Top Mistakes Edexcel IGCSE Physics Students Make (and How to Avoid Them)

Physics exams do not just test what you know. They test whether you can say it the way the mark scheme expects.

Every session, Edexcel IGCSE Physics candidates lose marks on questions they could have answered correctly. The errors are rarely about missing knowledge. They are about how answers are structured, how calculations are presented, how command words are interpreted and how time is spent. The same edexcel igcse physics common mistakes appear year after year, and the examiner reports document them in detail.

This guide covers the most persistent edexcel igcse physics errors, organised by topic area and exam skill. For each one, you will see the wrong approach, why it loses marks and the correct technique. Fixing even three or four of these habits can shift your mark by a full grade boundary on the IGCSE 4PH1 exam.

Mistake 1: Treating "explain" and "describe" as the same command word

This is the single most damaging habit across both papers. When a question says "describe," the mark scheme wants you to say what happens, step by step. When it says "explain," the mark scheme wants you to say why it happens, using physics principles.

The wrong approach: A question asks "Explain why a parachutist reaches terminal velocity." The candidate writes: "The parachutist falls faster and faster, then stays at the same speed." This is a description, not an explanation. It earns 0-1 marks out of 3-4.

The correct technique: "When the parachutist first jumps, their weight is greater than the air resistance, so there is a resultant downward force causing acceleration. As speed increases, air resistance increases. Eventually air resistance equals the weight, the resultant force is zero, and the parachutist travels at constant velocity (terminal velocity)."

Mistake 2: Missing units in calculations

This is one of the most consistent edexcel igcse physics mistakes in the examiner reports. Candidates perform the calculation correctly but leave the answer as a bare number.

The wrong approach: "Work done = 500 x 3 = 1500." Missing: the unit (J).

The correct technique: W = F x d = 500 N x 3 m = 1500 J. The formula, the substitution with units attached to each value and the answer with the correct SI unit. Every edexcel igcse physics exam tips guide repeats this advice because candidates keep losing this mark.

The most commonly forgotten units in the exam:

Mistake 3: Confusing mass and weight in forces questions

Forces, movement, shape and momentum is one of the most commonly examined areas in Edexcel IGCSE Physics. A recurring error is using mass (in kg) where weight (in N) is required, or the reverse.

The wrong approach: "The weight of the object is 5 kg." Weight is a force; it is measured in newtons, not kilograms.

The correct technique: Weight = mass x gravitational field strength. W = m x g = 5 kg x 9.8 N/kg = 49 N. Always check whether the question asks for mass or weight. Mass is how much matter an object contains (kg). Weight is the gravitational force acting on that mass (N).

Mistake 4: Incomplete explanations of energy transfers

Energy transfers is a topic that appears across both papers, and the most frequent error is naming energy stores without stating the transfer pathway.

The wrong approach: "Chemical energy becomes kinetic energy becomes thermal energy." This lists the stores but says nothing about how the energy moves between them.

The correct technique: "Energy is transferred from the chemical store (in the fuel) to the kinetic store (of the moving vehicle) mechanically by the engine. Energy is then transferred from the kinetic store to the thermal store of the brakes and surroundings by heating due to friction." Name the store, say where it is, and state the transfer pathway (mechanically, electrically, by heating, by radiation).

Mistake 5: Not converting temperature to kelvin for gas law calculations

This mistake appears every time the Edexcel IGCSE Physics exam includes a gas law question. The relationship p₁/T₁ = p₂/T₂ requires temperatures in kelvin. Substituting Celsius values gives an answer that is numerically wrong.

The wrong approach: "T₁ = 27, T₂ = 127, so p₂ = p₁ x 127 / 27." Wrong: this uses Celsius, and the ratio is incorrect.

The correct technique: Convert to kelvin first. T₁ = 27 + 273 = 300 K. T₂ = 127 + 273 = 400 K. Then p₂ = p₁ x 400 / 300. The kelvin conversion is worth a mark in its own right.

Mistake 6: Drawing incomplete or unlabelled ray diagrams

In light and sound questions, ray diagrams must include: the incident ray, the normal line, the reflected or refracted ray, and the angle labels (angle of incidence i and angle of refraction r or angle of reflection). Missing any one of these elements costs marks.

The wrong approach: Drawing a ray bouncing off a surface without drawing the normal or labelling the angles.

The correct technique: Draw the normal as a dashed line perpendicular to the surface at the point of incidence. Draw the incident ray to the point of incidence. Draw the reflected ray at the same angle on the other side of the normal. Label i and r clearly. For refraction, remember that the ray bends towards the normal when entering a denser medium.

Mistake 7: Confusing half-life with "halving the amount of substance"

In radioactivity questions, half-life is the time it takes for the activity of a radioactive source to fall to half its original value, or for half the radioactive nuclei to decay. It is not the time for half the substance to disappear.

The wrong approach: "After one half-life, half the material is gone." The atoms are still there; they have just become a different element or isotope.

The correct technique: "After one half-life, the activity of the source has decreased to half its initial value. The number of undecayed nuclei has halved." Use the word "activity" or "undecayed nuclei," not "material" or "substance."

Worked example: A source has an activity of 800 Bq. Its half-life is 3 hours. What is the activity after 9 hours?

• After 3 hours: 800 / 2 = 400 Bq
• After 6 hours: 400 / 2 = 200 Bq
• After 9 hours: 200 / 2 = 100 Bq

Mistake 8: Failing to distinguish between density and pressure

Density and pressure are among the most heavily examined concepts in the Edexcel IGCSE Physics specification. Candidates sometimes substitute density values into pressure equations without thinking carefully about what each formula requires.

Density = mass / volume (rho = m / V). It describes how much mass is packed into a given volume.

Pressure = force / area (p = F / A). It describes the force acting per unit area on a surface.

Pressure difference in a fluid = height x density x g (p = h x rho x g). This is where density and pressure appear in the same formula, and where candidates most often confuse the two. Make sure height is in metres, density is in kg/m³, and g is 9.8 N/kg (or 10 N/kg if the question tells you to use that value).

Mistake 9: Timing failures

The final category of edexcel igcse physics mistakes is not about physics at all. It is about time management. Candidates who spend too long on early questions leave themselves with insufficient time for the higher-mark questions at the end of the paper. A 6-mark extended response attempted in two minutes cannot reach Level 3 of the mark scheme, no matter how well the candidate knows the physics.

The fix: Before the exam, calculate your pace. Paper 1 has 110 marks in 120 minutes, so roughly 1 minute per mark. Paper 2 has 70 marks in 75 minutes, so roughly 1 minute per mark. A 4-mark question gets 4 minutes. A 1-mark question gets 1 minute. If you have spent 3 minutes on a 1-mark question, leave it and move on. Come back only if you have time at the end.