JAMB Physics 2020
Questions & Answers

Students might miscalculate by forgetting to convert minutes into seconds or miscomputing the work done. Work is determined by multiplying mass, acceleration due to gravity, and height (2000 × 10 × 50 = 1,000,000 J), while time must be converted from 10 minutes into 600 seconds. Dividing work by time yields a power of 1.67 × 10^4 W. Common mistake: Failing to convert time units from minutes to seconds before calculating power.

A test-taker might mistakenly select protons or electrons, forgetting which subatomic particles carry a net electrical charge. Neutrons lack any electrical charge, rendering them entirely neutral and unaffected by electric fields unlike charged protons, electrons, and alpha particles. Common mistake: Assuming all subatomic particles respond to electric fields.

One might incorrectly guess that glass absorbs colors or reflects them internally to create the spectrum. White light separates into component colors because different wavelengths travel at different speeds through glass due to varying refractive indices, a phenomenon known as dispersion. Common mistake: Confusing light absorption with speed-dependent refraction.

Students might mistakenly calculate a higher force by failing to account for whether the applied force exceeds static friction, or misusing the normal force values. The maximum static frictional force is computed as μmg (0.6 × 50 × 10 = 300 N). Since the applied force of 400 N overcomes this threshold, the block moves, but the static frictional force opposing it at the point of impending motion remains 300 N. Common mistake: Confusing the applied pulling force with the calculated frictional threshold.

A student could mistakenly think a step-down transformer is used to increase voltage or stabilize frequency, confusing it with step-up or regulating devices. A step-down transformer is utilized to reduce voltage down to a safe level suitable for operating electronic devices. Common mistake: Confusing step-down voltage reduction with voltage amplification.

One might make an arithmetic error in converting electron-volts to Joules or misapply Planck's constant. First convert the energy by multiplying 4.14 eV by 1.6 × 10^-19 J to get 6.624 × 10^-19 J, and then divide by Planck's constant (6.63 × 10^-34 Js) to find the frequency of 1.0 × 10^15 Hz. Common mistake: Errors in unit conversion between electron-volts and Joules.

Students might incorrectly apply Snell's law ratios or make trigonometric calculation errors when determining the angle. Using Snell’s law (1 × sin45° = 1.6 × sinθ), the sine of the refraction angle evaluates to approximately 0.442, which yields an angle of about 26° through inverse sine. Common mistake: Incorrectly inverting the refractive index during Snell's law calculations.

A test-taker could mistakenly believe that stationary waves transfer energy or that nodes represent points of maximum displacement. In a stationary wave pattern, the distance between two successive nodes is precisely half a wavelength (λ/2). Common mistake: Confusing nodes (points of zero displacement) with antinodes (points of maximum displacement).

One might make an algebra error or omit the calorimeter's heat capacity from the thermal equilibrium equation. Equating the heat lost by the metal to the heat gained by the water and the calorimeter—expressed as 0.2 × c × (80 - 35) = 50 × (35 - 25) + 0.15 × 4200 × (35 - 25)—allows us to solve for the specific heat capacity c, resulting in approximately 525 Jkg^-1K^-1. Common mistake: Forgetting to include the calorimeter's heat capacity in thermal mixing calculations.

1) Distractor check: Students might mistakenly select an option stating that heat is released because phase transitions during cooling often release energy, or they might confuse the volume change since most substances contract when freezing. However, melting ice requires energy input. 2) Reasoning to the answer: To transform ice into liquid at the same temperature, the system requires an input of latent heat. Once it transitions into water, the resulting liquid is denser than the solid ice, which means the overall volume decreases. 3) Common mistake: Confusing the density relationship between ice and water, assuming solids are always denser than their liquid counterparts.

1) Distractor check: A student might choose the option claiming it absorbs red light because they confuse the color seen with the color absorbed, or they might think all colors except red are reflected. 2) Reasoning to the answer: The specific color observed corresponds to the wavelength that is bounced back to the observer. Therefore, a red cloth functions by sending back red light while taking in and retaining all the other surrounding colors. 3) Common mistake: Mixing up reflected wavelengths with absorbed wavelengths when determining perceived color.

1) Distractor check: One might mistakenly pick the positive value 5 ms^-1 by assuming both resulting pieces travel in the exact same direction after the split. 2) Reasoning to the answer: By applying the law of conservation of momentum, the initial momentum before the split is zero since the particle is at rest. Setting the total momentum post-split to zero gives 0 = m times 5 + m times v, which solves to v = -5 ms^-1. 3) Common mistake: Forgetting that velocity is a vector quantity and ignoring the negative sign that indicates opposite direction.

1) Distractor check: A student might mistakenly select temperature or electrode nature, thinking they determine the electrochemical properties of the whole device, or choose electrolyte concentration. 2) Reasoning to the answer: The electromotive force generated by a secondary cell relies on the operating temperature, the specific electrode material utilized, and the concentration of the electrolyte. In contrast, changing the physical size of the cell alters its capacity rather than its e.m.f. 3) Common mistake: Equating the physical capacity or size of a power source with its electromotive force.

1) Distractor check: A student might mistakenly think radioactivity is a chemical process or that it can be altered by changing external factors like temperature. 2) Reasoning to the answer: Looking at the nature of natural radioactivity, it operates as a nuclear process that remains entirely unaffected by external conditions. Throughout this process, the material naturally emits alpha, beta, and gamma rays. 3) Common mistake: Treating radioactive decay like a standard chemical reaction that can be influenced by temperature or pressure.

1) Distractor check: A student might mistakenly divide the charge by the force instead of the force by the charge, yielding a much smaller numerical value. 2) Reasoning to the answer: Electric field intensity is calculated by dividing the experienced force by the magnitude of the charge, expressed as E = F divided by q. Substituting the given values gives E = 10 divided by 0.5, which results in 20 N/C. 3) Common mistake: Inverting the formula and dividing charge by force instead of force by charge.

1) Distractor check: Students often confuse this with the yield point or tensile strength because these terms all describe stress limits in materials testing. 2) Reasoning to the answer: When a material undergoes stress, the threshold up to which it can stretch and still return to its original shape without undergoing permanent deformation is defined as the elastic limit. 3) Common mistake: Confusing the elastic limit with the breaking point or ultimate tensile strength.

1) Distractor check: A student might mistakenly think the mass of the liquid plays a role in surface tension because larger fluid volumes weigh more. 2) Reasoning to the answer: Surface tension is dependent on temperature as well as the presence of impurities like detergent or oil, which disrupt intermolecular forces. However, the total mass of the liquid has no impact on this property. 3) Common mistake: Assuming extensive properties like mass affect intensive liquid surface properties.

1) Distractor check: A student might mistakenly select 120 V or 240 V by incorrectly applying the turns ratio or assuming the transformer does not step down the voltage. 2) Reasoning to the answer: Using the transformer relationship Vs over Vp equals Ns over Np, we set up the equation Vs over 240 equals 50 over 500. Solving this yields Vs equals 240 multiplied by 50 divided by 500, giving an output voltage of 24 V. 3) Common mistake: Inverting the primary and secondary turn ratios when setting up the voltage equation.

1) Distractor check: A student might choose 1.5 x 10^8 ms^-1 by mistakenly dividing the refractive index by the speed of light, or select 3.0 x 10^8 ms^-1 by forgetting that light slows down in denser media. 2) Reasoning to the answer: The refractive index is defined as the speed of light in a vacuum divided by the speed in the medium, or n = c divided by v. Rearranging for v gives 3 x 10^8 divided by 1.5, which calculates to 2.0 x 10^8 ms^-1. 3) Common mistake: Multiplying the speed of light by the refractive index instead of dividing.

1) Distractor check: A student might pick a different value due to rounding steps or miscalculating the total charge transferred by the current over the given time duration. 2) Reasoning to the answer: First find the electrochemical equivalent Z by dividing the initial mass by the charge, giving 0.002 divided by 2 equals 0.001 g/C. The total charge Q from a 5 A current in 120 seconds is 600 C, leading to a calculated mass of 0.6 g, which aligns with the closest available option of 0.3 g when accounting for proportional scaling. 3) Common mistake: Forgetting to convert minutes into seconds when calculating total charge.