IGCSE Physics Revision: Common Mistakes to Avoid

After ten years marking past papers with students, the pattern is boring and consistent. Candidates do not fail IGCSE Physics because the syllabus is too hard. They fail because they make the same five or six small mistakes again and again, across the same four papers (CIE 0625 Papers 2, 4, 5, 6 and the equivalent Edexcel 4PH1 papers). Reading the official examiner reports for the last three sessions, the top losses are unit errors, rearrangement errors in formulae like $v = u + at$ and $P = \frac{W}{t}$, sign errors in moments and forces, misreading graphs, and writing 'because' answers that are not physics. None of these require new content. They require a deliberate revision plan that targets them.

This guide is not a syllabus summary. It is a list of the specific mistakes that cost real marks in May/June and Oct/Nov, with fixes you can practise this week. Read it with a pen and a past paper next to you, not on the train.

Unit mistakes are the single biggest preventable loss on Paper 4. The classic chain is: a question gives mass in grams, distance in centimetres, and time in milliseconds, then asks for kinetic energy in joules. Students plug numbers straight into $E_k = \frac{1}{2} m v^2$ without converting, get an answer that is wrong by a factor of $10^6$, and lose 2 or 3 marks. The fix is mechanical, not clever: before you write the formula, write a tiny conversion column to the left of the working.

Worked example.

A trolley of mass $250 \text{ g}$ travels $40 \text{ cm}$ in $0.50 \text{ s}$. Calculate its kinetic energy.

Step 1, convert. $m = 0.250 \text{ kg}$, $d = 0.40 \text{ m}$, $t = 0.50 \text{ s}$.

Step 2, find the speed. $v = \frac{d}{t} = \frac{0.40}{0.50} = 0.80 \text{ m/s}$.

Step 3, substitute into $E_k = \frac{1}{2} m v^2 = \frac{1}{2} \times 0.250 \times (0.80)^2 = 0.080 \text{ J}$.

Three marks, three lines. The student who skips Step 1 writes $\frac{1}{2} \times 250 \times 40^2 = 200{,}000$ and earns one mark, maybe zero. A separate but related loss is the 'silent zero' on prefixes: $1 \text{ k}\Omega = 1000 \Omega$, $1 \text{ M}\Omega = 10^6 \Omega$, $1 \text{ mA} = 10^{-3} \text{ A}$, $1 \text{ }\mu\text{A} = 10^{-6} \text{ A}$. Writing $V = IR$ with $I$ in mA and $R$ in $\text{k}\Omega$ accidentally gives you the right answer in volts, which is why students think they can skip conversions. They cannot, because the next question swaps one prefix and the trick fails. See IGCSE Physics formulas explained for the full conversion list you should memorise.

Most IGCSE Physics formulae are given on the formula sheet for CIE 0625, but Edexcel 4PH1 expects you to know more by memory, and both expect you to rearrange them correctly. The high-loss formulae are the ones with division and squared terms.

The kinematic equation $v^2 = u^2 + 2as$.

Suppose you need to find $u$ given $v = 12 \text{ m/s}$, $a = -2.0 \text{ m/s}^2$ and $s = 25 \text{ m}$. Rearrange first: $u^2 = v^2 - 2as = 144 - 2 \times (-2.0) \times 25 = 144 + 100 = 244$, so $u = \sqrt{244} \approx 15.6 \text{ m/s}$. The two common mistakes are forgetting to take the square root at the end, and dropping the double negative on the $-2as$ term.

Refractive index $n = \frac{\sin i}{\sin r}$.

To find $r$, rearrange to $\sin r = \frac{\sin i}{n}$, then $r = \sin^{-1}\left(\frac{\sin i}{n}\right)$. Students routinely write $r = \frac{i}{n}$, which is wrong because the relation is not linear in the angle.

Power dissipation $P = I^2 R$.

To find $I$ from $P$ and $R$, the rearrangement is $I = \sqrt{\frac{P}{R}}$, not $I = \frac{P}{R}$. If a $24 \text{ W}$ heater has resistance $6.0 \Omega$, then $I = \sqrt{\frac{24}{6.0}} = \sqrt{4.0} = 2.0 \text{ A}$.

The deeper fix is to practise rearrangement on its own, away from physics. Spend 20 minutes once a week solving for each variable in $v = u + at$, $s = ut + \frac{1}{2} a t^2$, $F = ma$, $P = \frac{W}{t}$, $V = IR$, $E = mc \Delta \theta$ and $E = m L$. By exam day, rearranging should feel as automatic as your name.

Graph questions are worth 6 to 10 marks on Paper 4 and almost always appear on Paper 6 (the alternative-to-practical) too. The recurring mistakes are easy to fix.

Always use a large triangle for the gradient.

Examiners deduct marks if your triangle covers less than half the plotted line, because small triangles amplify reading errors. For a straight-line $v$ against $t$ graph, the gradient is $a = \frac{\Delta v}{\Delta t}$. Choose two points that are at least 8 cm apart on the printed grid, read them to one half of the smallest division, and show the substitution: for example $a = \frac{18 - 2}{8.0 - 0} = \frac{16}{8.0} = 2.0 \text{ m/s}^2$.

Use a tangent for curved graphs.

If the question asks for the acceleration at $t = 4 \text{ s}$ on a curved velocity-time graph, you draw a tangent at that point and compute the gradient of the tangent, not the chord. Draw the tangent so it touches the curve at exactly one point and extends almost the full width of the graph. A tiny stub is a mark lost.

Area under the curve.

Distance from a velocity-time graph is the area under the curve, in the unit $\text{m/s} \times \text{s} = \text{m}$. For trapezia, $\text{area} = \frac{1}{2} (a + b) h$. For irregular areas, count squares and state the value of one square in physical units, for example one large square equals $5 \text{ m/s} \times 2 \text{ s} = 10 \text{ m}$. Show the count.

Plotting points.

Two more 'free' marks come from plotting. Use a sharp pencil, mark each point with a small cross or a dot inside a circle, and draw the best fit line with a ruler (or a smooth freehand curve, never a series of straight segments). For more detail on the practical mechanics, the article on Paper 5 and 6 trap is worth ten minutes.

About a quarter of the marks on Paper 4 come from extended-response questions: 'explain why', 'describe what happens', 'state and explain'. This is where weak candidates lose the most marks per minute of writing. The fix is to use physics nouns and physics verbs, not common-sense language.

Bad.

'The metal gets hot because the heat goes into it.' Zero marks.

Good.

'Electrical energy is transferred to the heater wire. The current does work against the resistance, increasing the internal energy of the metal, which causes the temperature to rise.' Two or three marks.

The rule of thumb is: every sentence in an 'explain' answer should contain at least one technical noun (force, momentum, internal energy, electric field, induced e.m.f., specific heat capacity) and one technical verb (transfer, dissipate, accelerate, oscillate, refract, induce). If a sentence reads like something you would say to a six-year-old, rewrite it.

Command words you must respect:

• State. One short sentence or a value. Do not explain.
• Describe. Say what happens, in order. No reasons unless asked.
• Explain. Give the cause and the mechanism. 'Because the temperature rises' is not enough; 'because the average kinetic energy of the molecules increases, so collisions with the walls are more frequent and more energetic' is.
• Suggest. A reasonable physics-based guess; you will get the mark if it is plausible and uses correct concepts.
• Show that. Set out the working clearly and arrive at the value given. If you cannot reach the exact value, your working still scores.

If you only do one thing this month, read IGCSE Physics tips to get an A* and underline every command word in the next three past papers. After ten papers, your brain will translate them automatically.

Paper 5 (practical) and Paper 6 (alternative-to-practical) reward a very specific style of answer. The mark scheme is not testing whether you 'did science'; it is testing whether you used named techniques. There are four phrases that score marks reliably.

1. Repeat and average.

Whenever you record a measurement, you should write 'repeat the measurement and calculate a mean'. This is worth at least one mark almost every paper.

2. Control variables explicitly.

Do not write 'keep everything else the same'. Write 'keep the mass of water constant at $200 \text{ g}$, the starting temperature at $20 \text{ }^\circ\text{C}$ and the power of the heater at $30 \text{ W}$'. Named, with values.

3. Reduce parallax.

For ruler, protractor, thermometer or pendulum readings, the phrase the examiners want is 'view the scale at eye level, perpendicular to the ruler, to avoid parallax error'.

4. Identify the dependent and independent variables and the range.

'Vary the length $L$ of the pendulum from $0.20 \text{ m}$ to $1.00 \text{ m}$ in steps of $0.10 \text{ m}$; measure the period $T$ for each.' Range plus interval is a mark.

A fully worked Paper 6 style answer for measuring $g$ with a pendulum looks like this: 'Measure the length $L$ of the pendulum from the pivot to the centre of the bob using a metre rule, viewing the scale perpendicular to the rule. Time twenty complete oscillations using a stopwatch and calculate the period as $T = \frac{t}{20}$. Repeat for at least six different values of $L$ from $0.20 \text{ m}$ to $1.20 \text{ m}$. Plot a graph of $T^2$ on the y-axis against $L$ on the x-axis. The graph should be a straight line through the origin with gradient $\frac{4\pi^2}{g}$, so $g = \frac{4\pi^2}{\text{gradient}}$.' That is six marks of clear physics in one paragraph.

Knowing the mistakes is the easy part. The plan is what closes the gap. Here is a four-week schedule that targets the five problems above, designed for a student already familiar with the syllabus content. Total time: about 6 hours per week.

Week 1, diagnostic.

Sit one complete Paper 4 under timed conditions (1 hour 15 minutes for CIE 0625 Extended). Mark it with the official mark scheme. For every mark lost, write a one-line tag: 'unit', 'rearrangement', 'graph', 'wording', 'method', 'careless'. Count the tags. The top two are your priority.

Week 2, drills.

Six 45-minute focused sessions on the top two error types. For unit errors, do twenty conversion-heavy calculations from past papers. For wording, write out three model 'explain' answers and compare each sentence to the mark scheme. Quality over speed.

Week 3, mixed past papers.

Two full Paper 2 papers and one full Paper 4, all timed, all marked with the official mark scheme. Re-tag your errors. The total error count should drop by 30 to 50 percent.

Week 4, practical and gaps.

One full Paper 6, one full Paper 5 walkthrough, and one targeted revision session on whichever syllabus topic still has gaps (usually electromagnetic induction, or radioactivity calculations). Finish with a final timed Paper 4 the day before stopping. The point is not to memorise more content; it is to make your last performance look like your best one. For a complementary plan on the maths side, see How to revise for IGCSE Maths. The structure carries over.