Cambridge IGCSE Physics 0625: Frequent mistakes

Cambridge Principal Examiner Reports for Physics 0625 identify significant gaps in basic knowledge and recurring misconceptions. Addressing these improves your performance.

Basic mechanics

Constant motion and resultant force

Many candidates don’t recognise that zero resultant force means constant velocity (Newton’s first law). They think a moving object needs a constant force to keep moving.

Fix: No resultant force → constant velocity (including zero). Force is needed to change velocity (accelerate or decelerate).

Acceleration of falling objects

Candidates often think acceleration changes as an object falls or decreases with height. Acceleration due to gravity is approximately constant (9.8 m/s²) near Earth’s surface.

Fix: g ≈ 9.8 m/s² (constant). Velocity increases; acceleration does not (ignoring air resistance).

Density and pressure

Density calculations

Using total volume instead of change in volume when calculating density from displacement. Confusion between mass and weight in pressure.

Fix: Density = mass ÷ volume. For displacement: volume = volume of fluid displaced. Pressure = force ÷ area; weight = mg (force).

Mass vs. weight in pressure

Pressure involves force (e.g. weight = mass × g), not mass alone. Some use mass directly in P = F/A.

Fix: F = mg for weight. P = F/A. Use correct units (N, m², Pa).

Waves

Frequency definition

Candidates divide total time by number of oscillations instead of oscillations per unit time. Frequency = 1/T = number of oscillations ÷ time.

Fix: f = 1/T. Or: f = number of complete waves ÷ time (in seconds).

Wave properties

Weak grasp of transverse vs. longitudinal; diffraction; wavelength and amplitude. Confusion about how waves transfer energy without transferring matter.

Fix: Transverse: oscillations perpendicular to direction. Longitudinal: parallel. Diffraction: spreading when passing through gap or past obstacle.

Thermal physics

Radiation vs. conduction

Some think black surfaces are better conductors. Black surfaces are better absorbers and emitters of radiation; conduction is different.

Fix: Black/dark: good absorber and emitter of infrared. Conduction: transfer through solids; no black/white distinction in same material.

Electricity and magnetism

Series circuits and Ohm’s law

Failure to use total resistance when applying V = IR in series. Using resistance of one component for the whole circuit.

Fix: In series: R_total = R₁ + R₂ + … Use R_total for circuit current.

Motor effect

Weak grasp of Fleming’s left-hand rule; direction of force on current-carrying conductor in magnetic field.

Fix: Thumb = motion, First finger = field, Second finger = current. Practice with past paper questions.

Magnetic fields

Limited understanding of field direction around wires and magnets; field lines from N to S.

Fix: Right-hand grip rule for wire. Field lines: N → S externally.

Modern physics

Radioactivity

Poor understanding of half-life; randomness of decay; predicting future amounts. Some think decay can be triggered or stopped.

Fix: Half-life = time for half the nuclei to decay. Random process. Use N = N₀(½)^(t/T) for remaining amount.

Atomic structure

Weak knowledge of nucleus composition (protons, neutrons); isotopes; alpha, beta, gamma differences.

Fix: Nucleus: protons + neutrons. Isotopes: same protons, different neutrons. Alpha: ⁴He; beta: electron; gamma: electromagnetic.

Exam technique

• Read carefully—missing “not” or “except” changes the answer
• Ray diagrams—practise drawing and interpreting for lenses
• Show working—calculations need method + answer + units
• Use the resource—refer to diagrams and data given

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Based on Cambridge IGCSE Physics 0625 Principal Examiner Reports (2017–2024).