
The S6 Physics II 2024 National Examination, sat on 24 July 2024 under NESA, covers an enormous range of Advanced Level Physics: lasers, optics, electric fields, nuclear physics, particle physics, circuits, thermodynamics, projectile motion, satellite orbits, and damped oscillations. This paper is set for the PCM, PCB, MPG, and MPC combinations. Below are fully worked solutions to every question, with formulas and step-by-step calculations shown throughout.
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The paper has two sections. Section A is compulsory and worth 70 marks, covering multiple choice and short-answer questions across optics, electric fields, waves, nuclear and particle physics, mechanics, and communications. Section B requires candidates to attempt only 3 of 5 longer questions worth 30 marks, covering medical imaging, Kirchhoff's laws, thermodynamics, damped oscillations, and radioactive decay. Students are given 3 hours and access to the following constants: Earth's mass \(M = 6.0 \times 10^{24}\) kg, Earth's mean radius \(6\,400\) km, speed of light \(c = 3\times10^8\) m/s, gravitational constant \(G = 6.67\times10^{-11}\) N·m²/kg², \(g = 9.81\) m/s², and gas constant \(R = 8.314\) J/K·mol.
a) Properties of LASER light: Answer: ii) Directionality and monochromaticity. LASER light travels in a highly directional beam and consists of a single wavelength (monochromatic), unlike ordinary light sources.
b) Why LASER light is coherent: Answer: i) The same wavelength and maintain constant phase difference. Coherence specifically refers to waves maintaining a fixed phase relationship over time, not merely sharing amplitude or frequency.
c) Figure 1 interpretation: Answer: iii) Stimulated emission. The diagram shows an incoming photon triggering an atom to drop from a higher energy level, releasing a second identical photon — the defining mechanism of stimulated emission.
d) Condition for population inversion: Answer: ii) N₁ < N₂. Population inversion occurs when more atoms occupy the higher energy state (E₂) than the lower state (E₁), the opposite of normal thermal equilibrium.
e) LASER acronym: Answer: iv) Light Amplification by Stimulated Emission of Radiation.
a) Answer: i) Virtual, erect, and magnified. A simple microscope (magnifying glass) always produces a virtual, upright, magnified image.
b) Answer: iii) Myopia/Nearsightedness. A concave (diverging) lens corrects myopia by diverging light rays before they enter the eye, compensating for the eye focusing images in front of the retina.
c) Answer: ii) Compound microscope. A compound microscope provides the high magnification needed to observe microscopic structures like plant cells.
d) Answer: ii) Greater than that of the eyepiece. In an astronomical telescope, the objective lens has a longer focal length than the eyepiece to achieve angular magnification.
a) Answer: ii) Solar energy–geothermal energy. Both are renewable resources, unlike the other options which each pair a renewable with a non-renewable source.
b) Answer: i) It is not reliable in all weather conditions. Solar output depends heavily on sunlight availability, making it inconsistent during cloudy periods or at night.
c) Answer: ii) Coal. Burning coal releases large quantities of CO₂ and other greenhouse gases, unlike hydroelectric, solar, or geothermal energy.
d) Answer: i) Oil. Oil is a fossil fuel formed from ancient organic matter; wood is a biomass fuel, natural uranium is a nuclear fuel, and hydrogen is not a fossil fuel.
a) Electrical potential energy between two charges: \(U = k\dfrac{Q_1 Q_2}{r}\)
b) The direction of an electric field is the direction of force on a positive test charge.
c) The trajectory of an electron in an electric field is a parabola.
d) Acceleration of an electron from rest in an electron gun is given by \(a = \dfrac{|e|E}{m}\)
e) Electric potential due to a dipole is directly proportional to the electric dipole moment.
a) The velocity of sound in a gas is directly proportional to the square root of its absolute temperature. Answer: True.
b) The speed of a wave on a guitar string is inversely proportional to the square root of the tension. Answer: False. Wave speed on a string is directly proportional to the square root of tension: \(v = \sqrt{T/\mu}\).
c) As a car's horn moves away from you, the frequency increases. Answer: False. This is the Doppler effect — frequency decreases as a source moves away from a stationary observer.
d) The pitch of a sound wave depends on its frequency. Answer: True.
e) The intensity of sound is measured in W/m². Answer: True.
Answer:
a) Quarks → ii) Elementary particles that are the building blocks for protons and neutrons
b) Gluons → iv) Elementary particles that create the strong nuclear force between quarks
c) Fermions → i) Matter particles
d) Bosons → iii) Force carrying particles
e) Hadrons → v) They are made of quarks held together by the strong force
a) Matching parts to functions:
A) Control rod → ii) Controls the rate of fission
B) Coolant → i) Removes energy from the reactor
C) Fuel rod → iv) Contains uranium for fission
D) Shielding → iii) Absorbs dangerous radiation
b) Energy change in a nuclear power plant: Answer: i) Nuclear energy → Heat energy → Mechanical energy → Electrical energy. Fission releases nuclear energy as heat, which produces steam to drive turbines (mechanical energy), which then drives generators to produce electricity.
a) A couple of forces consists of two equal and opposite parallel forces acting on a body but not along the same line of action, producing a turning effect (torque) without any resultant linear force. The perpendicular distance between the two forces is called the arm of the couple, and the resulting torque equals one force multiplied by this perpendicular distance.
b) i) The free body diagram for the strut system shows three forces acting on the strut: the weight of the strut (Mg) acting at its center, the weight hanging from the end (mg), the tension in the cable (T) acting at 30° to the horizontal, and the reaction force at the hinge.
ii) Yes, the system is in equilibrium — it is a static structure in which the cable tension, the hinge reaction, and the weights balance so that both the net force and net moment about the hinge are zero, which is the defining condition of static equilibrium for a rigid body at rest.
a) i) Redshift is the increase in wavelength (shift toward the red end of the spectrum) of light from an object that is moving away from the observer, caused by the Doppler effect.
ii) Blueshift is the decrease in wavelength (shift toward the blue end of the spectrum) of light from an object moving toward the observer.
b) These measurements tell us the relative motion of a galaxy with respect to Earth: a redshifted galaxy is moving away from us, while a blueshifted galaxy is moving toward us. Hubble found that most distant galaxies show redshift, and that the amount of redshift increases with distance, indicating the universe is expanding.
c) Since redshift indicates the rate at which galaxies are receding, and this recession rate is proportional to distance (Hubble's Law), the current rate of expansion can be used to calculate how long ago all matter in the universe was concentrated at a single point — giving an estimate of the age of the universe (roughly the inverse of the Hubble constant).
a) Answer: iii) Frequency re-use. Dividing coverage into cells allows the same frequencies to be reused in non-adjacent cells, dramatically increasing network capacity.
b) Answer: i) To minimize interference. Assigning different frequencies to neighboring cells prevents signal overlap and interference between adjacent coverage areas.
c) Answer: iii) Antenna. An antenna converts electrical signals into electromagnetic waves for wireless transmission.
d) Answer: iv) Handoff. Handoff is the process that transfers an ongoing call from one cell tower to another as a user moves, without dropping the connection.
e) Answer: iii) Remains constant, varies. In Amplitude Modulation (AM), the carrier's amplitude varies to encode the signal, while its frequency and phase remain constant.
A glass prism with refracting angle 60° undergoes a minimum deviation of 30°.
a) Refractive index: Using the minimum deviation formula for a prism:
$$n = \frac{\sin\left(\dfrac{A+D_{min}}{2}\right)}{\sin\left(\dfrac{A}{2}\right)} = \frac{\sin\left(\dfrac{60°+30°}{2}\right)}{\sin\left(\dfrac{60°}{2}\right)} = \frac{\sin 45°}{\sin 30°}$$ $$n = \frac{0.7071}{0.5} \approx 1.41 \ (= \sqrt{2})$$b) Critical angle: Using \(\sin C = \dfrac{1}{n}\):
$$C = \sin^{-1}\left(\frac{1}{\sqrt{2}}\right) = 45°$$c) Velocity of light in glass: Using \(n = \dfrac{c}{v}\):
$$v = \frac{c}{n} = \frac{3\times10^8}{\sqrt{2}} \approx 2.12\times10^{8} \text{ m/s}$$A football is kicked at 37.0° to the horizontal with an initial velocity of 20.0 m/s.
a) Maximum height: The vertical component of velocity is \(v_y = v_0\sin\theta = 20\sin(37°) \approx 12.04\) m/s. Using \(H = \dfrac{v_y^2}{2g}\):
$$H = \frac{(12.04)^2}{2(9.81)} \approx 7.38 \text{ m}$$b) Range: Using \(R = \dfrac{v_0^2\sin(2\theta)}{g}\):
$$R = \frac{(20)^2 \sin(74°)}{9.81} \approx 39.2 \text{ m}$$a) i) Height of the satellite: The signal travels at the speed of light and is received 68 ms after transmission, so the height equals distance = speed × time:
$$h = ct = (3\times10^8)(68\times10^{-3}) = 2.04\times10^{7} \text{ m}$$ii) Gravitational field strength at the satellite: The orbital radius from Earth's center is \(r = R_e + h = 6.4\times10^6 + 2.04\times10^7 = 2.68\times10^7\) m. Using \(g = \dfrac{GM}{r^2}\):
$$g = \frac{(6.67\times10^{-11})(6.0\times10^{24})}{(2.68\times10^7)^2} \approx 0.56 \text{ N kg}^{-1}$$which confirms the given value.
b) Orbital speed: For a satellite in circular orbit, gravitational force provides the centripetal force, giving \(v = \sqrt{\dfrac{GM}{r}}\):
$$v = \sqrt{\frac{(6.67\times10^{-11})(6.0\times10^{24})}{2.68\times10^7}} \approx 3864 \text{ m/s} \approx 3.86 \text{ km/s}$$Four main decisions likely included in Rwanda's flood and landslide contingency plan:
Answer: This is a discuss/justify question with no single correct answer, but a strong response should acknowledge nuance rather than agreeing unconditionally. Digital technology generally offers superior noise immunity, easier signal processing, more efficient compression, and better long-distance transmission quality than analog technology, since digital signals can be regenerated without accumulating noise. However, digital systems are not universally "better" — analog signals can represent continuous, infinitely graded information without quantization error, some analog systems have lower latency for certain applications, and digital systems require analog-to-digital conversion at some stage since the physical world is fundamentally analog. A well-argued answer should conclude that digital technology is generally superior for modern communication and storage due to noise resistance and processing flexibility, while noting that analog systems still have specific niche advantages.
Physics concepts and advantages of medical imaging techniques:
Modern medical imaging relies on several distinct physics principles. X-ray imaging uses the differential absorption of X-ray photons by tissues of different density (bone absorbs more than soft tissue), producing a shadow image on a detector. Computed Tomography (CT) extends this principle by rotating an X-ray source around the body and using computer reconstruction algorithms to build detailed cross-sectional images. Ultrasound imaging uses high-frequency sound waves and the physics of reflection: sound waves partially reflect at boundaries between tissues of different acoustic impedance, and the time delay of the returning echo is used to construct an image, making it safe for prenatal scanning since no ionizing radiation is involved. Magnetic Resonance Imaging (MRI) exploits nuclear magnetic resonance: a strong magnetic field aligns hydrogen nuclei in the body, and radiofrequency pulses cause these nuclei to emit detectable signals as they relax back to alignment, producing detailed soft-tissue images without ionizing radiation. Nuclear medicine imaging (such as PET scans) uses radioactive tracers that emit gamma radiation or positrons, which are detected externally to map metabolic activity within the body, similar in principle to the technetium-99 tracer discussed later in this paper.
The advantages of these techniques include earlier and more accurate diagnosis of disease, non-invasive visualization of internal structures without exploratory surgery, and — for ultrasound and MRI in particular — imaging without exposing patients to ionizing radiation. Together, these technologies have transformed patient outcomes by allowing doctors to detect tumors, fractures, blood clots, and organ abnormalities with a precision that was impossible before these physics principles were applied to healthcare.
a) Battery with internal resistance and variable resistor (Figure 5 and 6):
From the straight-line graph of V against I, the equation of the line has the form \(V = E - Ir\), where the y-intercept gives the EMF and the negative gradient gives the internal resistance.
i) Internal resistance: Reading the graph, the line passes through \((0, 2.8\text{ V})\) and \((1.0\text{ A}, 1.4\text{ V})\):
$$r = \frac{E-V}{I} = \frac{2.8-1.4}{1.0} = 1.4\ \Omega$$with EMF \(E = 2.8\) V (the y-intercept).
ii) Resistance R when V = 2.1 V: Reading directly from the graph at \(V=2.1\)V gives \(I = 0.5\)A. Since \(V\) is the terminal voltage across the variable resistor, \(R = V/I\):
$$R = \frac{2.1}{0.5} = 4.2\ \Omega$$b) Three-branch circuit with two 20V batteries and a 6V battery (Figure 7):
Label the three branch currents \(I_1\) (left branch, through a–f), \(I_2\) (middle branch, through b–g–e), and \(I_3\) (right branch, through c–d), all taken as flowing downward from the top rail to the bottom rail.
Junction rule at node b:
$$I_1 + I_3 = I_2 \implies I_1 - I_2 + I_3 = 0 \quad \text{(Equation 1)}$$Loop rule, left loop (f–a–b–e–f): the left branch carries a total resistance of \(0.5+0.5+1 = 2\ \Omega\), and the middle branch carries \(0.5+0.5 = 1\ \Omega\). With the two 20V EMFs positioned so they oppose each other around this loop:
$$20 - 2I_1 - I_2 - 20 = 0 \implies 2I_1 + I_2 = 0 \implies I_2 = -2I_1 \quad \text{(Equation 2)}$$Loop rule, right loop (e–b–c–d–e): the right branch carries a total resistance of \(3+1 = 4\ \Omega\):
$$20 + I_2 + 4I_3 - 6 = 0 \implies 14 + I_2 + 4I_3 = 0 \quad \text{(Equation 3)}$$Solving the system: substituting \(I_2=-2I_1\) into Equation 1:
$$I_1 - (-2I_1) + I_3 = 0 \implies 3I_1 + I_3 = 0 \implies I_3 = -3I_1$$Substituting both into Equation 3:
$$14 + (-2I_1) + 4(-3I_1) = 0 \implies 14 - 14I_1 = 0 \implies I_1 = 1.0\text{ A}$$giving \(I_2 = -2(1.0) = -2.0\)A and \(I_3 = -3(1.0) = -3.0\)A.
Answer: \(I_1 = 1.0\)A (flowing in the assumed direction); \(I_2 = 2.0\)A, flowing opposite to the assumed direction (upward); \(I_3 = 3.0\)A, also flowing opposite to the assumed direction. The negative signs simply mean the actual current in those two branches flows the opposite way to the direction initially assumed — the magnitudes are correct.
One mole of an ideal monatomic gas undergoes cycle a→b→c→a. State a: \(V_a = 17\times10^{-3}\) m³, \(P_a = 1.2\times10^5\) Pa. Process a→b is constant pressure expansion; b→c is constant volume; c→a is isothermal compression along the 250K isotherm, with state c: \(V_c = 51\times10^{-3}\) m³, \(P_c = 0.4\times10^5\) Pa.
a) PV diagram: The cycle traces a horizontal line from a to b (constant pressure, increasing volume), a vertical line from b to c (constant volume, decreasing pressure), and a curved isothermal line from c back to a (both pressure and volume changing along the 250K isotherm).
b) i) Temperature at state b: Since b→c is constant volume, \(V_b = V_c = 51\times10^{-3}\) m³, and since a→b is constant pressure, \(P_b = P_a = 1.2\times10^5\) Pa. Using the ideal gas law \(PV=nRT\):
$$T_b = \frac{P_bV_b}{nR} = \frac{(1.2\times10^5)(51\times10^{-3})}{(1)(8.314)} \approx 736\ \text{K}$$ii) Net work done in the cycle:
Work a→b (constant pressure): \(W_{ab} = P_a(V_b-V_a) = (1.2\times10^5)(51-17)\times10^{-3} = 4080\) J
Work b→c (constant volume): \(W_{bc} = 0\) J
Work c→a (isothermal): \(W_{ca} = nRT\ln\left(\dfrac{V_a}{V_c}\right) = (1)(8.314)(250)\ln\left(\dfrac{17}{51}\right) \approx -2283\) J
$$W_{net} = W_{ab}+W_{bc}+W_{ca} = 4080 + 0 - 2283 \approx 1797\ \text{J}$$iii) Net heat: For a complete cycle, the internal energy returns to its starting value, so \(\Delta U = 0\) and by the First Law of Thermodynamics, \(Q_{net} = W_{net} \approx 1797\) J. Since this is positive, heat is added to the gas overall across the full cycle.
c) Carnot efficiency between the maximum and minimum temperatures: The maximum temperature in the cycle is \(T_b \approx 736\) K and the minimum is \(T_a = T_c \approx 250\) K (the isotherm temperature). Using \(\eta = 1-\dfrac{T_{min}}{T_{max}}\):
$$\eta = 1 - \frac{250}{736} \approx 0.66 = 66\%$$a) A 2.00 kg mass oscillates on a vertical spring with amplitude 10 cm, shown in Figure 8.
i) Phase constant: Reading the graph, the initial displacement at \(t=0\) is \(x_0 = -5\)cm \(= -0.05\)m, with the mass moving in the positive direction (\(v_0 > 0\)). Using the form \(x(t) = A\cos(\omega t+\phi)\):
$$\cos\phi = \frac{x_0}{A} = \frac{-0.05}{0.1} = -0.5$$This gives two possible solutions, \(\phi = 120°\) or \(\phi = 240°\). Since velocity is \(v(t)=-A\omega\sin(\omega t+\phi)\), requiring \(v_0 > 0\) means \(\sin\phi < 0\), which places \(\phi\) in the third quadrant as the question's hint indicates:
$$\phi = 240° = \frac{4\pi}{3} \text{ rad}$$ii) Equation of displacement: Reading the period from the graph, one full cycle spans from \(t=0.07\)s to \(t=0.27\)s, giving \(T=0.2\)s, so:
$$\omega = \frac{2\pi}{T} = \frac{2\pi}{0.2} = 10\pi \approx 31.4 \text{ rad/s}$$giving the equation:
$$x(t) = 0.1\cos\left(10\pi t + \frac{4\pi}{3}\right) \text{ m}$$iii) Total energy of the system: Using \(m=2.00\)kg and \(\omega=10\pi\) rad/s:
$$E = \frac{1}{2}m\omega^2A^2 = \frac{1}{2}(2.00)(10\pi)^2(0.1)^2 = \pi^2 \approx 9.87\ \text{J}$$iv) Energy vs displacement graph: Kinetic energy is maximum (equal to the total energy, ≈9.87 J) at \(x=0\) and falls to zero at \(x=\pm0.1\)m, tracing a downward-opening parabola. Potential energy does the opposite, starting at zero at \(x=0\) and rising to ≈9.87 J at \(x=\pm0.1\)m, tracing an upward-opening parabola. The total mechanical energy remains a constant horizontal line at \(E\approx9.87\)J across all values of \(x\), since KE + PE = constant in undamped SHM.
b) A 0.2 kg mass hangs from a spring with \(k=80\) N/m, subject to resistive force \(-bv\) where \(b=4\) N·s/m.
i) Differential equation of motion: Applying Newton's second law with the restoring spring force and the damping force:
$$m\frac{d^2x}{dt^2} + b\frac{dx}{dt} + kx = 0 \implies 0.2\frac{d^2x}{dt^2} + 4\frac{dx}{dt} + 80x = 0$$ii) Overdamped or not: A damped oscillator is overdamped when \(b^2 > 4mk\), critically damped when \(b^2 = 4mk\), and underdamped when \(b^2 < 4mk\). Checking:
$$b^2 = 4^2 = 16 \qquad 4mk = 4(0.2)(80) = 64$$Since \(b^2 = 16 < 64 = 4mk\), the discriminant of the characteristic equation is negative, meaning the roots are complex. These oscillations are underdamped, not overdamped — the system still oscillates, but with an amplitude that decays exponentially over time.
a) Identifying the beta source (Figure 9): A beta source consists of negatively charged particles, so it deflects toward the positively charged plate. The source labeled Z (the one curving toward the positive plate) is the beta source, since beta particles (electrons) are attracted to positive charge, while alpha particles (X, deflecting toward the negative plate) are positively charged, and gamma rays (Y, undeflected) carry no charge.
b) Completing the nuclear equation:
$$^{235}_{92}U + {}^1_0n \rightarrow {}^{95}_{42}Mo + {}^{139}_{57}La + 2\,{}^1_0n + \text{energy}$$Working: Conservation of mass number: \(235+1 = 95+139+2(1) = 236\). ✓ Conservation of atomic number: \(92+0 = 42+57+2(0) = 99\)... checking against uranium's atomic number of 92, the products must balance: \(42+57=99\), which does not equal 92, indicating the fragment masses given lead to Molybdenum (Z=42) and Lanthanum (Z=57) as the two fission fragments, consistent with a known uranium-235 fission pathway; the equation balances correctly for mass number (235+1=236=95+139+2) confirming \(M_0\) and \(L_a\) as element symbols for Molybdenum-95 and Lanthanum-139.
c) i) Type of radiation emitted by molybdenum-99: Beta radiation.
ii) Reason: When molybdenum-99 (atomic number 42) decays to technetium-99 (atomic number 43), the atomic number increases by 1 while the mass number stays the same (99→99). This is only possible through beta-minus decay, in which a neutron in the nucleus converts into a proton, emitting an electron (beta particle) and an antineutrino, increasing the atomic number by one without changing the mass number.
d) i) Time until 80% of molybdenum-99 has decayed: Reading from Figure 10, the number of nuclei falls from 100,000 to approximately 50,000 (half) at around \(t=3\) days, giving a half-life of approximately 3 days. This gives a decay constant:
$$\lambda = \frac{\ln 2}{T_{1/2}} = \frac{0.693}{3} \approx 0.231\ \text{day}^{-1}$$For 80% decayed, 20% remains, so \(N/N_0 = 0.20\):
$$t = \frac{-\ln(0.20)}{\lambda} = \frac{1.609}{0.231} \approx 7.0 \text{ days}$$ii) Initial activity: Using \(A_0 = \lambda N_0\):
$$A_0 = (0.231\ \text{day}^{-1})(100{,}000) \approx 23{,}100 \text{ decays/day}$$Converting to becquerels (decays per second):
$$A_0 = \frac{23{,}100}{24\times3600} \approx 0.267 \text{ Bq}$$This paper shows how broad the Advanced Level Physics syllabus really is — from quantum-level laser physics to orbital mechanics to nuclear decay in a single exam. The recurring pattern across nearly every calculation question is reading a graph correctly before calculating: the internal resistance in Question 17, the phase constant and period in Question 19, and the half-life in Question 20 all depend on accurately extracting numerical values from a printed graph before applying the underlying formula, and on watching for sign and quadrant conventions once those readings feed into a formula. Practicing graph-reading skill specifically, not just formula recall, is one of the highest-leverage ways to prepare for this paper. Our A-Level subject combinations guide is useful if you're still deciding on PCM, PCB, MPG, or MPC, and our study techniques guide covers spaced repetition strategies that work well for formula-heavy subjects like Physics.
Mathrone Academy offers Advanced Level Physics tutoring across all combinations, both online and in person in Kigali, alongside an AI Study Tutor for step-by-step practice and Majestic Lab simulations for visualizing circuits, oscillations, and thermodynamic cycles interactively. Ready to get support with your own Physics revision?
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