean) spacing of the gas particles.
(b)
Candidates lost marks due to careless errors such as referring to the mass of the gas
rather than the mass of the gas atoms/molecules/particles. Some candidates lost a mark
because they gave the specific charge as e / m rather than charge / mass or Q / m.
Question 2
(a)
Although many candidates knew the photoelectrons experience an attractive force towards
the metal surface, many thought photoelectrons leave the metal because they are attracted
to T. Candidates often failed to recognise the electrons leaving the surface arrive at T or
that the microammeter reading is due to the electrons reaching T. Very few candidates
recognised the photoelectrons have a range of speeds or kinetic energies. Common
misconceptions about photoelectricity were evident; some candidates thought the electrons
leaving the surface were diverted from moving to T and others thought the reduced
photocurrent is due to increased resistance.
(b)(i)
Many candidates scored both marks here although straight line graphs through the origin
caused loss of a mark.
(b)(ii) Few candidates gained full marks on this question. Many answers explained the equation
rather than using it to explain the graph. Descriptive answers without adequate use of the
equation rarely gave more than a mark. Candidates often recognised that the maximum
kinetic energy is equal to eVs but failed to convert the given equation into the equation for a
straight line graph. Candidates who were able to complete this step were usually but not
always able to score full marks.
(c)
Most candidates scored full marks on this calculation. The most common error was in the
calculation of the photon energy where the wavelength was sometimes converted
incorrectly from nanometers to metres.
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REPORT ON THE EXAMINATION – A-LEVEL PHYSICS A – PHA5D – JUNE 2014
Question 3
(a)(i)
Many candidates knew that the magnetic force is perpendicular to the electron’s velocity
although some failed to recognise the consequence that no work was done by the magnetic
force.
(a)(ii) Candidates scored well on this question. However, some candidates lost marks as they did
not give a 2 s.f. answer or failed to include the electron charge value in their calculation.
(b)
Many candidates knew diffraction and magnetic deflection were the key properties and
were able to relate at least one of these properties correctly to where they occur on the
electron microscope. Candidates who additionally discussed these properties in relation to
the screen or the image quality or magnification usually scored 4 or more marks.
Candidates who gave lengthy descriptions of the electron microscope and failed to bring
more than one or two key points into their answers often scored no more 2 marks.
Question 4
(a)
Many candidates gave vague answers that did not gain the mark, often referring to objects
moving at constant velocity in the frame of reference rather than constant velocity of the
frame of reference itself.
(b)(i)
Most candidates were able to score this mark although very few candidates arrived at it by
recognising the speed is equal to 0.86 c.
(b)(ii) Many candidates recognised that the proper time to is to be calculated and usually gained
full marks. Candidates who substituted an incorrect answer from bi into their equation could
gain full credit for their answer to bii. Those who substituted 5 years for to into their equation
to calculate t lost the last two marks.
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
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