S3 Physics I 2025 Past Paper: Full Worked Solutions (Rwanda O-Level)

The S3 Physics Paper I 2025 national examination was sat on 11th July 2025 under Rwanda's Ordinary Level (O-Level) programme, supervised by NESA (National Examination and School Inspection Authority). The paper, coded 011, carried 100 marks divided across three sections and tested students on forces, energy, electricity, pressure, motion, electromagnetism, thermodynamics, optics, and practical skills.

This article provides complete, step-by-step worked solutions for every question Sections A, B, and C  to help S3 students revise effectively for retakes and future examinations. If you are also preparing for your national exams this year, see our Rwanda National Exams 2026 Study Guide and Timetable for revision tips and scheduling advice.

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Section A: Attempt All Questions (55 Marks)

1) Multiple Choice Questions on Forces

• a) Friction force is a contact force.
  Explanation: Contact forces require direct, physical interaction between intersecting material boundaries. Gravitational, nuclear, and magnetic forces function across open distances via fields.
  Correct Choice: i) Friction force
• b) Magnetic force is a force of attraction between a magnet and a magnetic material.
  Correct Choice: ii) magnet
• c) Electric force is a non-contact force.
  Explanation: Upthrust, friction, and air resistance demand immediate macroscopic contact interfaces, whereas electric forces alter the spatial properties across a vacuum field.
  Correct Choice: iv) Electric force
• d) A push or a pull in a given direction that can change the motion of an object is called a force.
  Correct Choice: iii) force

2) Sentence Completion (Energy & Power)

• a) Kinetic energy of an object is the form of energy that it possesses due to its motion.
• b) Two forms of energy produced when wood burns are light energy and heat.
• c) Power is defined as the rate at which work is done.
• d) Solar energy is radiation from the Sun that is capable of generating electricity.

3) Matching Simple Machines

• a) Knife → iii) It is an example of a wedge
  Explanation: A wedge pairs two inclined planes back-to-back to translate an axial force vector laterally for splitting or separating materials.
• b) Lever → iv) It is a bar that pivots around a fulcrum used for lifting or moving heavy things.
• c) Pulley → ii) It is a type of simple machine that uses a wheel and rope to lift heavy loads.
• d) Ladder → i) It is an example of an inclined plane

4) Electricity Principles

• a) Which device is based on the heating effect of electric current?
  Correct Choice: ii) Electric cooker
  Explanation: High-resistance heating elements intentionally convert bulk kinetic electric paths into localized thermal energy via Joule dissipation ($P=I^{2}R$).
• b) What is an electric generator?
  Correct Choice: i) A device that generates electricity
• c) If $I$, $R$, and $V$ represent electric current, resistance, and voltage, the formula for electric power ($P$) is:
  Correct Choice: iv) VI
  Explanation: Power is defined as work completed per unit time. Mathematically, this evaluates to: $$P=V\times I$$

5) Electrical Systems and Tools

• a) Which metal is most commonly used to make electrical wires?
  Correct Choice: i) Copper
• b) What is the purpose of insulating electrical tools like screwdrivers?
  Correct Choice: i) To prevent user from getting electrocuted
• c) Which serves as protection against overload or short-circuit in a house electrical circuit?
  Correct Choice: iii) Circuit breaker
• d) What does the symbol represent (Figure 1)?
  Correct Choice: ii) Electric Fuse

6) Voltage Regulator Circuit Components (Figure 2)

• Q6: Bipolar Junction Transistor (Pass/Series BJT)
• D5: Zener Diode (Indicated by bent cathode lines, utilized for maintaining a stable reverse-bias reference voltage)
• D6: Standard Rectifier Diode
• R9: Fixed Current-Limiting Resistor

7) True or False (Fluid Mechanics)

• a) Hydraulic press works on Archimedes' principle. → False
  Explanation: It operates on Pascal's Principle of uniform fluid pressure transmission.
• b) The buoyant force is given by $\rho \times g\times V$. → True
  Explanation: Upthrust is always equal to the net weight vector of the displaced fluid.
• c) A block of iron dropped in water sinks because the weight of the water displaced is less than the weight of the block. → True
• d) SI unit of atmospheric pressure is newton (N). → False
  Explanation: The absolute SI base measurement is the Pascal ($\text{Pa}={\text{N/m}}^2$).

8) Fluid Pressure Properties

• a) Pressure in a fluid at rest depends on:
  Correct Choice: iii) density of the liquid
  Explanation: Modeled by the fundamental hydrostatic equation: $$P=\rho gh$$
• b) If the density of an object is less than water, it will:
  Correct Choice: iv) The object floats
• c) Three containers filled with water to the same level (Figure 3):
  Correct Choice: iv) P_1 = P_2 = P_3
  Explanation: Pressure is purely a function of depth ($h$) and fluid density ($\rho$). It remains entirely independent of container width, shape, or aggregate volume.
• d) The pressure exerted by a person on Earth is minimum when he/she:
  Correct Choice: ii) sleeps on the ground.
  Explanation: Pressure drops as the physical area of force distribution increases: $$P=\frac{F}{A}$$ Lying down flat maximizes the denominator surface area variable $A$, which minimizes total downward mechanical pressure.

9) Units, Dimensions, and Measurements

• a) If $v=at$, then $a=\frac{v}{t}$. Dimensional formula:
  Correct Choice: ii) [LT^{-2}]
  Explanation: Evaluating the dimensions yields: $$\frac{[L{T}^{-1}]}{[T]}=[L{T}^{-2}]$$
• b) An experimental value very close to the true value is called:
  Correct Choice: i) accurate
• c) How many significant figures are in 406.060?
  Correct Choice: iv) 6

10) Electromagnetic Induction

• a) In $ϵ=-\frac{\mathrm{\Delta }\varphi }{\mathrm{\Delta }t}$, the negative sign signifies that:
  Correct Choice: ii) the induced emf opposes the change in the magnetic flux. (Lenz's Law)
• b) Increasing the number of turns in a coil subjected to varying flux causes the emf to:
  Correct Choice: i) Induced emf increases
• c) The electric current induced in a coil while accelerating a magnet inside it:
  Correct Choice: iv) increases

11) Transformers

• a) Type of transformer in Figure 4:
  Answer: Step-down transformer (The internal input coil count exceeds downstream secondary output turns: $N_p>N_s$).
• b) Why real transformer efficiency is $<100\mathrm{\%}$:
  Correct Choice: i) Because some energy is lost as heat in the coils and core.
• c) Why a step-up transformer is used before long-distance transmission:
  Correct Choice: iv) It increases the voltage to reduce energy loss in the cables.
  Explanation: Raising the potential drops transmission line current values ($I$) significantly. This suppresses line energy waste caused by resistive Joule heating: $$P_{\text{loss}}=I^{2}R$$

12) Kinematics Graph Interpretation (Figure 5)

• a) Acceleration from O to A:
  $$a=\frac{\mathrm{\Delta }v}{\mathrm{\Delta }t}=\frac{6\text{ m/s}-0\text{ m/s}}{4\text{ s}-0\text{ s}}=1.5{\text{ m/s}}^2$$ Correct Choice: iv) 1.5 m/s^2
• b) Distance travelled from A to B (from $t=4\text{ s}$ to $t=10\text{ s}$):
  $$\text{Distance}=\text{Area under region }AB=\text{velocity}\times \mathrm{\Delta }t$$ $$\text{Distance}=6\text{ m/s}\times (10\text{ s}-4\text{ s})=6\times 6=36\text{ m}$$ Correct Choice: ii) 36 m

13) Momentum and Impulse Calculation

Given parameters: $m=10\text{ kg}$, $u=4\text{ m/s}$, $v=8\text{ m/s}$, $\mathrm{\Delta }t=0.2\text{ s}$.

• a) Initial momentum:
  $$P_1=m\times u=10\text{ kg}\times 4\text{ m/s}=40\text{ kg}\cdot \text{m/s}$$ Correct Choice: ii) 40 kgm/s
• b) Final momentum:
  $$P_2=m\times v=10\text{ kg}\times 8\text{ m/s}=80\text{ kg}\cdot \text{m/s}$$ Correct Choice: iii) 80 kgm/s
• c) Change in momentum ($\mathrm{\Delta }P$):
  $$\mathrm{\Delta }P=P_2-P_1=80-40=40\text{ kg}\cdot \text{m/s}$$ Correct Choice: iii) 40 kgm/s
• d) Impulse acting on the object:
  Explanation: By the Impulse-Momentum Theorem, total active net impulse corresponds exactly with total change in linear momentum: $$J=\mathrm{\Delta }P=40\text{ N}\cdot \text{s}$$ Correct Choice: iii) 40 Ns

14) System Mechanical Energy Conservation

Answer: The total mechanical energy will decrease.

Explanation: When non-conservative resistive interactions like air friction are present, they actively perform negative work against the translational motion of the system. This extracts energy out of the mechanical frame and transforms it into dispersed heat and structural sound vibrations.

15) Environmental Factors Affecting Plant Growth

1. Sunlight Intensity: Provides the fundamental electromagnetic wave photon packages needed to power chlorophyll structures during photosynthesis, synthesizing glucose molecules.
2. Water Availability: Serves as the critical transport fluid to dissolve and lift soil minerals through vascular tissue, while stabilizing cellular structural turgidity.

Section B: Attempt Any Three Questions (30 Marks)

16) Thermodynamics

• a) Phase matching:
  • i) Solid has definite shape and volume.
  • ii) Liquid has definite volume and takes the shape of its container.
  • iii) Gas does not have definite shape and volume.
• b) True or False:
  • i) True (Spontaneous heat transfer requires an external energy input or structural work loop).
  • ii) False (Final internal thermodynamic mixtures stabilize purely based on individual specific heat coefficients, not a simple geometric numerical average).
  • iii) False (Phase conversions pull heat energy completely at uniform structural temperature flatlines).
• c) First Law Analysis:
  Given parameters: Added heat $Q=+523\text{ J}$, expansion work done by system $W=+781\text{ J}$.
  • i) Using $\mathrm{\Delta }U=Q-W$: $$\mathrm{\Delta }U=523\text{ J}-781\text{ J}=-258\text{ J}$$
  • ii) The internal energy decreases because the expansion mechanical energy output spent by the gas exceeded the thermal influx energy supplied to the system.

17) Optics (Concave Mirror)

Given parameters: Object profile height $h=3\text{ cm}$, distance $u=15\text{ cm}$, focal point length $f=6\text{ cm}$.

• a) True (The universal law of reflection applies perfectly across spherical geometric curved boundaries).
• b) Geometric tracking principles:
  • i) Radius of curvature: $r=2f=2\times 6\text{ cm}=12\text{ cm}$.
  • ii) ...will reflect parallel to the principal axis.
  • iii) ...passing through the centre of curvature is reflected back along its original incident trajectory line.
• c) Image Position ($v$): $$\frac{1}{f}=\frac{1}{u}+\frac{1}{v}⟹\frac{1}{6}=\frac{1}{15}+\frac{1}{v}$$ $$\frac{1}{v}=\frac{1}{6}-\frac{1}{15}=\frac{5-2}{30}=\frac{3}{30}⟹v=10\text{ cm}$$
• d) Image Size ($h^{\prime }$): $$\text{Magnification ratio }(m)=-\frac{v}{u}=-\frac{10}{15}=-0.67$$ $$\text{Size }(h^{\prime })=|m|\times h=\frac{2}{3}\times 3\text{ cm}=2\text{ cm}$$
• e) Since the distance parameter $v$ resolves as a positive metric while magnification scales inverted, the identity aligns with choice iv) Real and inverted.

18) Parallel Electrical Circuits (Figure 7)

Given parameters: Loop resistor $R_1=3.0\mathrm{\Omega }$, supply rail voltage $V=12\text{ V}$, total input current line ammeter reading $I_{\text{total}}=5.0\text{ A}$.

• a) i) Linked completely in a parallel orientation. ii) Potential difference $V_{ab}=12\text{ V}$. iii) A standard voltmeter scans potential drops.
• b) Calculations:
  • i) Line current passing past ammeter indicator $A_1$: $$I_1=\frac{V}{R_1}=\frac{12\text{ V}}{3.0\mathrm{\Omega }}=4.0\text{ A}$$
  • ii) Current tracing across hidden parallel component resistor $R$: $$I_2=I_{\text{total}}-I_1=5.0\text{ A}-4.0\text{ A}=1.0\text{ A}$$
  • iii) Electrical resistance value of component $R$: $$R=\frac{V}{I_2}=\frac{12\text{ V}}{1.0\text{ A}}=12\mathrm{\Omega }$$
  • iv) Combined system network equivalent resistance value ($R_{\text{eq}}$): $$R_{\text{eq}}=\frac{V}{I_{\text{total}}}=\frac{12\text{ V}}{5.0\text{ A}}=2.4\mathrm{\Omega }$$

19) Thermal Physics & Calorimetry

• a) i) increases. ii) $\gamma =3\alpha$. iii) Conduction. Solid objects hold rigidly bound atomic lattice matrices, which optimizes kinetic vibrational transmission straight through neighboring nodes without shifting net macroscopic material mass boundaries.
• b) Calorimetry Problems:
  Given parameters: $m_c=0.050\text{ kg}$, $c_c=420{\text{ J/kg}}^{\circ }\text{C}$, $m_w=0.0507\text{ kg}$, $c_w=4200{\text{ J/kg}}^{\circ }\text{C}$, steady temperature step change $\mathrm{\Delta }T=22-20=2^{\circ }\text{C}$, metal sample mass $m_m=0.020\text{ kg}$ tracking downward from ${100}^{\circ }\text{C}$.
  • i) Energy pulled completely into the fluid volume: $$Q_w=m_w{c}_w\mathrm{\Delta }T=0.0507\times 4200\times 2=425.88\text{ J}$$
  • ii) Thermal energy absorbed by the housing vessel container: $$Q_c=m_c{c}_c\mathrm{\Delta }T=0.050\times 420\times 2=42\text{ J}$$
  • iii) Cumulative thermal energy shed by the hot insert sample block: $$Q_{\text{lost}}=Q_w+Q_c=425.88+42=467.88\text{ J}$$
  • iv) Resolving target metal specific heat capacity ($c_m$): $$Q_{\text{lost}}=m_m\times c_m\times (T_{\text{initial}}-T_{\text{final}})$$ $$467.88=0.020\times c_m\times (100-22)$$ $$467.88=1.56\times c_m⟹c_m=\frac{467.88}{1.56}=300{\text{ J/kg}}^{\circ }\text{C}$$

Section C: Compulsory Laboratory Practice (15 Marks)

21) Block Dimensions & Density Estimation

• a) Geometry Evaluations:
  • i) Physical measurements: Length $l=5.0\text{ cm}$, Width $w=2.0\text{ cm}$, Height $h=2.0\text{ cm}$.
  • ii) Calculated solid block volume parameter ($V_A$): $$V_A=l\times w\times h=5.0\times 2.0\times 2.0=20.0{\text{ cm}}^3$$
• b) Critique: A manual calculated geometric model remains a baseline approximation because hand-crafted clay masses possess edge irregularities and surface variations that depart from perfectly flat ${90}^{\circ }$ cuboids. To improve accuracy, collect several caliper dimensional checks across separate coordinates and use their direct mathematical mean.
• c) Method 1 Processing:
  • i) Mass value output check: $m_A=46.0\text{ g}$.
  • ii) Derived physical block density tracking (${\rho }_A$): $${\rho }_A=\frac{m_A}{V_A}=\frac{46.0\text{ g}}{20.0{\text{ cm}}^3}=2.3{\text{ g/cm}}^3$$
• d) Force Indicator Readout: Total baseline weight parameter: $W_B=0.25\text{ N}$.
• e) Fluid Starting Line: Measuring cylinder initial marking: $V_1=30.0{\text{ cm}}^3$.
• f) Submersion Tracking & Volume Displacements:
  • i) Submerged fluid rising target line check: $V_2=41.0{\text{ cm}}^3$.
  • ii) Evaluating alternate density metric (${\rho }_B$) scaling alongside configuration variable $k=100\text{ g/N}$: $${\rho }_B=\frac{k\times W_B}{V_2-V_1}=\frac{100\times 0.25}{41.0-30.0}=\frac{25}{11}\approx 2.27{\text{ g/cm}}^3$$
  • iii) ${\rho }_B$ is more accurate. The fluid displacement method contours immediately and uniformly around arbitrary surface defects or subtle structural depressions on the clay block, tracking the true three-dimensional volume perfectly.
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    Frequently Asked Questions

What topics are examined in S3 Physics Paper I 2025 Rwanda?
The paper covers forces, energy, electricity, pressure and fluids, motion (kinematics), electromagnetic induction, transformers, thermodynamics, optics (concave mirrors), electrostatics (Coulomb's law), and practical skills such as density determination. These all follow the REB O-Level Physics syllabus for Senior Three.

How many marks is the S3 Physics I 2025 paper out of?
The paper is out of 100 marks: Section A (compulsory, 55 marks), Section B (any three questions, 30 marks), and Section C (compulsory practical, 15 marks).

Where can I find more S3 past papers and resources?
Visit our Resources section for solved past papers across all O-Level and A-Level subjects including Mathematics, Chemistry, Biology, and Computer Science. You can also explore our study effectiveness guide for proven revision strategies.

Can I get a private tutor to help me with Physics in Rwanda?
Yes. Mathrone Academy offers qualified private tutors for Physics, Mathematics, and Sciences both online and at home in Kigali. Book a tutor here or reach us via WhatsApp +250786684285.

⚠️ Disclaimer

The solutions on this page are prepared by the Mathrone Academy team for revision and learning purposes only. This is not an official NESA marking scheme or REB-approved answer guide. While every effort has been made to ensure accuracy, answers and explanations may differ from the official examiners' marking guide. Always refer to your school teacher or the official NESA publications for authoritative marking guidance. Mathrone Academy accepts no responsibility for any discrepancies between these solutions and official results.

 

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