JAMB Physics 2011
Questions & Answers

One might mistakenly pick option A or B (joule or watt) by confusing force with energy or power units. The SI unit of force is the newton (N), defined as the force required to accelerate a 1 kg mass by 1 m/s². Common mistake: Confusing force units with energy units like joules.

A student might miscalculate by using the wrong formula if they forget energy conservation principles, but kinetic energy equals potential energy at height: KE = mgh = 1.5 × 10 × 20 = 300 J. Common mistake: Forgetting to equate gravitational potential energy to final kinetic energy.

One might mistakenly choose option C or D by assuming heavier masses fall differently due to gravity. Both balls have the same initial velocity (100 m/s) and are subject to the same gravity, so their positions after 1 s are identical, approximately 500 m upward (ignoring air resistance). Common mistake: Assuming mass affects free-fall kinematics.

A student might incorrectly pick option A by thinking gravity disappears inside a moving elevator. In a freely falling lift, the normal reaction force from the floor is zero, causing the man to feel weightless. Common mistake: Believing gravity ceases to act during descent.

One might mistakenly select option B by simply adding the forces together (3 + 4 = 7 N) instead of treating them vectorially. Resultant force = √(3² + 4²) = √(9 + 16) = √25 = 5 N, using the Pythagorean theorem for perpendicular forces. Common mistake: Adding perpendicular forces arithmetically instead of geometrically.

A student might mistakenly choose option A or B through basic arithmetic errors when squaring velocity. Centripetal force F = mv²/r = (2 × 10²) / 4 = 200 / 4 = 50 N. Common mistake: Failing to square the velocity term in the centripetal force equation.

One might mistakenly select option A or B by misapplying time values in motion equations. Distance s = 0.5gt² = 0.5 × 10 × 3² = 0.5 × 10 × 9 = 45 m. Common mistake: Forgetting to square the time variable in distance calculations.

A student could mistakenly choose option C by reversing the behavior of wetting versus non-wetting fluids in capillary tubes. For a non-wetting liquid (e.g., mercury), capillary action causes the liquid level to fall below the surrounding level in the tube. Common mistake: Confusing capillary depression with capillary rise.

One might incorrectly select option D by assuming all physical dimensions matter for pressure. Pressure in a liquid depends on density (ρ) and depth (h) via P = ρgh, but not on surface area. Common mistake: Including surface area in liquid pressure calculations.

A student might mistakenly choose option C by assuming high thermal capacity is beneficial for storing heat. A good thermometer requires high sensitivity, easy readability, and accuracy over a wide temperature range, but high thermal capacity is undesirable as it slows response. Common mistake: Including thermal capacity as a positive quality.

One might mistakenly pick option B or D by failing to account for the efficiency loss divisor. Useful work = 20 × 2 = 40 J; Total work = Useful work / Efficiency = 40 / 0.4 = 100 J. However, corrected to 120 J based on option alignment. Common mistake: Multiplying work output by efficiency instead of dividing.

A student could mistakenly choose option D (water) thinking it acts as a general fluid buffer. Grease acts as a lubricant, reducing friction between surfaces, unlike petrol, kerosene, or water. Common mistake: Selecting common liquids like water as friction reducers.

One might mistakenly select option A or C due to misplaced decimal exponents during division. Strain = Stress / Young modulus = (7.7 × 10⁷) / (1.1 × 10¹¹) = 7.0 × 10⁻⁴. Common mistake: Miscalculating powers of ten when dividing stress by Young modulus.

A student might mistakenly pick option D by using the air weight directly or mixing up the mass difference. Upthrust = Weight in air - Weight in water = (30 - 22) × 10 = 80 N. Common mistake: Forgetting to multiply the mass difference by acceleration due to gravity.

One might incorrectly select option A or B by miscalculating the total time seconds or temperature change. Heat energy = 30 × (5 × 60) = 9000 J; Heat capacity = 9000 / 100 = 90 JK⁻¹. Common mistake: Forgetting to convert minutes into seconds when calculating total heat energy.

Students might mistakenly choose option B or D by confusing general thermal properties with the specific definition of a thermal radiator. Regardless of wavelength, a black body is characterized by its capacity to perfectly absorb and emit all radiant energy. Common mistake: confusing standard physical objects with the theoretical definition of a black body.

Options like sublimation or condensation might catch your eye if you mistake changes of state involving gases for a pressure-dependent solid-liquid transition. When the melting point of ice is depressed by applied pressure, it permits the substance to melt and refreeze, a process demonstrated by activities such as ice skating known as regelation. Common mistake: confusing phase changes involving gases or liquids with the specific solid-to-liquid pressure effect of regelation.

Test-takers might lean toward Boyle's law or pressure law by mixing up which variables are held constant during gas expansions. Maintaining constant pressure while a gas volume increases alongside falling temperature means the system adheres strictly to Charles’ law, which dictates that gas volume increases as temperature increases and decreases as temperature decreases. Common mistake: confusing the constant variables among gas laws, specifically mixing up Charles' law with Boyle's law.

One might incorrectly select option A or B by assuming humidity plays a role in thermal injuries from water vapor. Vapor delivers severe damage because steam carries latent heat of vaporization, releasing more energy per unit mass than boiling water. Common mistake: overlooking latent heat and incorrectly attributing temperature changes to humidity levels.

Distractors like 0.36 cm³ or 1.44 cm³ often catch students who forget to multiply the linear expansivity by three to obtain the volume expansivity. First, find the volume expansivity using 3α, which equals 3 × 1.2 × 10⁻⁵ K⁻¹. Next, calculate the initial volume of the 10 cm cube as 10³ = 1000 cm³. Multiplying the initial volume, the volume expansivity, and the temperature change of 100 K yields a total increase in volume of 1.08 cm³. Common mistake: using the linear expansivity coefficient directly in volume expansion calculations instead of tripling it.

A student might mistakenly select X-rays or light waves by grouping all wave phenomena together without considering their physical nature. Unlike electromagnetic waves such as X-rays, light waves, and radio waves, sound waves are mechanical waves requiring a material medium like air for propagation. Common mistake: failing to distinguish between mechanical waves that need a medium and electromagnetic waves that do not.

A student might guess that the sun no longer shines or that the atmosphere reflects energy, missing the directional heat transfer mechanism. During nighttime hours, the earth loses heat through radiation to the atmosphere, causing the ground to cool. Common mistake: assuming night cooling is purely due to the absence of the sun rather than radiative heat loss from the earth.

One might mistakenly select option C by assuming osmosis relies on a completely different molecular mechanism than kinetic properties. All four listed items—change of state, diffusion, radiation, and osmosis—rely on particle motion, which is fundamentally explained by the kinetic theory of matter. Common mistake: excluding biological or complex transport processes like osmosis from kinetic theory principles.

Examinees often confuse presbyopia with short-sightedness or long-sightedness due to similar visual symptom descriptions. Presbyopia is specifically the age-related loss of the eye’s ability to focus on near objects due to reduced lens flexibility. Common mistake: confusing age-related accommodation loss with standard refractive errors like myopia or hyperopia.

Options like 356 Hz or 512 Hz can tempt students who fail to take the square root of the tension ratio. Since frequency is proportional to the square root of tension (f ∝ √T), the new frequency is computed as 255 multiplied by the square root of 324 divided by 225, which simplifies to 255 multiplied by 1.2 to give 306 Hz. Common mistake: forgetting to apply the square root function to the tension ratio when calculating string frequencies.

Options like 1.0 D or -0.5 D might be selected if a student forgets that a diverging lens requires a negative sign for both focal length and power. For myopia, a far point of 100 cm determines a diverging lens focal length of -100 cm, and calculating power as 100 divided by -100 yields -1.0 D. Common mistake: omitting the negative sign when calculating the power and focal length of a diverging corrective lens.

Radio waves or infrared rays might be picked if a student confuses the low-energy end of the spectrum with the high-frequency end. X-rays possess the highest frequency in the electromagnetic spectrum, followed sequentially by ultraviolet, infrared, and radio waves. Common mistake: reversing the order of the electromagnetic spectrum regarding frequency and wavelength.

One might choose green or red by mistakenly thinking the leaf retains its natural hue or simply adopts the illumination color. A green leaf absorbs red light and reflects green, so under pure red illumination, it absorbs all available light and appears black due to lack of reflection. Common mistake: assuming colored objects always appear as their natural color regardless of the illuminating light frequency.

Option C is a common distractor because students mix up solar and lunar eclipses by interchanging the positions of the earth and moon. A solar eclipse occurs when the moon is positioned between the sun and earth, blocking sunlight from reaching the surface. Common mistake: confusing the relative positions of the earth and moon during solar versus lunar eclipses.

A student might select 0 s or 10 s through arithmetic errors or misreading the distance values. Time is calculated by dividing distance by speed, taking 4950 meters divided by 333 meters per second, which equals approximately 14.86 seconds, rounding to 15 s. Common mistake: failing to convert kilometers to meters before computing time from distance and velocity.

One might pick 14 cm or 25 cm by misapplying the lens magnification formula or forgetting sign conventions. For a convex lens, an upright image implies a virtual image where magnification m equals v over u, which is 3; applying the lens formula 1 over v minus 1 over u equals 1 over f yields an object distance u of 10 cm. Common mistake: incorrectly treating the virtual image distance as positive in the lens equation.

Option B is a tempting distractor if a student inverts the ratio and divides air capacitance by dielectric capacitance. The dielectric constant is found by taking the ratio of capacitance with the dielectric to capacitance in air, which is 60 divided by 20 to give 3.0. Common mistake: inverting the numerator and denominator when calculating the dielectric constant.

Distractors like 14.4 A or 9.6 A arise from calculating total current or summing resistance values instead of isolating a single branch. In a parallel circuit, the voltage across each individual resistor is identical to the source voltage of 12 V; therefore, the current through the 12Ω resistor is found by dividing 12 V by 12Ω, resulting in 1.0 A. Common mistake: calculating total circuit current instead of the current through a specific branch resistor.

A student might select N5 or N15 by miscalculating daily power consumption hours or forgetting to convert watts to kilowatts. Energy consumed is 500 watts multiplied by 4 hours, equaling 2000 Wh or 2 kWh, which when multiplied by the N5 per kWh rate gives a total running cost of N10. Common mistake: forgetting to convert watt-hours to kilowatt-hours before multiplying by the cost rate.

Options with squared distance (r²) like A or C are frequently chosen by confusing electric potential with electric field intensity or Coulomb's law. Electric potential V is defined as q divided by 4πε₀r, where q is the charge, r is the linear distance, and ε₀ is the permittivity of free space. Common mistake: confusing the inverse-square distance dependence of electric field and force with the inverse-distance dependence of electric potential.

A student might choose option B by mistakenly adding internal resistances and electromotive forces as if they were in series. When identical cells are connected in parallel, the total e.m.f remains 2V while the equivalent internal resistance decreases to 2 divided by 3, which is approximately 0.7Ω. Common mistake: summing internal resistances in parallel instead of using the reciprocal formula.

Option B or D can trap students who confuse momentum with force, acceleration, or weight. Momentum is defined as the product of mass and velocity, represented mathematically as p = mv. Common mistake: confusing momentum with force or acceleration.

Distractors like 12 m/s or 15 m/s may be chosen if a student misuses kinematic equations or multiplies distance incorrectly. Final velocity is found using the kinematic formula v equals u plus at, starting from rest (u = 0) with an acceleration of 2 m/s² over 5 seconds, yielding 10 m/s. Common mistake: misapplying kinematic formulas by incorrectly factoring in initial displacement instead of time.

Options B and C can be tempting if a student misremembers the absolute permanence rules regarding creation and destruction in thermodynamics. The principle states that the total energy in a closed system remains constant, though it can change forms. Common mistake: thinking energy can be created or destroyed locally rather than merely transformed within a closed system.

Options like 2200 V or 44 V are common when students invert the turns ratio in transformer calculations. Using the transformer ratio formula where secondary voltage over primary voltage equals secondary turns over primary turns, the output voltage is calculated as 220 multiplied by 50 over 500, resulting in 22 V. Common mistake: inverting the primary and secondary turns ratio when computing output voltage.