WAEC Physics 2020
Questions & Answers

The assumption that an object was initially at rest or that an inelastic collision applies might confuse students, but conservation depends strictly on the collision type. In an elastic collision, kinetic energy along with momentum is fully conserved, whereas inelastic collisions convert kinetic energy into other forms like heat or deformation regardless of initial rest states. Common mistake: believing initial rest conditions dictate kinetic energy conservation rather than the elasticity of the collision.

Acceleration or velocity might seem plausible because they relate to motion kinematics, but quantity of motion has a specific dynamic definition. Momentum represents the quantity of motion as the product of an object's mass and velocity, whereas acceleration measures velocity changes and displacement measures position changes. Common mistake: confusing the dynamic quantity of motion with kinematic measures like velocity or acceleration.

Charles' law or the pressure law might be selected by students mixing up gas variables, but inverse volume-pressure behavior defines Boyle's law. Boyle's law establishes that for a fixed mass of gas at constant temperature, pressure multiplied by volume remains constant, meaning volume varies inversely with pressure. Common mistake: confusing the inverse pressure-volume relationship of Boyle's law with the temperature-volume relationship of Charles' law.

Virtual or upright images might tempt students thinking of plane mirrors, but screen projection imposes strict optical rules. Real images can be projected onto a physical screen because they are formed by converging light rays, and they always appear inverted relative to the source object. Common mistake: assuming images formed on projection screens share the upright orientation of virtual mirror images.

A decrease in capacitance or a drop to zero might be guessed incorrectly if the formula is misapplied, but plate separation has an inverse effect. Capacitance equals epsilon-zero times area divided by distance, so reducing the separation distance increases capacitance because the electric field strength rises for a given charge. Common mistake: assuming decreasing the distance between capacitor plates reduces rather than increases capacitance.

Claw hammers, wheelbarrows, and sugar tongs might be recognized as standard lever classes, but pulleys operate differently. A single pulley relies on redirecting force using a rope over a grooved wheel rather than pivoting around a fulcrum with effort and load arms like levers do. Common mistake: grouping simple machines like pulleys together with mechanical levers that utilize a fulcrum.

1) Distractor check: Students might mistakenly select 'intermolecular distance' because it sounds technically descriptive of the space between molecules, but it is not the specific term for the path traveled. 2) Reasoning to the answer: The average path length traversed by a gas molecule between successive impacts is formally called the mean free path. This parameter is governed by the gas density as well as the physical size of the molecules involved. 3) Common mistake: Confusing descriptive terminology with the formal physics definition.

1) Distractor check: Test-takers often guess light waves because they are familiar with waves traveling through space, forgetting that electromagnetic waves do not need a medium. 2) Reasoning to the answer: Sound waves operate as mechanical longitudinal waves, meaning they must pass through a material medium (whether solid, liquid, or gas) by relying on the physical vibration of particles. In contrast, radio waves, light waves, and X-rays are electromagnetic waves capable of traveling through a vacuum. 3) Common mistake: Assuming all wave phenomena share the exact same propagation requirements regardless of their physical classification.

1) Distractor check: A student might choose ammonium chloride because it serves as the electrolyte solution in the cell, or the carbon rod because it acts as the cathode, rather than identifying the specific chemical additive responsible for chemical reduction. 2) Reasoning to the answer: Manganese(IV) oxide (MnO₂) fulfills the role of a depolarizer within a Leclanche cell by oxidizing the hydrogen gas that accumulates at the cathode. This chemical action prevents polarization from occurring, which successfully sustains the cell's voltage output. 3) Common mistake: Confusing the electrolyte or electrode components with the specific chemical agent responsible for preventing polarization.

1) Distractor check: A student could mistakenly choose boron because they associate it with nuclear reactors, overlooking the fact that boron acts as an absorber rather than a slowing agent. 2) Reasoning to the answer: Graphite acts as a moderator in nuclear reactors, reducing the velocity of fast neutrons through elastic collisions to enhance the probability of fission in U-235. Meanwhile, boron functions by absorbing neutrons, uranium provides the fissile fuel, and copper is not typically employed for this role. 3) Common mistake: Mixing up the moderating material used to slow down neutrons with the control materials used to absorb them.

1) Distractor check: An examinee might pick the option claiming heat increases the surface tension of water, incorrectly reasoning that hotter water becomes more active in a way that raises tension. 2) Reasoning to the answer: Soap functions as a surfactant that lowers water's surface tension, enabling superior wetting capabilities and the emulsification of oil droplets. Introducing heat further decreases this surface tension, which significantly enhances the overall cleaning performance. 3) Common mistake: Believing that increasing the temperature of water raises its surface tension rather than reducing it.

1) Distractor check: A student might select the option stating all three are deflected, assuming any radiation passing through a magnetic field must react to the magnetic force. 2) Reasoning to the answer: Charged particles such as positive alpha particles and negative beta particles experience a Lorentz force given by F = q(v × B), causing them to deflect in opposite directions. Gamma rays are neutral photons and therefore pass straight through without any deflection. 3) Common mistake: Assuming uncharged ray types will experience magnetic deflection just because they travel alongside charged particles.

1) Distractor check: A student could easily select a step-up transformer by misinterpreting how the number of coil turns relates to voltage conversion. 2) Reasoning to the answer: The operational voltage ratio of a transformer follows V_s / V_p = N_s / N_p. Because the secondary turns count (20) is less than the primary turns count (50), the secondary voltage must be less than the primary voltage, indicating a step-down operation. Generators do not utilize this specific primary and secondary coil configuration. 3) Common mistake: Reversing the roles of primary and secondary coil turns when determining whether a transformer steps voltage up or down.

1) Distractor check: A test-taker might select sound to electrical, confusing the function of the earpiece with that of the microphone mouthpiece. 2) Reasoning to the answer: The earpiece functions as a speaker, transforming incoming electrical signals from the telephone line back into audible sound waves through electromagnetic or piezoelectric transduction so the user can listen. 3) Common mistake: Confusing the direction of energy conversion between the telephone mouthpiece and the earpiece.

1) Distractor check: An examinee might choose rapid cooling due to the large initial temperature gap, failing to account for the gradual asymptotic nature of the thermal exchange. 2) Reasoning to the answer: Both cups continuously transfer heat with the surrounding room via conduction and convection until they achieve thermal equilibrium at 25°C. The hot water undergoes cooling while the cold water undergoes warming, leading to temperatures that approach room temperature gradually in accordance with Newton's law of cooling. 3) Common mistake: Assuming temperature changes stop abruptly or follow a non-asymptotic path when reaching ambient conditions.

1) Distractor check: A student might miscalculate by inverting the refractive indices in Snell's law, leading to an incorrect angle value. 2) Reasoning to the answer: Applying Snell's law gives n₁ sin i = n₂ sin r, which rearranges to sin r = (n₁ / n₂) sin i. Substituting the given values yields (1.5 / 1.36) × sin 30° ≈ 0.55145, and taking the inverse sine results in an angle of refraction of approximately 33.5°. This matches the physical expectation that light bends away from the normal when moving into a less dense medium where n_glass > n_alcohol. 3) Common mistake: Inverting the ratio of the refractive indices when setting up Snell's law calculation.

1) Distractor check: A student might calculate 600 N by only factoring in the stationary weight and forgetting to add the upward acceleration component. 2) Reasoning to the answer: The scale registers an apparent weight calculated by the formula m(g + a). Substituting the mass of 60 kg along with g = 10 ms⁻² and a = 5 ms⁻² yields 60 × (10 + 5), which equals 900 N. The upward acceleration of the elevator serves to increase the normal force exerted by the scale. 3) Common mistake: Forgetting to add the acceleration term to gravity when computing apparent weight in an accelerating elevator.

1) Distractor check: A test-taker might forget to add the initial reference temperature of 20°C to the calculated temperature change, leaving only the delta T value. 2) Reasoning to the answer: Using the thermal expansion relation ΔL / L = α ΔT, a percentage increase of 0.01% corresponds to a fractional change of 0.0001. Dividing this by the linear expansivity of 1.2 × 10⁻⁵ °C⁻¹ gives a temperature change ΔT of approximately 8.33°C, which must then be added to the initial 20°C to reach a final temperature of 28.3°C. 3) Common mistake: Reporting only the temperature change rather than adding it to the starting temperature.

1) Distractor check: An examinee could mistakenly select 250.00 Ω by misplacing a decimal place during the resistance subtraction step. 2) Reasoning to the answer: The total resistance of the converted voltmeter configuration is calculated as R_total = V / I_g, which evaluates to 5 / 0.005 = 1000 Ω. Subtracting the external resistance of 975 Ω from this total gives the internal meter resistance of 25 Ω. 3) Common mistake: Incorrectly performing arithmetic when combining total circuit resistance and external series resistance.

1) Distractor check: A student might think capacitance increases because the physical distance between the plates becomes larger. 2) Reasoning to the answer: Disconnecting the battery ensures that the stored charge Q remains constant. Based on the formula C = ε₀A/d, increasing the separation distance d results in a smaller capacitance value. 3) Common mistake: Confusing the behavior of capacitance with voltage when plate separation is altered while isolated from a power source.