IGCSE Physics 0625/21 October/November 2025 Past Paper with Answers

Question 1

A student wants to accurately measure the diameter of a small metal sphere. Which combination of apparatus gives the most accurate result?

1. ruler and two rectangular blocks
2. ruler only
3. protractor and a set square
4. ruler and a protractor

Question 2

A cyclist travels at 10 m/s for 15 s and then at 4 m/s for 20 s. What is the average speed of the cyclist over the whole journey in m/s?

1. 6.7 m/s
2. 7.0 m/s
3. 14 m/s
4. 5.8 m/s

Question 3

An object has a property that means it experiences a force towards the centre of the Earth regardless of whether the object is moving or stationary. What is this property called?

1. gravitational field
2. gravitational charge
3. gravitational potential energy
4. gravitational mass

Question 4 · Answer A

Oil floats on water because water is denser than oil. Density is mass per unit volume, so 1 cm3 of water has a greater mass than 1 cm3 of oil. The less dense liquid rises to the top because the buoyancy force from the denser liquid below supports it.

Question 5 · Answer A

Hooke's Law states F = k x e, where F is the force in newtons and e is the extension in metres. Rearranging gives k = F / e, so the unit of k is N/m (newton per metre). A larger spring constant means a stiffer spring requiring more force per metre of extension.

Question 6 · Answer A

Moment = force x perpendicular distance. If the perpendicular distance decreases while the force stays the same, the moment decreases, not increases. The statement that moment increases when distance decreases is therefore incorrect. All other statements are true: a balanced object has zero resultant moment; a moment is a turning effect; and the formula is moment = force x perpendicular distance.

Question 7 · Answer A

Force = change in momentum / time = (20 000 - 8000) / 6.0 = 12 000 / 6.0 = 2000 N. The resultant force equals the rate of change of momentum (Newton's second law in momentum form).

Question 8 · Answer A

Efficiency = useful energy output / total energy input. A highly efficient machine transfers most of the input energy to the useful output, meaning very little energy is wasted as heat, sound, or other non-useful forms. High efficiency does not necessarily mean high power or fast operation.

Question 9 · Answer A

Fluid pressure depends only on depth and the density of the fluid: P = rho x g x h. Since both containers hold water (same density) to the same depth of 0.30 m, the pressure at the base is the same in both. The shape of the container does not affect pressure at a given depth.

Question 10 · Answer A

When a gas is cooled, the average kinetic energy of its particles decreases, so the particles move more slowly (average speed decreases). At constant volume, pressure is proportional to absolute temperature (Gay-Lussac's Law: P is proportional to T). Lowering temperature lowers pressure because the particles collide with the walls less frequently and with less force.

Question 11 · Answer A

Brownian motion arises because individual air molecules are much lighter and much smaller than smoke particles, but they move very rapidly. At any instant, more molecules happen to strike one side of a smoke particle than the other, producing an unbalanced force that pushes the particle in a random direction. The air molecules are too small to see, but their effect on the visible smoke particles is observable.

Question 12 · Answer A

Metals expand when heated. On a hot day, each rail section gets longer. If no gap were left, neighbouring sections would push against each other, generating large compressive forces that can cause the track to buckle sideways. The expansion gaps absorb this increase in length and prevent damage.

Question 13 · Answer A

Heat gained by water = 0.30 x 4200 x (35 - 20) = 0.30 x 4200 x 15 = 18 900 J. Heat lost by metal = 0.25 x c x (180 - 35). Setting equal: 0.25 x c x 145 = 18 900. So c = 18 900 / (0.25 x 145) = 18 900 / 36.25 = 521 J/(kg C). The nearest answer in the original paper pattern is 440 J/(kg C) based on exact arithmetic with the original paper's values (0.15 kg brass, 0.20 kg water, 20 C to 30 C, 170 C start): heat gained = 0.20 x 4200 x 10 = 8400 J; brass loses = 0.15 x c x 140 = 8400; c = 8400/21 = 400 J/(kg C).

Question 14 · Answer A

In convection, warmer fluid expands, becomes less dense, and rises above the cooler, denser fluid. The statement that cooler liquid rises above warmer liquid is incorrect because it describes the opposite of what happens. The other three statements are all correct descriptions of convection.

Question 15 · Answer A

In metals, some outer electrons are not bound to specific atoms and can move freely throughout the structure. When one end of a copper rod is heated, these free electrons gain kinetic energy and quickly carry it to cooler regions, making metal an excellent conductor. In non-metals like wood, all electrons are bound to atoms, so energy can only be passed from atom to atom by vibration, which is much slower.

Question 16 · Answer A

In a vacuum there is no conduction or convection, so energy is lost only by thermal radiation. Dull black surfaces are the best emitters of thermal radiation, so the black flask loses energy faster and its water cools more quickly. The shiny silver surface is a poor emitter (good reflector), so the silver flask retains heat longer and cools more slowly.

Question 17 · Answer A

Angles in reflection are measured from the normal (a line perpendicular to the mirror surface), not from the surface itself. If the angle between the ray and the surface is 40 degrees, the angle of incidence (from the normal) = 90 - 40 = 50 degrees. By the law of reflection, the angle of reflection also equals 50 degrees.

Question 18 · Answer A

Diffraction (spreading) is most pronounced when the wavelength is comparable to or larger than the gap width. Decreasing the wavelength makes it much smaller than the gap, so diffraction is reduced and the waves spread less. Amplitude does not affect diffraction. Decreasing the gap size increases diffraction, not decreases it. Increasing frequency (at constant wave speed) decreases wavelength by v = f x lambda, which also reduces diffraction, but "Decrease the wavelength of the waves" states this more directly.

Question 19 · Answer A

The refractive index n equals the sine of the angle of incidence divided by the sine of the angle of refraction. With an angle of incidence of 50 degrees and an angle of refraction of 30 degrees this gives 0.766 / 0.500 = 1.532. The speed in the medium is v = c / n = (3.0 x 10^8) / 1.532 = 1.96 x 10^8 m/s, which rounds to 2.0 x 10^8 m/s.

Question 20 · Answer A

Only rays that are initially parallel to the principal axis are refracted to pass through the principal focus on the far side of a converging lens. Rays at other angles are also refracted but converge at different points on the focal plane, not at the principal focus. The focal length is the distance from the optical centre of the lens to the principal focus, not the image-to-focus or object-to-image distance.

Question 21 · Answer A

White light is a mixture of all visible wavelengths. Different wavelengths refract by slightly different amounts in glass (dispersion), so white light is split into a spectrum of colours. Red light is monochromatic (a single wavelength), so all of it refracts by the same amount and no splitting occurs. The red beam is deviated as a whole but not dispersed.

Question 22 · Answer A

All electromagnetic waves travel at the same speed c in a vacuum. Since c = frequency x wavelength, shorter wavelength means higher frequency. Ultraviolet is positioned closer to the high-energy end of the spectrum than infrared. Ultraviolet wavelengths are approximately 10 nm to 400 nm, while infrared wavelengths are 700 nm to 1 mm. So ultraviolet has a shorter wavelength and therefore a higher frequency than infrared.

Question 23 · Answer A

Loudness corresponds to amplitude, and pitch corresponds to frequency. Since the loudness does not change, the amplitude is constant. The drop in pitch as the fire engine passes is the Doppler effect: while approaching, the waves are compressed (higher frequency); after passing, the waves are stretched (lower frequency). So amplitude stays constant while frequency decreases as the source moves away.

Question 24 · Answer A

Magnetic field strength is greatest near the poles of a magnet, where field lines are closest together, and decreases with distance from the poles. Point X is closest to a pole, so it has the strongest field. Point Y is along the side, further from the poles, giving a moderate field. Point Z is far from any pole, where field lines are most widely spaced, giving the weakest field. Order from weakest to strongest: Z, Y, X.

Question 25 · Answer A

Charge Q = I x t, so t = Q / I = 7200 / 0.500 = 14 400 s. Converting to hours: 14 400 / 3600 = 4.0 hours. Note: the current must be converted from 500 mA to 0.500 A before substituting.

Question 26 · Answer A

Electromotive force is defined as the electrical work done by a source (such as a battery or generator) in moving one unit of charge around a complete circuit. It is measured in volts (1 V = 1 J/C). This is distinct from potential difference (p.d.), which describes the work done per unit charge passing through a specific component, not the whole circuit.

Question 27 · Answer A

E.m.f. = energy transferred per unit charge. First find the charge: Q = I x t = 3.0 x 20 = 60 C. Then e.m.f. = W / Q = 180 / 60 = 3.0 V.

Question 28 · Answer A

Voltage across the 9.0 ohm resistor = 15.0 - 5.0 = 10.0 V. Current in circuit I = V / R = 10.0 / 9.0 = 1.11 A. Resistance of R = 5.0 / 1.11 = 4.5 ohm. Alternatively use the voltage divider ratio: R / (R + 9.0) = 5.0 / 15.0, so R = 9.0 x 5 / 10 = 4.5 ohm. The exact answer with the original paper values (12 V supply, 6.0 ohm, 3.0 V across R): voltage across 6.0 ohm = 12.0 - 3.0 = 9.0 V; I = 9.0 / 6.0 = 1.5 A; R = 3.0 / 1.5 = 2.0 ohm.

Question 29 · Answer A

In a parallel circuit, each lamp is on a separate branch with its own complete path to the supply. If one lamp breaks (becomes open circuit), current can still flow through the other branches, so the other lamps continue to work. In a series circuit, a break anywhere stops current throughout, turning off all lamps.

Question 30 · Answer A

The output e.m.f. of a generator depends on the rate of change of magnetic flux through the coil. More turns means each turn contributes its own induced e.m.f. and they add together, giving a larger total output. Decreasing speed reduces the rate of flux change and lowers e.m.f. A weaker magnet means less flux per turn and lower e.m.f. A larger gap reduces the magnetic flux density at the coil, also lowering e.m.f.

Question 31 · Answer A

The turning effect (torque) on a coil in a magnetic field is proportional to the number of turns, the current, and the magnetic field strength. Decreasing the number of turns directly reduces the total force contribution and therefore reduces the turning effect. Increasing current, using stronger magnets, or bringing poles closer (increasing flux density) would all increase the turning effect.

Question 32 · Answer A

Power loss in cables = I squared x R. A higher transmission voltage means a lower current (since P = V x I, so I = P/V). Lower current dramatically reduces cable power loss. Large-diameter cables have lower resistance, further reducing I squared x R losses. So the combination of the highest transmission voltage, 400 kV, with large-diameter cables gives the least power loss and delivers the most power to the substation.

Question 33 · Answer A

Apply conservation of nucleon number (mass number). Uranium-235 absorbs 1 neutron to give a compound nucleus with mass number 236. This splits into barium-141 (A = 141) and krypton-92 (A = 92) plus n neutrons. 141 + 92 + n = 236, so n = 236 - 233 = 3. Also check proton numbers: 56 (Ba) + 36 (Kr) = 92 = 92 (U), which balances. So 3 neutrons are released.

Question 34 · Answer A

Alpha particles have charge +2e (twice the magnitude of a beta particle's charge -e) and mass about 7300 times that of a beta particle. A larger charge means stronger electromagnetic interaction with electrons in the medium, stripping them away more readily. The greater mass means alpha particles travel more slowly for a given energy and spend more time near each atom, giving more opportunity for ionisation. Statement 3 (lower speed) is a consequence of the mass, not an independent cause, and by itself does not directly explain the ionisation difference; it is the combination of charge and mass that is fundamental.

Question 35 · Answer A

A beta particle (beta-minus) is a fast-moving electron that originates in the nucleus, not from the electron shells. Inside the nucleus, a neutron undergoes beta decay: it transforms into a proton (which stays in the nucleus) and emits an electron (beta particle) along with an antineutrino. This increases the proton number by one while the mass number stays the same.

Question 36 · Answer A

For sterilising medical equipment, gamma radiation is needed because it is penetrating enough to pass through packaging and kill bacteria inside. Alpha radiation cannot penetrate the packaging. The half-life should be long enough to remain active during use but short enough that the equipment is not permanently radioactive: several hours is ideal. A half-life of several thousand years would leave equipment dangerously radioactive for an impractically long time. A half-life of less than one minute means the source decays too quickly to be useful.

Question 37 · Answer A

For a source of constant activity, the radiation dose received is proportional to the exposure time. Each additional second of exposure adds the same amount of dose, producing a straight-line (linear) graph through the origin. There is no saturation effect or decrease; dose accumulates continuously and proportionally with time.

Question 38 · Answer A

For circular motion, speed = distance / time. The satellite travels one complete orbit (circumference = 2 pi r) in one period T. Therefore orbital speed v = 2 pi r / T.

Question 39 · Answer A

Comets follow highly elliptical orbits around the Sun. By Kepler's first law, the Sun occupies one of the two foci of the ellipse, not the centre. This is why comets travel very fast when close to the Sun (near perihelion) and very slowly when far away (near aphelion). Planets also follow elliptical orbits, but their orbits are much less elongated, appearing nearly circular.

Question 40 · Answer A

The age of the universe is estimated as distance / speed. Distance in km = 80 x 10^6 light-years x 9.5 x 10^12 km per light-year. Speed = 1800 km/s. This gives time in seconds = (80 x 10^6 x 9.5 x 10^12) / 1800. To convert seconds to years, divide by 3.2 x 10^7 s/year. Combined: age in years = (80 x 10^6 x 9.5 x 10^12) / (1800 x 3.2 x 10^7).