Turning points in physics (A-level only) questions

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• AQA 7408 · June 2025 · Paper 3BD · Q1 · 16 marks

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  [01.1] Most students were able to gain at least one mark on this question, however very few were able to gain full marks. A significant minority did not appear familiar with the process of thermionic emission at all. Marks could be gained from the diagram or text. When drawing these diagrams students need to take care to label diagrams and make clear what lines are wires and which refer to the glass enclosure, as some diagrams were ambiguous. Most students were aware that a filament needed to be heated. However, a small minority did not realise that that was achieved with a power supply, with some students heating the filament with a heater or other source of heat: this did not gain a mark. Most students realised that a potential difference was required to accelerate the electrons, although some did not explicitly indicate that the anode required a hole to allow the electrons to pass through. Those who drew the power supply the wrong way round or drew incomplete circuits with only one side of the filament attached to the power supply, could not access this mark. Very few students realised that thermionic emission requires either a vacuum or a low pressure gas, with this mark being rarely awarded. Only a few students fully attempted the question and provided values for the potential differences; of these, most did not give suitable values. Many thought the low pd accelerated the electrons while a high pd was required to heat the metal, while others attempted to use just one pd to both heat the filament and accelerate the electrons. 01.2-01.5 Students had to determine a value for the specific charge, using Thomson’s original approach which involved measuring the deflection angle of the electron beam. It was not expected that all students would have seen Thomson’s approach before; however, it was expected that students would be able to apply their knowledge of fields and their understanding of measuring the specific charge to this technique. The question explicitly told students that in this determination of 𝑒𝑒 𝑚𝑚 it was not appropriate to use the values of 𝑒𝑒 and 𝑚𝑚 from the formulae booklet, so the many attempts which did use these values scored 0 for the part of the question where these values were used. However, this did not prevent other parts from scoring. Questions 01.2 to 01.4 were “show that” questions where students are expected to provide the equations used as they appear on the formulae sheet, a substitution, a rearrangement (if required) and the answer to at least one more significant figure that the approximate value given in the question. While most students appeared to follow this, some lost marks by missing out some of these steps, most commonly only quoting the answer as the approximate value given in the question. Most students found 01.2 accessible and were able to gain full marks. 01.3 was reasonably accessible with around two thirds realising that they could use trigonometry with their previous answer to get the mark. 01.4 proved more challenging, with the most common © 2025 AQA approach being to use 𝑚𝑚𝑚𝑚= 𝑄𝑄𝑄𝑄 𝑑𝑑 with the values from the formulae sheet, which scored zero. Unfortunately, most of those students did not realise they could use this approach in 01.5 where it provided the correct route to the answer. In 01.4 the correct approach was to use the horizontal motion to determine a time and then an equation of motion to determine the vertical acceleration. Of those that used this approach, around half correctly used 𝑣𝑣ℎ= 𝑎𝑎𝑎𝑎 with their answer to 01.3. However, the other half used 𝑣𝑣2 = 2𝑎𝑎𝑎𝑎, either using 2.5 cm as 𝑠𝑠 or estimating a value smaller than this, as they realised the beam did not reach the top plate. These students were awarded 1 mark. Very few were able to determine the specific charge using 𝑚𝑚𝑚𝑚= 𝑄𝑄𝑄𝑄 𝑑𝑑 in 01.5. Most students either used 𝑒𝑒 and 𝑚𝑚 from the formulae booklet or mixed up the accelerating pd with the pd across the plates. A small number of students attempted to use the vertical deflection with the magnetic field.

  Text version

  This question concerns Thomson's determination of the electron's specific charge e/m. Do not use the electron charge, mass or specific charge from the data booklet. In the apparatus, a horizontal electron beam passes between two horizontal parallel plates and then strikes a fluorescent screen carrying an angular scale.

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• AQA 7408 · June 2025 · Paper 3BD · Q2 · 4 marks

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  [02.2] While most students were familiar with the idea of quantisation, most answers failed to give enough detail for the mark, most commonly missing out the reference to energy being proportional to frequency or an equivalent. It was pleasing to see some students quote Planck’s original 𝑛𝑛ℎ𝑓𝑓. Students who mixed up Planck’s and Einstein’s contribution and referred to photons or the photoelectric effect were not able to gain this mark.

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  In the early 1900s, the mismatch between measured black-body intensity spectra and the classical theoretical prediction was called the ultraviolet catastrophe. Consider intensity plotted vertically against wavelength horizontally.

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• AQA 7408 · June 2025 · Paper 3BD · Q3 · 6 marks

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  The levels of response question was generally more accessible than in previous years, with some excellent answers seen. Most students were able to make good progress with around 60% scoring at least 3 marks. The question still discriminated well. Many, but not all, students seemed familiar with the concepts. This question brought together different experiments concerning the speed of light and asked students what the significance of these measurements was. © 2025 AQA Most students were able to gain 3 marks by: • stating that Newton’s corpuscular theory predicted that light was faster in water than in air • stating that Huygen’s wave theory predicted that light was faster in air than in water • commenting that Foucault’s experiment demonstrated Newton was wrong or Huygen was correct. Students were not expected to be familiar with the work of Foucault, with the relevant information being given in the question, but were expected to apply their knowledge of Newton and Huygen’s work to realise its significance. Most students were able to do this. Many students were able to go on to give some details about Maxwell’s theory of electromagnetism, although most did not provide enough information to fully address this area. A minority of students did not mention the electromagnetic nature of Maxwell’s theories and stated that he predicted the speed of light, rather than the speed of electromagnetic waves. Many students were also able to go on to express the idea that Fizeau’s measurement of the speed of light matching Maxwell’s prediction for the speed of electromagnetic waves, suggested that light was an electromagnetic wave. However, the vast majority of students did not realise the significance of Hertz’s measurement of the speed of radio waves, incorrectly stating that this proved radio waves were electromagnetic. Hertz had already established the nature of radio waves as electromagnetic through his other experiments. His measurement confirmed that Maxwell had correctly predicted the speed of electromagnetic waves. The following were commonly seen but did not gain credit, although their inclusion was not penalised but ignored: • Repeating information from the question or formulae sheet (e.g. just quoting 𝑐𝑐= 1 ඥ𝜀𝜀0𝜇𝜇0 and stating the names of 𝜀𝜀0 and 𝜇𝜇0 from formulae sheet, or repeating what Fizeau, Foucault and Hertz did). • Giving details about how the experiments were done. The question said, “Do not include any experimental details in your answer.” • Referring to other experiments performed by others (e.g. Young’s double slits) or by the scientists named in the question (e.g. Hertz’s other experiments with radio waves). • Giving more details of Newton’s and Huygen’s theories (e.g. how they explained refraction). A small minority mixed up Einstein’s photon model of light with Newton’s corpuscular theory of light, with many incorrectly thinking Einstein lived prior to 1800. Other common misunderstandings were suggesting that some or all of these experiments or scientists thought that the speed of light was invariant and attempting to link them to relativity. While the concept of an invariant speed of light is consistent with and could be implied by Maxwell’s formula, this was not a view that was expressed prior to Einstein’s paper on special relativity. © 2025 AQA

  Text version

  Fizeau measured the speed of light in air. Foucault confirmed that light travels faster in air than in water. Hertz measured the speed of radio waves in air. Maxwell developed a theory from which the speed of electromagnetic waves could be obtained.

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• AQA 7408 · June 2025 · Paper 3BD · Q4 · 9 marks

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  [04.3] This question proved surprisingly challenging with just under 50% getting the mark. While most students can quote Einstein’s postulate “the speed of light in free space is invariant” or “the speed of the light in free space is the same regardless of the speed of the observer or source,” only around half could apply it to this context. The radio wave, as a form of electromagnetic wave, would have to be measured as travelling at 𝑐𝑐 by all observers, including those on spaceship B. The most common wrong answers were 0.80𝑐𝑐 (the spaceships’ speeds added together, or the difference in speeds subtracted from 𝑐𝑐), and 0.20𝑐𝑐 (the difference between the spaceships’ speeds, or the sum of the spaceships’ speeds subtracted from 𝑐𝑐).

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  In a science-fiction scenario, planet O and planet P are stationary in the same reference frame. Spaceship A travels from O toward P at constant speed 0.50c as measured on P. Use c = 3.00 x 10^8 m s^-1.

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• AQA 7408 · June 2024 · Paper 3BD · Q2 · 12 marks

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  Question 2 was about Millikan’s experiment to determine the charge on the electron.

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  Millikan observes a charged oil droplet through a microscope between horizontal parallel plates X and Y separated by 6.0 mm. A variable voltage supply and voltmeter are connected across the plates through a switch. Opening the switch removes the electric field; closing it allows the potential difference to be adjusted. Millikan first determines the droplet radius.

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• AQA 7408 · June 2024 · Paper 3BD · Q3 · 5 marks

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  Question 3 was about Hertz’s determination of the speed of radio waves. While many students were familiar with this experiment, there was a large number of students who did not seem to know how Hertz was able to make this measurement. Some of these students were able to gain a mark for describing or naming the equipment Hertz used to create and detect radio waves. These students were obviously familiar with Hertz’s work but did not know how he measured the speed. There were two marks for describing Hertz’s experimental setup; generally students who were familiar with Hertz’s work were able to score here. Common mistakes were calling the reflector a mirror or placing the reflector between the transmitter and the detector. As Hertz did many experiments over a number of years, any reference to equipment that could have been used by Hertz was credited here, with either a spark-gap transmitter or a dipole aerial used to create radio waves and a dipole aerial or loop with a small gap used to detect radio waves being accepted. This detail could be provided in a labelled diagram or the text. The method marks were more challenging although many who knew the experiment were able to gain three marks. In order to score here, there had to be a setup which could allow the measurements to be made. So, answers which did not include some form of reflector could only score the third marking point since maxima and minima did not exist and could not therefore be measured. The third marking point was for correctly identifying the wavelength; in this case answers had to refer to the distance between adjacent nodes, not just nodes. The fourth marking point was for establishing what was measured; many students missed this point by just describing a detection of the maxima and minima but not measuring the distance between them. The last mark was only awarded when there was an experimental setup which allowed the wavelength to determined. This did not need to be described in detail for this mark and answers which failed to score either the third or fourth marking point could gain this mark if the correct experiment could be identified but was poorly described. Some answers required a measurement of the radio wave intensity at the transmitter and reflector, which is impossible due to the size of the equipment given. Answers of this type, including those which stated that the first harmonic was used, were not able to gain the fourth marking point, since this referred to the measurements made, but were able to access the other four marking points. Those students who attempted to measure the distance between the transmitter and the time delay between sparks being produced by the transmitter and receiver did not score any of the last three marks. Some students were aware that stationary waves were involved but described waves on a string. These were not able to gain credit; only answers which involved radio waves were able to score. Some students did attempt to describe a different experiment to determine the speed of light. That did not score here. This part of the specification is about historical experiments and so setups which were able to measure the speed of radio waves were only able to score marks where they described situations that were accessible to Hertz. References to data logging, computers and oscilloscopes did not gain credit, although they did not prevent marks being scored in the rest of the answer.

  Text version

  Hertz used radio waves, a transmitter, a detector and a reflecting surface to determine the speed of radio waves.

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• AQA 7408 · June 2024 · Paper 3BD · Q4 · 12 marks

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  [04.4] Nearly two-thirds of students gave the correct response.

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  In a modern version of Bertozzi's experiment, a pulse of electrons from a particle accelerator passes a detector and travels to an aluminium block. The detector starts a timer and the arrival of the pulse at the block stops it. The electron pulse heats the block.

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• AQA 7408 · June 2023 · Paper 3BD · Q3 · 8 marks

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  [03.4] This question proved challenging with only around 60% of students scoring anything. However, around two-thirds of those who gained one mark were able to score at least two. Only a few were able to score full marks. The first mark was given for a description of an experiment involving diffraction and/or interference. For this mark some description either in the text or a labelled diagram was required; simply naming an experiment, e.g. Young’s double slits, was not enough. For the second mark, it was required to describe or sketch the results of the experiment, e.g. a description of the diffraction pattern. It was not enough to refer to a diffraction pattern; a description of what was observed was required. It was also not enough to refer to maxima alone but rather maxima and minima or a series of fringes, since the particle model also predicted (two) maxima. For the third mark, responses need to link diffraction or interference to wave behaviour and give a description in either writing or a diagram of what Newton’s model would have predicted. Most students described a variation of the double-slit experiment; however, a single slit or diffraction grating was also accepted. Although not on the specification, references to Poisson’s spot were accepted as the idea that the wave theory predicted a bright fringe in the centre of the double-slit pattern. Producing a spectrum from a diffraction grating was accepted, but not from a prism. Any response in terms of refraction did not score, as Newton’s theory was able to explain this phenomenon.

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  An electromagnetic wave is propagating through space. Figure 3 shows the variation of the magnetic flux density of the wave with distance. The magnetic field is in the xz plane. The y-axis is at right-angles to the xz plane. Figure 3

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• AQA 7408 · June 2023 · Paper 3BD · Q4 · 10 marks

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  [04.4] Many students struggled with drawing the graph, with around 40% failing to score. Only around a third were able to gain two marks, with only a small number gaining full marks. The first mark was for following the 2 1 2 mv line up to around 0.1c; the acceptable range of divergence was from 0.3 × 108 to 1.1 × 108 m s−1. The second mark was for going through the point established in question 04.3, (2.5 × 108, 122 × 10−12) to within one square. In order to score this mark, the curve drawn had to show an increasing gradient, straight lines drawn through this point did not score. The third mark for was showing that the speed of the particle could not exceed 3.0 × 108 m s−1. Lines which clearly would exceed 3.0 × 108 just off the graph paper did not gain this mark.

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  Einstein developed his theory of special relativity from two postulates. One postulate states that physical laws have the same form in all inertial frames.

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• AQA 7408 · June 2022 · Paper 3BD · Q1 · 11 marks

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  Most students were able to pick up one mark in 01.1. This was often for mentioning the absence of diffraction in the images. Fewer students obtained both marks, with the point that corpuscles were postulated to travel in straight lines being missed frequently. Although most students were able to pick up a few marks in 01.2, answers of high quality were rare. The changes that had to be made to the arrangement usually lacked detail, with only one or two points being mentioned. The descriptions of Huygen’s wavelets were vague and incomplete, with many students demonstrating little understanding of what the theory suggests. The explanations of the pattern were often based on basic core knowledge with little more than references to constructive and destructive interference, and with no mention of how the waves combined. Many students also confused path differences and phase differences in their discussions. 01.3 proved to be similarly challenging. The fact that the arrangement drawn would not produce sharp shadows was missed by most students. The best answers gave diagram annotations that showed the paths of the corpuscles extending beyond the area directly below the gaps. This omission meant that comparisons with Huygens based on diffraction alone rarely gained credit, as both Newton and Huygens would predict a shaded area around the central bright spot. Answers were often limited to some idea of a pattern of fringes around the bright centre, with some discussion of separation of colours.

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  White light illuminates two relatively wide separated slits and forms two sharp bright white lines on a screen. The apparatus is then altered so overlapping light from the two openings produces alternating red and dark fringes.

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• AQA 7408 · June 2022 · Paper 3BD · Q2 · 12 marks

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  The best answers to 02.1 annotated the diagram to show the directions of the forces on the droplet and made correct reference to the direction of the electric field. Often answers were vague, with little attempt to describe the equilibrium position, with terms like ‘counteract’ being used to try to give some sense of the relationship between the weight and the force due to the electric field. The calculation in 02.2 proved to be much more accessible with most students being able to gain all three marks. Partial credit was available for incorrect answers, but the quality of presentation made some students’ work very difficult to follow. Having the relationship to aim for was a mixed blessing in 02.3. It was clear that many students attempted to work backwards but, without a clear indication of the difference between the two velocities, these approaches did not gain credit. Despite being directed to use the equation in 02.4, many students attempted to work out Q from first principles. Most students who correctly determined the charge on the oil drop spotted that it was a multiple of the electronic charge. In order to obtain full marks in 02.5 students had to have a clear argument about the relationship between the viscosity and the charge. The most commonly missed mark was the second, which required some clarity about how the mass or radius of the droplet was determined.

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  A Millikan oil drop lies between horizontal plates with the upper plate positive. With pd zero it falls at terminal speed v_1=3.8 x 10^-5 m s^-1. Oil density is 910 kg m^-3 and air viscosity is 1.8 x 10^-5 N s m^-2. When voltage is applied, the drop can rise at terminal speed v_2.

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• AQA 7408 · June 2022 · Paper 3BD · Q3 · 6 marks

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  Many students gave a very simple answer to 03.1 without any consideration of the significance of the frequency in that equation. Complete answers were rare. In 03.2 many students simply worked out a speed and made a comment about how close it was to the speed of light, giving little consideration to the question itself. The best answers compared one of the properties of the experiment with what it would have to be in order to measure the speed of light. The frequency was the most common parameter, but the path length or number of teeth were also discussed by some students. There is a statement (3.12.2.3) in the specification about the permittivity and permeability constants. It was clear in 03.3 that many students had very little idea about how these are related to the properties of an electromagnetic wave and therefore the speed of light.

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  Figure 5 shows the arrangement used by Fizeau to determine the speed of light. Figure 5 The toothed wheel W is rotated and the reflected light from a distant mirror M is observed. The speed of light is calculated from the equation c = 4dnf0 where d is the distance from W to M and n is the number of teeth on the rotating wheel W.

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• AQA 7408 · June 2022 · Paper 3BD · Q4 · 6 marks

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  Most students correctly identified the graph in 04.1. 04.2 proved to be much more challenging, with some students attempting to use the non-relativistic kinetic energy formula and using a speed greater than that of light. The relationship between energy and mass is an important one in relativity and in 04.3 examiners expected to see some awareness of the implications of that relationship. This was rarely seen, even in a basic form.

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  Question 4 from AQA A-level Physics Paper 3BD, June 2022.

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• AQA 7408 · November 2021 · Paper 3BD · Q1 · 13 marks

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  This question discriminated extremely well, with students picking up a range of marks on each question part. Answers which were written clearly and contained the appropriate level of detail were able to score well. In question 01.2 it was clear that students continue to be confused by equations that include both a V for p.d. and a v for velocity. It was pleasing to note how many students were able to identify a value for the wavelength, however. There was evidence of a significant lack of experience with the use of electron diffraction to investigate the de Broglie hypothesis in question 01.3.

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  An electron-diffraction tube contains heated component A, an accelerating anode B, a crystalline target and a fluorescent screen. Potential difference V1 heats A and V2 accelerates electrons from A toward B.

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• AQA 7408 · November 2021 · Paper 3BD · Q2 · 10 marks

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  In order to score both marks in question 02.1 a comment was needed comparing the calculated value for the speed of light with that in the data booklet. This was commonly missed. In question 02.2, many students demonstrated some knowledge of Maxwell’s model but few could relate it to the experiment. Question 02.4 assessed Assessment Objective 3 (AO3) and was not answered well by many students.

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  Maxwell predicted electromagnetic-wave speed from vacuum permeability and permittivity. Hertz used an oscillating-current two-rod transmitter and a conducting-loop detector, then stationary waves between a transmitter and reflector.

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• AQA 7408 · November 2021 · Paper 3BD · Q3 · 6 marks

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  As usual the levels of response question produced a range of marks and discriminated very well. Few students were able to pick up all six marks, however. There was much confusion between this experiment and the one used to determine the speed of light.

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  A Michelson-Morley interferometer splits monochromatic light into perpendicular paths to mirrors M1 and M2, recombines the returning beams at a telescope, and can be rotated through 90 degrees.

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• AQA 7408 · November 2021 · Paper 3BD · Q4 · 6 marks

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  This question on relativity proved to be quite difficult for many students. As has been seen in many series in the past, there was much confusion about which reference frame was being considered in the calculation. There were also some poor descriptions of proper time and inertial frames. Concluding Remarks The level of difficulty, range and style of questions used in this paper were similar to those of the questions used in previous series. The mathematical difficulty and AO coverage was also very similar. Almost all of the questions proved to be challenging, but it was clear that many students were unable to cope with the questions designed to assess AO3 in particular. There were also many answers that indicated poor knowledge of the specification content. In general, the question on the electron gun and charge on the electron was the most accessible. The questions on proper time and the discussion of the determination of the speed of electromagnetic waves by interference caused the most difficulties. Whilst these have caused problems in the past, the evidence of difficulty is stronger in this series.

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  Two detectors are 45 m apart in the laboratory. Unstable particles travel at 0.97c; beam intensity at detector 2 is 12.5% of that at detector 1.

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• AQA 7408 · November 2020 · Paper 3BD · Q1 · 8 marks

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  This question required students to be familiar with JJ Thomson’s work and the discovery of the electron. In 01.1, many students could not explain how the cathode rays were produced in Thomson’s experiment; a description of thermionic emission was a common incorrect answer. Despite 01.2 and 01.3 requiring little more than knowledge from the core physics specification, answers that demonstrated an understanding of the processes involved were rare. By contrast, 01.4 was well answered with over 40% of students obtaining both marks. A common error here was to reverse the relative magnitude of the specific charge for the proton and electron.

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  JJ Thomson used a low-pressure discharge tube with a cathode C, accelerating anode/aperture assembly A and B, parallel deflection plates P and Q, and a fluorescent screen to investigate cathode rays. The electron beam travels left to right. Plate P is above the beam and positive; Q is below and negative. A transverse horizontal line XY is perpendicular to the tube axis, with X at the upper-right/far-side end and Y at the lower-left/near-side end as drawn in the source perspective.

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• AQA 7408 · November 2020 · Paper 3BD · Q2 · 12 marks

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  This question was about the development of quantum theory. 02.1 was the poorest performing question on the paper. Many incorrect answers attempted to explain the nature of a black-body rather than to answer the question as written. It was clear from some answers that a significant number of students were unfamiliar with the term. Despite not being answered by over 15% of the students, 02.2 did yield a good spread of marks. It was common to see evidence of some familiarity with the ultra-violet catastrophe. Many students made no attempt to annotate the graph despite the explicit prompt to do so. Answers to 02.3 suggested that many students were unable to express clearly Max Planck’s contribution to quantum theory. Vague and incomplete answers were common. 02.4 had a maximum of 6 marks and, in common with previous series, the level of response mark scheme discriminated very well, with answers seen across the full mark range. Students whose knowledge of the photoelectric effect was limited to what is on the core specification were still able to pick up some marks. In the best answers, students made it very clear whether a point referred to an observation, a failure of classical theory, or to Einstein’s solution. Answers scoring low marks were usually too vague and incomplete.

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  Late-19th-century measurements of black-body radiation and the photoelectric effect challenged classical physics. The supplied black-body graph has intensity on the vertical axis and wavelength on the horizontal axis; the measured spectrum starts near zero at short wavelength, rises to one finite peak, and then falls gradually towards zero at long wavelength.

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• AQA 7408 · November 2020 · Paper 3BD · Q3 · 8 marks

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  This question required knowledge and understanding of quantum tunnelling, a specialist area of this Option. It was rare to see an answer that suggested the student had any familiarity with the process or its application in the scanning tunnelling microscope (STM). In particular, few students demonstrated an appreciation of the role that probability or wave-particle duality plays in the STM. In 03.3, many students were able to obtain a value for the de Broglie wavelength of the electrons but few managed to make a correct judgement linking this value to the size of an atom. Again it should be noted that the de Broglie wavelength equation is part of the core specification. A surprisingly common error was the use of the speed of light for the speed of the electron.

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  A scanning tunnelling microscope operates in constant-current mode and maps atoms on a conducting surface.

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• AQA 7408 · November 2020 · Paper 3BD · Q4 · 7 marks

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  This question dealt with special relativity and the behaviour of muons. It was disappointing to see how many students attempted to answer 04.1 without any reference to relativity at all. Many of the students who made use of the correct equation unfortunately confused L0 and L . Similarly, answers to 04.2 indicated that many students were not familiar with the muon-decay experiment, with answers frequently suggesting that the speed or the time taken were measured directly. Many answers to 04.3 either made no reference to relativity or suggested that the muons were decelerating so that the theory of relativity no longer applied. The best answers focused on one frame of reference, either the muon or the observer, and made a clear argument in terms of length contraction or time dilation. A common error was the suggestion that the half-life of the muons changed. Concluding Remarks In terms of difficulty, this paper was broadly similar to previous papers on this option. It is worth pointing out, however, that there may have been a greater emphasis on contexts unique to this Option, with less overlap with core physics than there has been in the past. This means that, for example, specific knowledge was expected on Thomson’s experiment, black-body radiation and quantum tunnelling. Whilst the questions were not in themselves more difficult, the need for a better understanding of some of the detail specific to the Option may have made the paper less accessible to many of these students. In common with previous series, question parts that assessed AO3 and required students to make a judgement were generally answered less well. The AO1 question part on the significance of Thomson’s discovery and the AO2 question part that required the calculation of a relativistic length proved to be the most accessible.

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  Cosmic-ray muons travel through Earth's atmosphere at relativistic speeds.

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• AQA 7408 · June 2019 · Paper 3BD · Q4 · 9 marks

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  [4.3] This calculation proved more difficult than the other calculations in this paper. A common error was the use of 0.95c in the calculation of E0. There was also evidence of students taking no account of relativistic effects and simply calculating 2 1 2 mv .

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  Bertozzi data for electrons give (v/10^8 m s^-1, Ek/MeV): (2.60,0.5), (2.73,0.7), (2.88,1.3), (2.96,2.6), (2.99,5.8).

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• AQA 7408 · June 2018 · Paper 3BD · Q1 · 9 marks

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  [01.3] Students needed to show all data substituted to gain full marks. Most appreciated that the answer needed to be given to two significant figures for consistency with the significant figures in the data used. 58.8% of students were awarded both marks.

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  Electrons thermionically emitted from a filament accelerate through V then follow radius r perpendicular to magnetic flux density B. Data: V=320 V,r=4.0 cm,B=1.5 mT.

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• AQA 7408 · June 2018 · Paper 3BD · Q2 · 5 marks

  5 marks

  2

  
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  [02.1] There were two particular reasons why students failed to gain marks in this question. The first was that the amplitude cannot be negative, and the second was a failure to include any scale on the angle axis. Sketches were often quite poor, but these were treated generously providing the important features were shown. Some attempt to show curvature was expected to gain the final mark. Full marks were awarded to 42.5% of students.

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  A Hertz-style dipole transmitter continuously emits EM waves to a rotatable dipole receiver. Maxwell obtained c=1/sqrt(mu_0 epsilon_0).

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• AQA 7408 · June 2018 · Paper 3BD · Q3 · 9 marks

  9 marks

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  [03.2] A large number of students seemed not to have read this question carefully enough – there were relatively few convincing responses. Many responses explained why the photon model could explain why there is threshold frequency, which is not what was asked. In this question, there is no change to the incoming radiation other than intensity. Some students commented that as intensity increased there would be more photons arriving, so the number of photoelectrons produced would increase, so current would increase. However, what determines the current is not the number of electrons that are emitted but the number that can reach T. As the photon energy is fixed, students needed to appreciate that current could only increase if more electrons could reach T. As the maximum KE of the photons is the same, even though there are more of them, none can reach T so the current is unchanged. A high intensity wave could transfer, resulting in electrons with more KE, so these could reach T, producing a higher current. Over half of the students failed to score on this question.

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  A photocell metal work function is6.2 eV. Monochromatic light produces stopping potential0.50 V.

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• AQA 7408 · June 2018 · Paper 3BD · Q4 · 12 marks

  12 marks

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  [04.4] This was answered well, with just over 50% of students gaining three or four marks. Both approaches attracted roughly equal numbers of students. A read-off from the graph was adequate for the final step, but many did correct but unnecessary calculations of the number left. There was a significant minority of students who gave the number that decayed as their final answer having correctly determined the number remaining.

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  Michelson-Morley used equal perpendicular interferometer arms and predicted a0.4-fringe shift on90-degree rotation. A muon example has speed0.990c and lab path1310 m; rest half-life graph gives about2.2 microseconds.

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• AQA 7408 · June 2017 · Paper 3BD · Q1 · 7 marks

  7 marks

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  [01.2] This question was not well answered by significant numbers of students. Common mistakes included describing the processes of thermionic emission and photon emission following de-excitation. A small number of students recalled, to a high level of detail, the process of ionisation and subsequent role of the positive ions in generating the electrons that go on to become cathode rays. A good number of students were able to describe the acceleration of the electrons toward the anode as an important factor in generating cathode rays.

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  In Crookes's gas-discharge tube a large potential difference makes a paddle wheel rotate and travel along rails toward the anode.

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• AQA 7408 · June 2017 · Paper 3BD · Q2 · 9 marks

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  [02.1] Although many responses were detailed and showed understanding of Fizeau’s experiment, few students explained with sufficient precision observations A and B. For observation A, a response explaining that light returned through the same gap was expected, while for observation B the correct response was that the light was blocked by the adjacent tooth.

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  In Fizeau's rotating-toothed-wheel experiment, light passes through a wheel gap to a mirror 8.6 km away and returns. At low wheel speed it is seen; at 12 revolutions per second with 720 teeth it is first extinguished. The derived relation is c=4dnf.

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• AQA 7408 · June 2017 · Paper 3BD · Q3 · 10 marks

  10 marks

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  [03.2] The calculation was done well by many students. Some students used an energy argument rather than the more straightforward direct substitution from the formula sheet. Where students did follow an energy argument, a common mistake was to confuse potential difference with velocity. Students in general were reasonably successful in arguing whether the electrons could resolve atoms. Few students were able to recall and use the approximate size of an atom and compare their value for the de Broglie wavelength with this; for those who did make some progress in this regard, a common mistake was to confuse the dimensions of the atom and nucleus.

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  A transmission electron microscope accelerates electrons of kinetic energy 4.1 x 10^-16 J; relativistic effects are negligible at this energy.

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• AQA 7408 · June 2017 · Paper 3BD · Q4 · 6 marks

  6 marks

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  [04.1] A significant number of answers to this question scored full marks. Some students neglected the instruction to support their answer with a time-dilation calculation. In addition, where students’ strategy was sound but marks were lost, this tended to be for not fully answering the question which required a judgement on both aspects of the student’s prediction, i.e. the time being longer and the magnitude of several days.

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  A spacecraft journey lasts one year in the spacecraft frame at speed 1.2 x 10^7 m s^-1 relative to an observer. In practice the Sun's gravity changes the spacecraft's speed/direction.

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• AQA 7408 · June 2017 · Paper 3BD · Q5 · 3 marks

  3 marks

  5

  
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  Nearly all students were able to convert the energy given in eV into J. A good number of students went on to complete the calculation successfully. A common error was the incorrect use of the EK = mc2 – moc2 equation. Use of statistics Statistics used in this report may be taken from incomplete processing data. However, this data still gives a true account on how students have performed for each question.

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  Cosmic-ray protons detected on a spacecraft have total energy 3.7 x 10^9 eV.

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• AQA 7408 · sample1 · Paper 3BD · Q4 · 8 marks

  8 marks

  4

  
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  Two stationary-frame detectors are 34 m apart. Pi mesons travel between them at 0.95c. Their proper half-life is 18 ns, and experiments find that about 75% of mesons passing the first detector decay before the second.

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