Common Mistakes (part 2)

This is the second set of tips for avoiding some of the mistakes made by students sitting AQA GCSE Trilogy science papers. It is based on the exam board’s advice, which can be downloaded from https://filestore.aqa.org.uk/resources/science/AQA-GCSE-SCI-TG-MISUNDERSTANDINGS.PDF, together with observations and explanations of my own. To read the first part of this summary, see https://physbang.com/2023/05/01/common-mistakes-part-1/.

7. UNFAMILIAR SITUATIONS… Questions sometimes use a context that you haven’t been taught and this can be confusing. One useful tip is to ignore the bits you don’t know about and answer the information that you recognise. For example, if you are asked what happens to the movement of the particles inside a pressurised can of hair spray when the can is put into a freezer, you can shorten this question to, “what happens to the movement of particles when the temperature goes down?” Hopefully, you know that colder particles have less energy so they move slower. You might add, though there might not be any extra marks for this given what the question has asked, that the particles collide with the inside surface of the can less frequently, causing the pressure inside the container to decrease.

8. DRAW A DIAGRAM WHENEVER POSSIBLE… Long-answer questions often say you are allowed to draw a diagram if you wish. This usually means that a good diagram can get you marks! Make sure that your diagram is labeled and drawn carefully, using a rule to get straight lines, for example. When it comes to explaining the diagram, use bullet points to keep your answer short. If you can’t remember the exact purpose of everything you have drawn, just explain what you can.

9. EQUATIONS AND NUMBERS… For this year (2023) only, students sitting AQA GCSE science exams will be provided with an equation sheet for the two physics papers. You can download the equation sheet in advance, to see what it contains, from https://filestore.aqa.org.uk/resources/science/AQA-8464-ES-INS-JUN23.PDF. Warning! The equation linking distance, speed and time contains potentially confusing symbols:

s = v t

Remember that s is the symbol for distance, not speed! And v is the symbol for speed (think, velocity). The reason is just a matter of convention but don’t get caught out! And remember that although you will be given an equation sheet, you could be asked to rearrange at least one equation before doing the calculation. If you are not confident in your maths skills then an easy way to do the rearrangement is by using an equation triangle: an explanation of how to do this can be downloaded here.

9a. INVENT AN EQUATION (if you have to!)… This sounds crazy so bear with me… If you can’t remember how to rearrange an equation then make up something that fits. Provided that it uses all the numbers you have been given, your invented equation could get you at least one mark. Here’s how it works:

• Suppose you are asked to calculate current (in amps) using numbers for potential difference (in volts) and power (in watts). You will be given an equation that links these three quantities in the form; P = I x V. If you can’t remember how to do the rearrangement then, rather than skipping the question, invent a new version of the equation with current on the left-hand side, such as current = power + voltage.
• Now you can get a mark for substituting values into your (wrong) equation. To be safe, you should write your equation in full, using words for the quantities, then insert the numbers you are given and calculate your answer. If the question is worth three marks, which will normally be the case, you will get the “middle” mark for doing a correct substitution even though your equation and your answer will both be wrong. This works because exam questions often have independent marks, so you can get the method mark even if the rest of your answer is wrong.

9b. OTHER INDEPENDENT MARKS… There are three other common independent marks that you should look for;

• You could be asked to state the unit used to measure a particular quantity, such as newtons for force or hertz for frequency. This will be worth one mark and will be completely separate from the calculation itself. It might also take the form of a multiple-choice question. The equation sheet does not give the units so you need to learn them! I have created two revision sheets to help you; if you are less confident in Physics then choose the “easy” list (download it here) but if you are a keen physicist then get the complete list (here).
• If a question asks for the answer to be written as three significant figures then there will be a mark for knowing what that means. Just writing 1.23 will get you a mark even if that number has nothing to do with the calculation! Be careful not to confuse significant figures (which are normally required) with decimal places. If you aren’t sure, 0.123 is both three significant figures and three decimal places.
• Finally, remember to watch for conversions that are necessary when values are given with unit-multipliers, such as kN, kJ or kΩ, all of which indicate thousands (of newtons, joules or ohms respectively). So 1.2 kN is the same as 1200 N and writing that conversion will get you a mark. Harder conversions include “mega”, such as MW, and “nano” such as nm. Mega means millions, so a 300 MW power station produces 300 000 000 W of electricity. Nano is a tiny amount and is normally written as 10^-9, so 550 nm would be 550 x10^-9 m. This can be simplified to 5.5 x10^-7 m but just writing 550 x10^-9 should get you the conversion mark.

When you have finished answering the exam paper, it is a good idea to go through and check every calculation to see if any of these independent marks appear, especially in questions you might have skipped because you thought you couldn’t answer them.

10. GRAPH SKILLS… If you are asked to plot points, always use a pencil so you can rub-out mistakes and re-plot if necessary. The pencil need to be sharp and plotting must be done carefully as you will only get the mark if your plot is accurate to the nearest square on the grid. Lines of best fit must have points scattered equally to both sides of the line, which should normally be straight. If the line looks like it could be curved then it should bend only one way and not have lots of wiggles. If you are asked to predict a value that goes beyond the end of a graph, think whether your answer is sensible. A few years ago students were asked to look at a heating curve and predict the temperature of water after a long period of time: quite a lot of students gave numbers around 110 ⁰C. What is wrong with that? Water can’t reach 110 ⁰C because it boils and turns to steam under normal room conditions! So the correct answer was any number VERY close to 100 ⁰C but definitely not 110 ⁰C.