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

The pattern behind lost marks

Most marks lost in IGCSE Physics exams are not lost on hard questions. They are lost on preventable errors: wrong units, confused definitions, missing working, sloppy graph reading. The pattern repeats across every exam session, and examiners' reports flag the same problems year after year.

What follows covers the most damaging categories of mistake across the Cambridge IGCSE Physics (0625) syllabus. Each section shows the error, explains why it costs marks, and demonstrates the correct approach.

Confusing mass and weight

This error appears more often than any other in the forces and motion section. Students treat mass and weight as interchangeable. They are not.

Mass is a scalar quantity measured in kilograms. It does not change with location. Weight is a force measured in newtons. It depends on the gravitational field strength: W = mg. A 60 kg astronaut has a weight of approximately 588 N on Earth but only about 96 N on the Moon. The mass stays 60 kg in both places.

Confusing speed and velocity

Speed is a scalar. Velocity is a vector. The distinction matters on paper because IGCSE mark schemes penalise candidates who define velocity as "how fast something moves" without mentioning direction.

• Speed = distance / time (scalar, always positive)
• Velocity = displacement / time (vector, has direction)
• An object moving in a circle at constant speed has a changing velocity because its direction changes continuously

That third point catches many students. Circular motion at constant speed still involves acceleration because the velocity vector is always changing direction. If a question asks "Is the object accelerating?", the answer can be yes even when speed is constant.

Failing to convert units

Physics equations demand SI units. The exam routinely gives values in non-SI units to test whether candidates convert before calculating.

Worked example: unit conversion gone wrong

A 2 kW heater runs for 5 minutes. Calculate the energy transferred.

Wrong: E = Pt = 2 x 5 = 10 J. (Time was not converted. Power was not converted.)

Right:

1. Convert power: 2 kW = 2000 W
2. Convert time: 5 minutes = 300 s
3. E = Pt = 2000 x 300 = 600 000 J = 600 kJ

Two conversions missed, answer off by a factor of 60 000. This type of error appears in energy, work, power, and electrical calculations alike.

Incomplete or vague explanations

IGCSE Physics mark schemes are built on specific key phrases. Vague answers score zero even when the student genuinely understands the concept.

The fix is simple: every explanation needs a mechanism. State what happens, then state why it happens using correct physics terminology. "The metal spoon feels cold" is an observation. "Metal is a good conductor, so thermal energy transfers rapidly from your hand to the spoon" is physics.

Misreading command words

Cambridge command words carry precise expectations. Ignoring them is one of the fastest ways to lose marks across any IGCSE Physics paper.

• State: give a fact with no explanation needed
• Describe: say what happens in detail, step by step
• Explain: give reasons using physics principles; cause and effect
• Calculate: show working and give a numerical answer with units
• Suggest: apply physics knowledge to an unfamiliar context; more than one answer may be acceptable
• Compare: identify both similarities and differences; always cover both

A student who writes a full paragraph when the question says "state" wastes time. A student who writes a one-word answer when the question says "explain" loses the explanation marks. Read the command word before writing anything.

Circuit diagram errors

Electric circuits are tested heavily, and three errors dominate the mark losses.

Error 1: confusing series and parallel rules. In a series circuit, current is the same everywhere and voltage divides across components. In a parallel circuit, voltage is the same across each branch and current divides. Mixing these rules up produces wrong answers in every subsequent calculation.

Error 2: placing meters incorrectly. An ammeter goes in series with the component it measures. A voltmeter goes in parallel across the component it measures. Swapping them gives nonsensical readings and costs marks in both theory and practical papers.

Error 3: forgetting that adding a resistor in parallel reduces total resistance. This is counterintuitive but follows directly from 1/R_total = 1/R₁ + 1/R₂. More paths for current mean less overall opposition to flow.

Worked example: parallel resistance

Two resistors, 6 Ω and 3 Ω, are connected in parallel. Find the total resistance.

Wrong: R_total = 6 + 3 = 9 Ω. (This is the series formula.)

Right:

1. 1/R = 1/6 + 1/3 = 1/6 + 2/6 = 3/6 = 1/2
2. R = 2 Ω

The parallel combination has a lower resistance than either individual resistor. If that surprises you, revisit the parallel rules before exam day.

Sign and direction errors in forces

Resultant force problems require consistent sign conventions. Students who assign positive and negative directions inconsistently within the same calculation get the wrong answer even when they know the physics.

Pick a direction as positive at the start. Stick with it. If rightward is positive, then a leftward force is negative. If upward is positive, weight acts downward and is negative.

Worked example: resultant force

A 500 N engine force drives a car forward. Friction is 200 N backward. What is the resultant force?

Wrong: 500 + 200 = 700 N. (Friction was added instead of subtracted.)

Right: 500 - 200 = 300 N forward. Friction opposes motion, so it acts in the opposite direction to the engine force and must be subtracted.

Poor graph skills

Graph questions appear on nearly every IGCSE Physics paper. Candidates lose marks for four specific reasons:

1. No ruler for straight lines. Freehand curves are acceptable for smooth curves (cooling curves, I-V characteristics). Straight-line graphs demand a ruler.
2. Awkward scales. Each small square should represent a round number: 1, 2, 5, or 10. Never 3 or 7. A poor scale makes plotting inaccurate and reading off values unreliable.
3. Ignoring anomalous results. Circle the anomaly. Draw the line of best fit through the remaining points. Do not force the line through an obvious outlier.
4. Wrong gradient calculation. Use two points far apart on the best-fit line, not raw data points from the table. Read their coordinates carefully. Gradient = change in y / change in x.

Not showing working

Cambridge mark schemes award marks at each step: selecting the right equation, substituting values correctly, and stating the final answer with units. A candidate who writes only the final number and gets it wrong scores zero. A candidate who shows working but makes an arithmetic slip at the last step still picks up method marks.

The non-negotiable routine for any calculation:

1. Write the equation you are using
2. Substitute the known values (with correct units)
3. Calculate and state your answer
4. Include the correct unit

Skipping step 1 or step 4 costs at least one mark per question. Over a full paper, that adds up to a full grade boundary.

Thermal energy transfer: mixing up the mechanisms

Students frequently confuse conduction, convection, and radiation, or describe them in vague, non-physics language.

The classic error: "Heat rises." Heat does not rise. Hot air rises because it is less dense than the surrounding cooler air. The energy transfer mechanism is convection, and it requires a fluid. If a question involves a vacuum flask, convection and conduction are both eliminated. Only radiation remains.

Energy, work, and power mix-ups

Three equations, three quantities, three sets of units. The mistakes come from conflating them.

• Work done = force x distance moved in the direction of the force (W = Fd), measured in joules
• Kinetic energy = 1/2 x mass x velocity² (KE = 1/2 mv²), measured in joules
• Power = work done / time taken (P = W/t), measured in watts

A frequent error: using the wrong distance in work calculations. Work done requires the distance in the direction of the force, not the total path length. If you push a box 10 m along a flat floor, the work done against gravity is zero because the vertical displacement is zero.

Another: forgetting to square the velocity in kinetic energy. KE = 1/2 mv², not 1/2 mv. Doubling the speed quadruples the kinetic energy. This has direct implications for stopping distances and road safety questions that appear regularly on the exam.

Test yourself: spot the error

Each problem below contains a deliberate mistake. Identify it, then solve correctly.

1. A 70 kg person stands on the Moon (g = 1.6 N/kg). A student writes: "Weight = 70 kg." What is wrong?
2. A car travels 1500 m in 2 minutes. A student calculates speed = 1500 / 2 = 750 m/s. What went wrong?
3. A 12 V battery powers a 4 Ω and 8 Ω resistor in parallel. A student adds them: R = 4 + 8 = 12 Ω. Why is this wrong?
4. A student explains convection by writing: "Heat rises because it is hot." What is missing?

Time management on exam day

Poor time allocation turns solid physics knowledge into wasted potential. A practical rule: spend roughly one minute per mark. A 4-mark question gets about four minutes. A 1-mark "state" question gets one minute, not five.

• Read the whole paper first. Flag questions you can answer quickly.
• Answer the straightforward ones first. Bank those marks early.
• If stuck on a question for more than two minutes, move on. Return to it at the end.
• Leave five minutes at the end to check units, significant figures, and whether you actually answered what was asked.

The fix list

Every mistake on this list is fixable with practice. Work through past papers under timed conditions, check your answers against the mark scheme, and pay attention to where marks were lost. The candidates who improve fastest are the ones who treat each lost mark as a specific, correctable habit rather than a general lack of understanding.