Key messages from the PERT
ASA Physics (9702)
Papers 2 & 4
General Comments
Candidates need to take care to ensure that they read the question properly, understand what is
being asked and give responses that answer the question that is asked. It is not uncommon to find
candidates giving answers to questions that were not asked, but that have been asked in recent
past papers.
Candidates should choose their wording carefully, especially when giving a definition or describing a
situation. Often answers are made unclear through the omission of certain key words and
inappropriate use of everyday language. Examples of words that are often confused by candidates
are atom, molecule and particle, nuclide and nucleus, and force and field. Candidates are not able
to obtain full credit if they use an inappropriate word that makes the response technically incorrect.
Candidates should pay particular attention to the prefixes of units. If these are ignored or interpreted
incorrectly, it can lead to a power-of-ten error in the final answer.
In ‘show that’-type questions, credit is given for showing the calculation as well as the final answer.
Therefore, candidates must carefully present each step of the calculation with full substitution
including values of physical constants such as g or c. It is important to remember that when an
equation is rearranged, the new subject should also be given.
Answers to numerical questions should be given to an appropriate number of significant figures; the
precision of the data provided in the question is generally indicative of the appropriate number of
significant figures for an answer. When performing intermediate calculations within a question,
candidates should take care to avoid premature rounding; as a general rule, any intermediate
calculated values should always carry at least one more significant figure than will be used in the
final answer.
Candidates should be made aware that giving answers to an inappropriate number of significant
figures, or that are inaccurate as a result of rounding intermediate values prematurely, can both lead
to full credit not being awarded.
Candidates should be reminded that full credit is not awarded for answers inappropriately given as
fractions.
© Cambridge University Press & Assessment 2024
PERT key messages
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Particular points: Paper 2
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In Q 1b, a common mistake was to assume that the difference in pressure (p2 – p1) had no
base units instead of the base units of pressure.
Some candidates confused “mass” and “weight” in Q2
In Q3, a significant minority of the candidates incorrectly defined force as ‘mass ×
acceleration’. Only the stronger candidates could explain that there was constant velocity
because there was no resultant force acting on the block.
In Q4, a common mistake was to substitute the value of the total length of the spring instead
of its extension, or to make power of ten errors in unit conversion
In Q6a, many candidates made confused and inappropriate attempts to apply Ohm’s law
Paper 4
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In Q1 some of the weaker candidates confused the Earth’s radius with the height of the
satellite, or did not know the difference between angular speed and linear speed
Many candidates did not define T as absolute temperature in 2(a), and quoted absolute zero
as -273 rather than the precise syllabus value.
Q2b(ii) is a question where candidates should pause to think about the plausibility of their
answer. It was not uncommon to see incorrect answers giving the mass of a molecule of the
order of 1026 kg. Candidates who carried out a quick check of the sense of their answer
would have realised that a mistake must have been made in such a calculation. In 2(iii) it
was common to confuse the mass of a molecule with the mass of a gas.
Candidates found Q3 difficult. Only a minority of candidates could give a fully correct
statement of the law, with the directions of transfer clearly and consistently stated for all
three quantities. There were many misconceptions in applying the ideas consistently in the
remaining parts of the question
Q4 was well answered. A common misconception among weaker candidates was to confuse
the potential energy of the oscillations with gravitational or elastic potential energy
individually (not appreciating that the potential energy of the oscillations is the sum of the
two). Some candidates also confused the terms amplitude and displacement.
Q5 was a demanding question which discriminated well between higher achieving
candidates
In Q6 most produced correct definitions and calculations but struggled with the explanation
in part (c). This question is a good example of the need to answer the question that was
asked, and not attempt to reproduce the mark scheme for a question from a previous paper.
This question did not ask candidates to explain why there is an induced current, and it did
not ask for a statement of Lenz’s law. The question asked candidates to use Lenz’s law to
determine the direction of the current in the disc. The key to answering the question was to
appreciate that the current in the disc, being perpendicular to the external magnetic field,
causes a force to act on the disc. The point where Lenz’s law is applied is to determine that
the direction of this force must be to oppose the rotation of the disc (which is the change that
leads to the induced e.m.f.). Once the direction of the force on the disc is known, Fleming’s
left-hand rule can then be used to determine the current direction. Many responses were
very muddled.
In Q7(b) the difference between the rectified voltage–time graph and the power–time graph
is not understood by many candidates, and many found part (iii) very challenging to explain.
Q8 required accurate use of the technical terms energy levels, difference in energy levels,
photon energy and radiation frequency. Many responses could not be awarded credit
because these terms were not used correctly.
© Cambridge University Press & Assessment 2024
PERT key messages
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The definition of mass defect in Q9 appears to be not as well known as most definitions.
Q9c(i) was a challenging question. It was not possible to answer just by recalling an
equation and plugging numbers into it.
Many answers were seen to Q10a(ii) in which candidates attempted to answer a different
question from the one asked, and offered responses that attempted to replicate previous
mark schemes for a question asking why the emitted X-rays have a range of energies.
Paper 3
General Comments
Candidates need to use a sharp pencil to plot points and draw lines of best fit. Points should be
drawn as neat crosses and not as ‘blobs’ (circles greater than half a square in diameter), and lines
should be continuous, thin and straight. A transparent 30 cm ruler should be used for drawing lines
of best fit.
To be successful answering Question 2, candidates should be reminded that their identified
limitations and suggestions for improvement must be focused on the particular experiment being
carried out and relate in turn to the measurements taken. To practise and develop this skill,
candidates should be encouraged to look at the experiment chronologically stating the limitations as
they encounter them or focus on the difficulties of the measurements that they are asked to collect.
Many candidates correctly record their raw values for lengths to the nearest 0.1 cm. Some
candidates stated their measurement to the nearest cm. The ruler used to measure can be read to
the nearest mm, so the number of decimal places in the measurement must be consistent down the
column.
Particular points
Question 1
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The table work was done well by candidates. The mark for the range was the most difficult
mark to obtain.
Many candidates find it difficult to choose scales that make good use of the graph grid. The
aim should be to occupy the majority of the graph grid. If less than half of the grid is
occupied in one direction, the range of the corresponding axis can be halved to make better
use of the grid. The scale should be chosen so that it is easy to use, and should not be
based on multiples of 3, 7 etc. It was very common for candidates using such scales to lose
further credit for subsequent incorrect read-offs.
If a point seems anomalous, candidates should be encouraged to check the plotting and to
repeat the measurement if necessary. If such a point is ignored in assessing the line of best
fit, the anomalous point should be labelled clearly, e.g. by circling the point. There is no
credit specifically for identifying an anomalous point, so candidates should be reminded that
they do not need to identify an anomalous point if they do not think they have one.
© Cambridge University Press & Assessment 2024
PERT key messages
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Question 2
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The stronger candidates calculate the percentage difference between their two values of k,
and then test it against the specified 5% percentage uncertainty as a criterion, rather than
using 10% or another value. Some candidates omitted a criterion, or gave a general
statement such as ‘this is valid because the values are close to each other’ or ‘strongly
supported’ without any working, which could not be accepted.
Credit is not given for generic suggested improvements that could be carried out in the
original experiment unless detailed reasoning given, such as ‘repeat measurements’, ‘do
more readings to get an average value’, ‘look perpendicularly onto the ruler’, etc. Unrealistic
solutions were also not given credit.
Candidates should be encouraged to write about four different problems (perhaps relating to
the different measurements undertaken or chronologically going through the experiment)
stating how these difficulties impact on the experiment. Candidates should then try to think of
associated solutions that address each of these problems.
Paper 5
General Comments
In Question 1, candidates’ responses should include detailed explanations of experimental
procedures such as how to control variables, how to take measurements and how to analyse the
data.
The numerical answers towards the end of Question 2 require candidates to show all their working.
A full understanding of significant figures and the treatment of uncertainties is required.
The practical skills required for this paper should be developed and practised with a ‘hands-on’
approach throughout the course.
Particular points
Question 1
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The diagram must be of an appropriate workable experiment e.g. with clamp stands and
metre rules correctly positioned in relation to the relevant apparatus, and protractor shown
clamped in position. Candidates should ensure that full labelling of the components is
provided.
Candidates should be encouraged to consider the control of variables and to explicitly state
the quantities that need to be kept constant to make the experiment a fair test. A number of
candidates use the incorrect term ‘control’ rather than 'constant'. Candidates should identify
the variables to be controlled from the given relationship.
Candidates must explicitly state the quantities to be plotted on each axis either in the text or
on drawn axes – credit is not given for just writing y = mx + c under an expression. Having
suggested an appropriate graph, candidates needed to explain how the graph would confirm
the suggested relationship. This required the words ‘relationship is valid if’ and the word
‘straight’ to describe the line - passing through the origin if applicable.
© Cambridge University Press & Assessment 2024
PERT key messages
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Question 2
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Completing the table appeared to be challenging for candidates. Many candidates did not
determine the absolute uncertainty correctly. When readings are repeated, the absolute
uncertainty is equal to half the range of the values.
A significant number of candidates drew large blobs for the plotted points which could not be
awarded credit – the diameter of each point should be less than half a small square.
Candidates need to take greater care over the accuracy of the error bars and ensure that the
error bars are symmetrical.
The worst acceptable line was drawn well in general, and many stronger candidates drew a
line which passed through all error bars. Candidates should clearly indicate the lines drawn.
Where a dashed line is used to represent the worst acceptable line, the dashed parts of the
line should cross the error bars.
When finding the gradient of the line a number of candidates chose data points that did not
lie on the line, often using data from the table that is close to the line instead. Candidates
should be encouraged to select two points which are easy to read from the graph.
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© Cambridge University Press & Assessment 2024
PERT key messages
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