WAEC Physics 2021
Questions & Answers

1) Distractor check: Options B, C, or D are likely to be passed over because students readily recognize them as types of forces, making mass stand out as a fundamental quantity. 2) Reasoning to the answer: Tension, weight, and friction are all vector forces that act upon objects in various physical contexts. Mass, however, is a fundamental scalar quantity that measures the amount of matter in an object, expressed in kilograms, and is therefore not a force itself. 3) Common mistake: Confusing mass with weight.

1) Distractor check: Options B, C, or D might be mistakenly chosen by assuming that changing the core structure directly alters the primary magnetic flux or acts to reduce device efficiency. 2) Reasoning to the answer: A solid iron core would experience significant circulating currents induced by changing magnetic fields. By laminating the core into thin, insulated sheets, the electrical resistance perpendicular to the magnetic field is increased, which successfully reduces these eddy currents and minimizes energy loss as heat, thereby improving overall efficiency. 3) Common mistake: Thinking lamination is meant to stop magnetic induction entirely rather than suppress eddy currents.

1) Distractor check: A student could mistakenly choose B, C, or D by confusing cell chemistry terms with broader electrical or chemical phenomena like electrolysis or local action. 2) Reasoning to the answer: During the discharge of a primary cell, hydrogen gas bubbles accumulate on the cathode, which is the copper plate. This buildup creates an insulating barrier and increases internal resistance, a phenomenon known as polarization that reduces the voltage output of the cell. 3) Common mistake: Confusing polarization with local action in a primary cell.

1) Distractor check: Options A, C, or D might be incorrectly selected if a student assumes all physical properties of a system influence evaporation rates. 2) Reasoning to the answer: The rate at which a liquid evaporates is influenced by temperature because higher heat increases molecular kinetic energy, by wind because it removes saturated vapor from the surface, and by surface area because it exposes more molecules to the air. However, the total volume of the liquid does not affect the evaporation rate, as volume only dictates how long the liquid will last, not how fast individual molecules escape. 3) Common mistake: Confusing the rate of a process with the total capacity or volume of the substance.

1) Distractor check: Students frequently confuse the pupil with the iris or cornea due to their close anatomical association in the structure of the eye. 2) Reasoning to the answer: Light enters the eye from external sources and must pass through an adjustable aperture located within the iris. This specific opening is called the pupil, which regulates the amount of light traveling through to the lens and ultimately focusing onto the retina. 3) Common mistake: Confusing the pupil (the opening) with the iris (the colored muscle controlling the opening).

1) Distractor check: Options A, B, or D are plausible distractors because they involve temperature measurement or related scientific terminology, but they do not describe the specific semiconductor resistor component. 2) Reasoning to the answer: A semiconductor component whose electrical resistance varies significantly and predictably with changes in temperature is specifically known as a thermistor, which is widely utilized for temperature sensing applications in electronic circuits. 3) Common mistake: Confusing a thermistor with a thermostat, which is a control device rather than a sensing resistor.

1) Distractor check: Options A, B, or D might be picked if a student confuses electric motors with generators, chemical cells, or simple heating elements. 2) Reasoning to the answer: An electric motor operates by utilizing electromagnetic induction and magnetic forces on a current-carrying coil. When current is supplied, it interacts with magnetic fields to produce torque, thereby converting electrical energy into mechanical energy of rotation. 3) Common mistake: Mixing up the energy conversion of an electric motor with that of a generator.

1) Distractor check: Options A, B, or C might be chosen if a student assumes liquids share gas-like random motion or that solids and liquids share weak bonds and wide separations. 2) Reasoning to the answer: Both solid and liquid states consist of molecules that are closely packed together due to strong intermolecular forces. Rather than moving around randomly like gases, the molecules in both solids and liquids exhibit vibratory motion, though particles in solids vibrate about fixed positions while those in liquids can move slightly more freely. 3) Common mistake: Assuming that because liquids flow, their molecules exhibit completely random translational motion like gases.

1) Distractor check: A student might choose B or A by confusing the distance between a node and an antinode with a full wavelength or a half-wavelength. 2) Reasoning to the answer: In a transverse standing wave, a node represents a point of zero displacement while an adjacent antinode represents a point of maximum displacement. Since a complete wavelength spans from one node through an antinode, another node, another antinode, and back to a node, the distance spanning from a single node to its immediate neighboring antinode represents exactly a quarter of the wavelength (λ/4). 3) Common mistake: Confusing the node-to-antinode distance with the node-to-node distance, which is half a wavelength.

1) Distractor check: Options A, B, or D might be selected if a student confuses gravitational principles with Newton's laws of motion or electrostatic forces. 2) Reasoning to the answer: The foundational principle describing the attractive pull between two masses states that the gravitational force is directly proportional to the product of their masses and inversely proportional to the square of the distance separating them, which is formally known as Newton's Law of Universal Gravitation. 3) Common mistake: Confusing Newton's Law of Universal Gravitation with Coulomb's Law for electrostatic charges.

1) Distractor check: Options B, C, or D are attractive if a student misremembers how plate separation, surface area, and dielectrics mathematically affect the value of capacitance. 2) Reasoning to the answer: The capacitance equation is given by C = ε₀A/d, which shows that capacitance increases when the surface area of the plate overlap (A) is increased, and also increases when the distance between the plates (d) is decreased. Conversely, increasing distance or omitting a dielectric decreases capacitance, confirming that only actions I and III increase it. 3) Common mistake: Reversing the proportional relationship between plate distance and capacitance.

1) Distractor check: Options A, C, or D are likely selected if a student ignores thermal expansion principles or incorrectly assumes density remains constant when an object expands thermally. 2) Reasoning to the answer: When a metallic sphere is heated, it undergoes thermal expansion, meaning its volume increases according to the relation V = V₀(1 + αΔT). Because the overall mass of the sphere remains constant while its volume expands, the density, calculated as mass divided by volume (ρ = m/V), must decrease as a result. 3) Common mistake: Assuming density stays constant during thermal expansion because the mass of the object does not change.

1) Distractor check: Options A, B, or D might be chosen by confusing the graphical representations of distance-time versus velocity-time relationships in kinematics. 2) Reasoning to the answer: Acceleration is mathematically defined as the rate of change of velocity with respect to time (a = Δv/Δt). On a velocity-time graph, this rate is represented graphically by the slope, calculated as the rise over the run, whereas the area under a velocity-time graph yields displacement. 3) Common mistake: Confusing the slope of a velocity-time graph with the slope of a distance-time graph.

1) Distractor check: Options A, C, or D might be chosen if a student confuses measuring potential difference with controlling current or measuring current directly. 2) Reasoning to the answer: To measure the potential difference across a resistor, a device specifically designed to be placed in parallel with high resistance is required, which is a voltmeter. Although a galvanometer can be converted into one, a voltmeter is the direct instrument needed for this measurement. 3) Common mistake: Suggesting an ammeter, which must be connected in series to measure current rather than potential difference in parallel.

1) Distractor check: Options B, C, or D are plausible if a student confuses reactance units with units of potential difference, current, or capacitance. 2) Reasoning to the answer: Inductive reactance represents opposition to alternating current flow in an inductor, calculated as X_L = 2πfL. Because it serves a role analogous to resistance in AC circuits, its standard unit of measurement is the ohm (Ω). 3) Common mistake: Confusing inductive reactance units (ohms) with capacitance units (farads).

1) Distractor check: Options A, C, or D are plausible if a student makes a calculation error while rearranging the formula or misplaces the decimal exponent during division. 2) Reasoning to the answer: Begin with the formula for inductive reactance, X_L = 2πfL, and rearrange it to solve for inductance: L = X_L / (2πf). Substitute the given values into the rearranged equation, yielding L = 1 / (2 × 3.14 × 100), which simplifies to 1 / 628. Evaluating this division gives approximately 0.00159 H, which in scientific notation is 1.59 × 10⁻³ H. 3) Common mistake: Dropping the negative exponent or miscalculating the frequency multiplier in the denominator.

Students might mistakenly choose options like A, B, or C due to calculation errors or inverted formulas when dividing the load by the velocity ratio. To find the required effort, first compute the velocity ratio using the formula 2 times pi times the radius divided by the pitch. With a radius of 0.35 meters and a pitch of 0.005 meters, the velocity ratio works out to approximately 440. Dividing the 2000 N load by this velocity ratio yields an effort of roughly 4.55 N, which rounds to 4.5 N. Common mistake: Forgetting to convert the handle length from centimeters to meters before calculating the radius.

Students might mistakenly select options like B, C, or D, confusing alpha decay with beta emissions, gamma rays, or neutron radiation. When an atom undergoes alpha decay, it releases a helium nucleus designated as ^4_2He. This emission carries away two protons and two neutrons, causing the parent nucleus to decrease its atomic number by 2 and its mass number by 4. Common mistake: Confusing the change in atomic number with the change in mass number.

Students may mistakenly pick B, C, or D by confusing current with charge, potential difference, or time derivatives. Current is defined as the rate of flow of electric charge, expressed mathematically as current equals charge divided by time. Because charge is measured in coulombs and time in seconds, the resulting unit for current is coulombs per second, which is equivalent to the ampere. Common mistake: Mixing up the numerator and denominator when relating fundamental electrical units.

Students might incorrectly select B, C, or D by multiplying the values instead of dividing, or by misplacing decimal places during the computation. According to Ohm's law, current equals voltage divided by resistance. Substituting the given values yields 100 V divided by 500 ohms, which calculates directly to 0.2 A. Common mistake: Inverting the Ohm's law formula and dividing resistance by voltage.