Revision tips for Paper 3A

It’s a fool’s game to try to predict what will be in an examination paper but… looking back over past years, there are three practicals that have been used very little in Paper 3A and one of those hasn’t appeared since 2020. So, in my opinion, there is a good chance that this year’s paper could include a question about investigating simple harmonic motion (RP7) and most likely based on a spring rather than a pendulum. The question could well ask you to analyse the motion of different masses that are displaced vertically on a hanging spring, as covered on pages 71-73 of the AS/A-Level Student Guide for Practical Physics.

As an extension, the question could link to the Young’s Modulus practical (RP4) by asking about the connection between period of oscillation and the stiffness of different springs. This could be the sort of task where you are told that an unfamiliar equation applies and you are asked to work out a value for one of the terms in the equation by analysing some data points shown on a graph (but probably without a line of best fit).

The other two practicals that are due for inclusion are determination of g by free-fall (RP3) and investigating magnetic fields (RP11).

I wouldn’t be surprised if the inverse-square law for gamma radiation also appeared (RP12). In particular, it is about time a question asked why the inverse-square law applies to gamma radiation but not to alpha and beta radiation. The answer is that gamma is electromagnetic radiation that is released as photons and displays wave behaviour, so its intensity decreases as the radius of the sphere surrounding the source gets larger.

By contrast, alpha and beta are particle emissions that travel in specific directions and can be absorbed by the air and other substances. As such, there are limits to the distances that alpha and beta particles can travel whereas gamma radiation can travel indefinitely through free space, which is why we can detect gamma ray bursts from distant galaxies here on Earth. That said, the detection of gamma radiation from a lab source is limited by the distance at which the intensity (counts per second) falls to the level of the background radiation.

My other suggestion is that there could be a question asking you to interpret an oscilloscope display. Remember that the time-base tells you the interval (often in milli-seconds) corresponding to each horizontal division and the gain tells you the magnitude of the signal indicated by each vertical division (often in volts).

One thing that I don’t think has ever been asked is to compare the voltage displayed on an oscilloscope with a measurement of the same potential difference indicated on a voltmeter. We always say ideal voltmeters have infinite resistance but this isn’t true in practice. Digital voltmeters are likely to have a higher resistance than analogue (needle-based) voltmeters but oscilloscopes have even higher resistances so they give the truest readings. The real resistance of a non-ideal digital voltmeter means its reading will be lower than that shown on an oscilloscope.

The disadvantage of oscilloscopes is they require a bit more skill to read and readings take more time than when using a digital voltmeter. It is not true to say that oscilloscopes aren’t portable as you can get pocket-size, battery powered models.

If you want to check your ability to interpret an oscilloscope display then head over to LiveWorksheets (https://www.liveworksheets.com/worksheet/en/physics/618212) where there is a very short interactive test I created some years ago for GCSE students. To test your understanding further, you can experiment with Farid Minawi’s excellent interactive oscilloscope simulator at https://physics-zone.com/virtual-oscilloscope/.

Another thing that might come up is the fairly common task of using percentage uncertainties to work out the absolute uncertainty in a final value. Remember that we add percentage uncertainties when we multiply or divide values whereas we add absolute uncertainties when we add or subtract values. To convert from a percentage uncertainty back to an absolute uncertainty, multiple the percentage uncertainty by the mean value. There is an entire post all about uncertainties and random/systematic errors at https://physbang.com/2025/04/18/random-and-systematic-errors/.

I have also recently covered micrometers and Vernier callipers, which lend themselves to questions about uncertainty, so be sure to re-read those posts (and do the exercises) if you are in any way hesitant about their use. The posts are at https://physbang.com/2025/04/09/how-to-read-a-micrometer/ and https://physbang.com/2025/04/20/how-to-read-vernier-callipers/ respectively.

Apart from all of this, the best advice for Paper 3A is, keep calm. Questions often look harder than they really are. You are bound to be asked to analyse at least one equation (relationship) you have never seen before – and the chances are you will be asked to do this using a graph. In particular, you could be asked to analyse a logarithmic graph to determine a constant that you have never met before but which might be comparable to a time constant or half-life.

You might also be asked how you could reduce the uncertainty in measurements. Please don’t simply mention doing repeats! Yes, repeats can be useful but be sure to talk about taking readings at different settings too, either within the existing range or beyond it. And if the relationship is non-linear, think about the most appropriate spacings for measurements, which often isn’t “at regular intervals”. If appropriate, decide which end of the data range needs more information and focus measurements there.

When trying to reduce parallax errors, you might get one mark for minimising the distance between the object (or a pointer attached to it) and the scale but remember the options to use a set-square or a mirror. Set-squares are useful for ensuring that a reading is taken horizontally against a ruler that is held vertically. They are also good for ensuring the ruler is vertical when it is placed on a horizontal surface. Mirrors are more versatile because if the reflection of the pointer is hidden behind the pointer itself then you can be certain you are reading the scale exactly perpendicular to the line – and that will be true regardless of angle.

Finally, remember to look through the whole paper and to manage your time appropriately as you will have two hours to complete both section 3A (data analysis) and 3B (the specialist topic).