May/June 2025 Paper 23 Worked Answers (IGCSE Physics 0625 Extended)

40 questions · 40 marks · 45 minutes

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1. Question 1

   1 marksMeasurement techniques
   A ruler measures length directly. The distance the car travels is a length, so it can be read straight from a ruler. Time requires a stopwatch, temperature requires a thermometer, and volume requires a measuring cylinder or calculation from dimensions.
   Method:

   A ruler measures length. Distance is a length, so it is the correct answer.
   Examiner tips

   • Identify what physical quantity each instrument measures before choosing.

2. Question 2

   1 marksAverage speed
   Average speed = total distance divided by total time = 24 km / 2 h = 12 km/h. Velocity requires displacement (straight-line distance) not total path length, and would also need a direction. Acceleration involves change in velocity, and kinetic energy involves mass and speed squared.
   Method:

   Recall that speed = distance / time. The winding road means distance not displacement, so this is average speed.
   Examiner tips

   • Average speed uses total path length; velocity uses displacement.

3. Question 3

   1 marksSpeed-time graphs
   The area under a speed-time graph has units of (m/s) x s = m, which is distance. Acceleration is the gradient (slope) of a speed-time graph. Speed is the value on the y-axis, and force is not directly read from a speed-time graph.
   Method:

   Units of area = m/s x s = m (distance). Therefore the area gives distance.
   Examiner tips

   • Area = distance; gradient = acceleration on a speed-time graph.

4. Question 4

   1 marksWeight and gravity
   Statement 1 describes mass, not weight. Mass is the quantity of matter. Weight is the gravitational force on an object: it is the effect of a gravitational field on the object (statement 2) and is the force of attraction between the object and the Earth (statement 3). Both statements 2 and 3 are correct descriptions of weight.
   Method:

   Eliminate statement 1 (that is mass), confirm statements 2 and 3 describe weight as a gravitational force.
   Examiner tips

   • Mass = quantity of matter; weight = gravitational force on an object.

5. Question 5

   1 marksDensity and floating
   The densest liquid sinks to the bottom; it has the smallest volume for the same mass (80 cm3). The next densest liquid sits in the middle (100 cm3), and the least dense floats on top (120 cm3). The first boundary is at 80 cm3. The second boundary is at 80 + 100 = 180 cm3. The total level is 80 + 100 + 120 = 300 cm3.
   Method:

   Rank by density (smallest volume = densest = bottom). Add volumes: boundary 1 = 80, boundary 2 = 80 + 100 = 180.
   Examiner tips

   • Densest liquid has smallest volume for equal mass and sinks; add volumes upward for boundaries.

6. Question 6

   1 marksNewton's second law and resultant force
   Resultant force = driving force - resistive forces = 2000 - 800 - 500 = 700 N forwards. A positive (forward) resultant force means the car accelerates in the direction of motion, so it speeds up. For constant speed the resultant force would need to be zero.
   Method:

   Sum opposing forces: 800 + 500 = 1300 N backwards. Net = 2000 - 1300 = 700 N forwards. Forward net force means the car speeds up.
   Examiner tips

   • Add all forces in each direction first, then find the net (resultant) force.

7. Question 7

   1 marksCircular motion and centripetal force
   For circular motion at constant speed the resultant force is centripetal: directed towards the centre of the circle. This force continuously changes the direction of the ball without changing its speed. The tension in the string provides this centripetal force, always pointing inward.
   Method:

   State that circular motion requires a centripetal force directed towards the centre of the circle.
   Examiner tips

   • In circular motion at constant speed, the resultant force is always centripetal (towards the centre).

8. Question 8

   1 marksConservation of momentum
   Total momentum before = 5000 x 3.0 + 10 000 x 0 = 15 000 kg m/s to the right. Combined mass = 5000 + 10 000 = 15 000 kg. Velocity after = 15 000 / 15 000 = 1.0 m/s to the right. Momentum is conserved in the absence of external forces.
   Method:

   Calculate total momentum before (5000 x 3.0 = 15 000 kg m/s). Divide by combined mass (15 000 kg) to get 1.0 m/s.
   Examiner tips

   • Define a positive direction first, then apply p(before) = p(after).

9. Question 9

   1 marksFree fall and kinematics
   Using energy conservation: the kinetic energy gained equals the gravitational potential energy lost. So 0.5 x m x v^2 = m x g x h. Cancelling mass: v^2 = 2 x g x h = 2 x 9.8 x 20 = 392. Therefore v = square root of 392 = 19.8 m/s, which rounds to 20 m/s. Alternatively using kinematics: v^2 = u^2 + 2as = 0 + 2 x 9.8 x 20 = 392, giving v = 19.8 m/s which is closest to 20 m/s.
   Method:

   Apply v^2 = 2 x 9.8 x 20 = 392, then v = sqrt(392) = 19.8 m/s.
   Examiner tips

   • Use v^2 = 2gh for a free-fall drop from rest.

10. Question 10

    1 marksParticle model of matter
    In a gas, particles are far apart (much greater separation than in a solid or liquid) and move randomly in all directions at high speed, colliding with each other and the container walls. In a solid, particles are close together and vibrate about fixed positions. In a liquid, particles are close together but can move past each other.
    Method:

    Recall gas properties: far apart, random motion. Eliminate options describing solids and liquids.
    Examiner tips

    • Remember: solid = close + fixed; liquid = close + free; gas = far apart + random.

11. Question 11

    1 marksGas laws and absolute temperature
    When particles gain kinetic energy their average speed increases. Higher speed means more frequent and harder collisions with the container walls, so pressure increases. Temperature is directly proportional to the average kinetic energy of the particles, so temperature also increases. Both increase together in a fixed volume.
    Method:

    More KE means faster particles, so higher temperature and more forceful collisions with walls, so higher pressure.
    Examiner tips

    • Temperature is a measure of average kinetic energy of particles; more KE = higher T and higher p in fixed volume.

12. Question 12

    1 marksGas pressure and momentum
    When a gas particle bounces off a wall its momentum changes. By Newton's second law, a change in momentum over time is a force. That force, distributed over the area of the wall, gives pressure. So: change in momentum leads to force, and pressure = force / area. The correct chain is: change in momentum produces a force; pressure = force / area.
    Method:

    Bouncing particles change momentum, producing a force. Pressure = force divided by area of the wall.
    Examiner tips

    • p = F/A; F comes from rate of change of momentum. Never mix up multiplication and division.

13. Question 13

    1 marksChanges of state
    Melting is the change of state from solid to liquid. Boiling is the change from liquid to gas at the boiling point. Condensation is the change from gas to liquid. Freezing is the change from liquid to solid.
    Method:

    Recall that melting is the change from solid to liquid.
    Examiner tips

    • Learn all six change-of-state names and their directions.

14. Question 14

    1 marksSpecific heat capacity experiment
    To find specific heat capacity using c = E / (m x delta-T), we need: energy supplied E = V x I x t (voltage from voltmeter, current from ammeter already present, time from stop-watch), mass m of the block (from balance), and temperature change delta-T (from thermometer already present). Therefore the additional items needed are a voltmeter (to find voltage), a stop-watch (to measure time) and a balance (to measure mass).
    Method:

    Use c = E/(m x delta-T). Need voltmeter for V, stop-watch for t, balance for m. These are the three additional items.
    Examiner tips

    • List every quantity in the formula and check which measuring instrument provides each one.

15. Question 15

    1 marksThermal radiation
    A surface emits radiation most slowly when it is a poor emitter (white and shiny) and has a small area. Black, dull surfaces are the best emitters. White, shiny surfaces are the worst (slowest) emitters. A small area also reduces total emission. Therefore a white, shiny surface with only 10 cm2 emits at the slowest rate.
    Method:

    Identify that white and shiny emits slowest; then choose the smallest area among white shiny options.
    Examiner tips

    • Two factors reduce emission rate: white/shiny surface and small area. Choose the option with both.

16. Question 16

    1 marksThermal equilibrium
    A constant temperature means the internal energy of the object is not changing. This happens when energy is transferred to the object at the same rate as it is transferred away: the two rates are equal. This is thermal equilibrium. If energy gain exceeded loss the temperature would rise; if loss exceeded gain the temperature would fall.
    Method:

    Constant temperature means no net change in internal energy, so energy input rate equals energy output rate.
    Examiner tips

    • Constant temperature = thermal equilibrium = energy in rate equals energy out rate.

17. Question 17

    1 marksWave properties
    Light is an electromagnetic wave and is transverse. In a transverse wave the vibration (oscillation of the electric and magnetic fields) is perpendicular to the direction of energy travel. Sound, by contrast, is a longitudinal wave where vibration is parallel to the direction of travel.
    Method:

    Recall that light is transverse electromagnetic wave. Transverse means perpendicular vibration.
    Examiner tips

    • Transverse: vibration perpendicular to travel. Longitudinal: vibration parallel to travel.

18. Question 18

    1 marksDiffraction
    Diffraction is greater when the wavelength is longer (or when the gap width is closer to the wavelength). Wave X has the longer wavelength, so it diffracts more than wave Y through the same gap.
    Method:

    State that diffraction is greater for longer wavelengths. Wave X (longer wavelength) diffracts more.
    Examiner tips

    • More diffraction occurs when wavelength is longer relative to the gap.

19. Question 19

    1 marksReflection of light
    A plane mirror always produces an image that is the same size as the object and is located as far behind the mirror as the object is in front. The object is 30 cm in front, so the image is 30 cm behind the mirror. The distance between object and image is therefore 30 + 30 = 60 cm.
    Method:

    Image is 30 cm behind mirror. Object is 30 cm in front. Object-to-image distance = 60 cm. Image same size.
    Examiner tips

    • Plane mirror: image distance = object distance. Object-to-image distance = twice the mirror-to-object distance.

20. Question 20

    1 marksTotal internal reflection
    Light travels more slowly in glass than in air because glass is optically denser. This difference in speed is the fundamental reason why total internal reflection can occur: light bends away from the normal when going from glass to air, and above the critical angle it cannot escape. The critical angle is NOT simply proportional to refractive index; in fact the sine of the critical angle equals 1 divided by n. For total internal reflection the angle of incidence must be greater than (not equal to or less than) the critical angle.
    Method:

    Recall that glass is optically denser so light travels slower in glass than in air. This is always true.
    Examiner tips

    • Light slows down in denser media. TIR needs angle of incidence > critical angle.

21. Question 21

    1 marksElectromagnetic spectrum
    All electromagnetic waves, regardless of frequency or wavelength, travel at exactly $3.0 \times 10^{8}$ m/s in a vacuum. Radio waves and gamma rays are both electromagnetic waves, so both travel at $3.0 \times 10^{8}$ m/s in a vacuum.
    Method:

    Recall that all electromagnetic waves travel at the same speed in a vacuum.
    Examiner tips

    • All EM waves have the same speed in a vacuum: $c = 3.0 \times 10^{8}$ m/s.

22. Question 22

    1 marksSound waves and echoes
    The sound travels from the student to the wall and back, covering a total distance of 2d in time t. So 2d = v x t, which gives d = vt / 2. The factor of 2 accounts for the return journey.
    Method:

    Total distance = v x t. Distance to wall = v x t / 2.
    Examiner tips

    • Echo: sound goes out and comes back. Divide the total distance vt by 2 for one-way distance.

23. Question 23

    1 marksLongitudinal waves
    In a longitudinal wave (such as sound), compressions are regions where particles are pushed closer together, giving higher pressure. Rarefactions are regions where particles are spread further apart, giving lower pressure. Compressions and rarefactions are features of longitudinal waves, not transverse waves.
    Method:

    Recall that a compression is a high-pressure, particles-close-together region in a longitudinal wave.
    Examiner tips

    • Compression: close together (high pressure). Rarefaction: far apart (low pressure). Both in longitudinal waves.

24. Question 24

    1 marksMagnetic fields
    By convention, magnetic field lines point from north pole to south pole outside the magnet. A weak magnet has a weak field, so the field lines are spread far apart (widely spaced). A strong magnet would have densely packed field lines near the poles. The direction is always N to S outside.
    Method:

    Direction is always N to S outside. Weak magnet = wide spacing of field lines.
    Examiner tips

    • Field lines: N to S outside; close together = strong; far apart = weak.

25. Question 25

    1 marksResistance and wire dimensions
    Resistance = resistivity x length / area. Cross-sectional area is proportional to diameter squared. Wire 2 has diameter 0.50 mm, which is double that of wire 1 (0.25 mm). Area is proportional to $d^2$, so area of wire 2 is $2^2 = 4$ times the area of wire 1. For the same resistance and same material (same resistivity): $R = \rho L / A$, so $L$ must also be 4 times larger to compensate for the 4 times larger area. Length of wire 2 = 4 x 0.60 = 2.40 m.
    Method:

    Area ratio = $(0.50/0.25)^2 = 4$. Same resistance requires length x4. New length = 0.60 x 4 = 2.40 m.
    Examiner tips

    • Area proportional to $d^2$. If diameter doubles, area quadruples. For same R, length must also quadruple.

26. Question 26

    1 marksElectrical energy and power
    Energy transferred = e.m.f. x charge = e.m.f. x current x time. Time must be in seconds: 5.0 minutes = 300 s. Energy = 6.0 x 2.0 x 300 = 3600 J.
    Method:

    Convert: 5.0 min = 300 s. E = 6.0 V x 2.0 A x 300 s = 3600 J.
    Examiner tips

    • Always convert time to seconds. Energy = e.m.f. x I x t.

27. Question 27

    1 marksE.m.f. and potential difference
    E.m.f. is defined as the energy transferred (supplied) per unit charge that passes through the source. Its unit is joules per coulomb (J/C), which is the same as the volt (V). Charge per unit time is current; total charge is a quantity not a rate; and total energy depends on how much charge flows, not the energy per charge.
    Method:

    Recall the definition: e.m.f. = energy transferred per unit charge by the source.
    Examiner tips

    • e.m.f. = energy per charge (J/C = V). Do not confuse with current (charge/time).

28. Question 28

    1 marksSeries and parallel circuits
    If a switch that belongs to only one of the three parallel lamps is opened, current can still flow through the other two parallel branches, so only that one lamp goes out. If the series lamp's switch or the main switch were opened, all lamps would go out. The switch that puts out exactly one lamp is the one in series with just one of the parallel branches.
    Method:

    Identify that only one lamp going out means only one branch is broken: a switch in series with a single parallel-branch lamp.
    Examiner tips

    • Opening a switch in a parallel branch only breaks that branch. Opening a series switch breaks the whole circuit.

29. Question 29

    1 marksElectrical safety
    Both the fuse and the switch must be placed in the live wire. This ensures that when the switch is open or the fuse blows, the appliance is completely isolated from the high-voltage live supply. If placed in the neutral wire the casing could still be at live potential and be dangerous to touch.
    Method:

    Recall that both fuse and switch interrupt the live supply and must be in the live wire.
    Examiner tips

    • Fuse and switch in the live wire. Earth wire is a safety ground, not a current path for the switch/fuse.

30. Question 30

    1 marksElectromagnetic induction and force on a conductor
    Applying Fleming's left-hand rule: the First finger points in the field direction (back to front = towards viewer), the seCond finger points in the current direction (left to right), and the thuMb gives the force direction. With these orientations the force acts upwards.
    Method:

    Apply Fleming's left-hand rule with field into page and current left to right to get force upwards.
    Examiner tips

    • Fleming's LEFT-hand rule for motors/forces: First finger = Field, seCond = Current, thuMb = Motion.

31. Question 31

    1 marksA.c. generator
    The induced e.m.f. is proportional to the rate at which flux is cut. When the coil is horizontal (parallel to the field) the coil sides are cutting through the field lines at the greatest rate, giving maximum e.m.f. When the coil is vertical (perpendicular to the field, as at t = 0) the coil sides move parallel to the field lines, so the rate of flux cutting is zero and the e.m.f. is zero.
    Method:

    When the coil is horizontal the sides move perpendicularly through the field: maximum cutting rate = maximum e.m.f.
    Examiner tips

    • Coil perpendicular to field (vertical) = zero e.m.f. Coil parallel to field (horizontal) = maximum e.m.f.

32. Question 32

    1 marksD.c. motor
    As the coil rotates it passes through the position where the forces would reverse direction if the current stayed the same. The split-ring commutator reverses the direction of current in the coil every half rotation, so the forces on the coil sides always act in the same rotational direction. This keeps the turning effect (torque) in the same direction and the motor continues to spin consistently.
    Method:

    The commutator reverses current in the coil every half rotation so the turning effect (torque) always acts in the same direction.
    Examiner tips

    • Commutator swaps current direction every half rotation to keep rotation continuous in one direction.

33. Question 33

    1 marksAlpha, beta and gamma emission
    Kinetic energy = 0.5 x m x v^2. Although beta particles travel at about 20 times the speed of alpha particles, alpha particles are much more massive (about 8000 times the mass of a beta particle). The combined effect gives alpha particles significantly more kinetic energy. More kinetic energy, larger charge (+2 vs -1 or +1) and larger size make alpha particles far more ionising. However, alpha particles are less penetrating because they lose energy rapidly through dense ionisation.
    Method:

    KE of alpha is greater (large mass). Greater KE and charge make alpha more ionising but less penetrating.
    Examiner tips

    • Alpha: more ionising, less penetrating. Beta: less ionising, more penetrating. Large mass of alpha gives it more KE despite slower speed.

34. Question 34

    1 marksAtomic structure and nuclear model
    Rutherford's gold foil experiment showed that most alpha particles passed straight through, a small number were deflected at large angles, and a very few bounced almost straight back. This proved that most of the atom is empty space and nearly all the positive charge and mass is concentrated in a tiny, dense nucleus. This is the nuclear model of the atom.
    Method:

    Identify Rutherford's alpha-particle scattering as direct evidence for the nuclear (planetary) model of the atom.
    Examiner tips

    • Rutherford's experiment: alpha scattering proves most mass/charge concentrated in a tiny nucleus.

35. Question 35

    1 marksRadiation safety
    Ionising radiation damages cells by ionising molecules in the cell. This can cause cancer (uncontrolled cell division), cell death, and mutation (changes to DNA). Infection by bacteria is caused by pathogenic microorganisms entering the body, which is completely separate from the effects of ionising radiation.
    Method:

    Recall the three harmful effects of radiation (cancer, cell death, mutation). Infection is caused by microorganisms, not radiation.
    Examiner tips

    • Radiation effects: cancer, mutation, cell death. Infection is a biological process unrelated to radiation.

36. Question 36

    1 marksRadioactive decay
    A radioactive nucleus is, by definition, one that is unstable and will spontaneously decay. Radioactive decay can involve emission of alpha particles, beta particles, gamma rays, or a combination of these. No single type of emission is guaranteed for all radioactive isotopes; the only guaranteed statement is that the nucleus is unstable.
    Method:

    Recall definition: radioactive isotope has an unstable nucleus. This is the only universal statement.
    Examiner tips

    • Radioactive = unstable nucleus. The type of radiation emitted varies; the instability does not.

37. Question 37

    1 marksHalf-life
    After each half-life the corrected count rate halves. After 10 minutes (1 half-life): 960 / 2 = 480 counts per minute. After 20 minutes (2 half-lives): 480 / 2 = 240 counts per minute.
    Method:

    20 min / 10 min = 2 half-lives. Count rate after 2 half-lives = 960 x (1/2)^2 = 240 counts/min.
    Examiner tips

    • Count half-lives in the given time. Divide initial count rate by 2 for each half-life.

38. Question 38

    1 marksEarth, Moon and day and night
    The Earth rotates from west to east (anticlockwise when viewed from above the North Pole). This makes the Sun appear to move in the opposite direction across the sky, rising in the east and setting in the west, so the apparent motion is east to west.
    Method:

    Earth rotates west to east, making the Sun appear to travel east to west across the sky.
    Examiner tips

    • Earth spins west to east, so the Sun appears to rise in the east and set in the west: east-to-west apparent motion.

39. Question 39

    1 marksHubble constant and expanding universe
    The Hubble law states that v = H0 x d, so d = v / H0. Substituting: d = (4.4 x 10^6) / (2.2 x 10^{-18}) = 2.0 x 10^{24} m.
    Method:

    Rearrange Hubble's law: d = v / H0 = (4.4 x 10^6) / (2.2 x 10^{-18}) = 2 x 10^{24} m.
    Examiner tips

    • Hubble law: v = H0 x d, so d = v / H0. Take care with powers of 10 when dividing.

40. Question 40

    1 marksSolar system and orbits
    Gravitational field strength decreases with distance from the Sun, so closer to the Sun the field is stronger. A stronger gravitational field provides more centripetal force, requiring a higher orbital speed to stay in a circular orbit at a smaller radius. Planets closer to the Sun orbit faster (Kepler's third law). Both the gravitational field strength and the orbital speed are greater for planet X than for Earth.
    Method:

    Gravitational field strength decreases with distance, so closer in = stronger. Stronger gravity at smaller radius requires higher orbital speed.
    Examiner tips

    • Closer to Sun: stronger gravity, faster orbit. Use inverse square law for gravity and Kepler's law for speed.