WAEC Physics 2018 Theory Past Questions

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Strain is the relative deformation produced by stress; it is the ratio of extension to original length, and has no units (dimensionless): strain = extension/original length.

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Extension = 2l₀ − l₀ = l₀. Strain = extension/original length = l₀/l₀ = 1 (i.e. 100%).

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Silicon dioxide (silica glass), sapphire, and chalcogenide glass (plastic/polycarbonate fibres are also used in some applications).

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Ferromagnetic materials, paramagnetic materials, and diamagnetic materials.

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An intrinsic semiconductor is a chemically pure semiconductor containing no impurities, whose (low) conductivity arises only from thermally generated electron-hole pairs.

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N-type: majority carriers are electrons, minority carriers are holes; formed by adding pentavalent (donor) impurities; donor density exceeds acceptor density. P-type: majority carriers are holes, minority carriers are electrons; formed by adding trivalent (acceptor) impurities; acceptor density exceeds donor density.

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Maximum range occurs at θ=45°. Maximum height H = u²sin²θ/2g = (200²×sin²45°)/(2×10) = (40000×0.5)/20 = 1000m.

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T = 57+273 = 330K. λmax = 2.9×10⁻³/330 ≈ 8.79×10⁻⁶m. f = c/λmax = (3×10⁸)/(8.79×10⁻⁶) ≈ 3.41×10¹³Hz.

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LASER stands for Light Amplification by Stimulated Emission of Radiation.

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A laser is a device that produces a highly coherent, nearly monochromatic and intense beam of light as a result of the cooperative stimulated emission of radiation from many atoms.

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Uniform acceleration is the constant (steady) time rate of increase of velocity of a body.

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Horizontal forces: the driving/thrust force (forward, in the direction of motion) and the frictional/air-resistance force (backward, opposing motion). Vertical forces: the weight of the car (downward) and the normal reaction from the road (upward).

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(i) The graph is a trapezium: velocity rises linearly from 0 to 25ms⁻¹ over 0–20s, stays constant at 25ms⁻¹ from 20–50s, then falls linearly to 0 from 50–60s. (iii) Total distance = area under the graph = ½(60+30)×25 = ½×90×25 = 1125m. (iv) Deceleration = (0−25)/(60−50) = −2.5ms⁻² (i.e. a deceleration of 2.5ms⁻²).

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Radiation: surface area of the body; nature (colour/texture) of the surface; temperature of the body/excess temperature over surroundings. Convection: exposed surface area; density of the surrounding fluid; temperature difference between body and fluid.

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The mass of the body and its specific heat capacity (and its temperature/temperature change).

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This means that 2.72×10⁵ joules of heat energy are required to change 1kg of mercury from liquid to vapour at its boiling point without any change in temperature.

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Heat supplied in first 4min (240s): Q=V²t/R = 220²×240/35 ≈ 331,886J. This heats the jug (ΔQ=250×72=18,000J) and the water (mcΔT=m×4200×72=302,400m). 331,886 = 18,000 + 302,400m → m ≈ 1.04kg. (ii) Heat supplied in next 5min (300s): Q₂=220²×300/35 ≈ 414,857J. This evaporates 0.3kg of water: L = Q₂/mass = 414,857/0.3 ≈ 1.38×10⁶ J/kg.

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Diffraction is the spreading and bending of waves as they pass through a narrow opening or around an obstacle; a narrow gap produces a larger diffraction effect.

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(i) The critical angle is the angle of incidence (in the denser medium) for which the angle of refraction in the less dense medium is 90°. (a) ic + 44° = 90° → ic = 46°. (b) Refractive index η = 1/sin(ic) = 1/sin46° ≈ 1.39.

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Closed pipe: f₀=200Hz, first overtone = 3f₀ = 600Hz. (i) For the open pipe, first overtone = 2f₀' = 600Hz → f₀' = 300Hz. (ii) Length of closed pipe: f₀=v/4L → L = 330/(4×200) = 0.4125m. (iii) Length of open pipe: f₀'=v/2L' → L' = 330/(2×300) = 0.55m.

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(i) Reactance is the opposition which an inductor or capacitor offers to the flow of a.c. current without dissipating heat, measured in ohms. (ii) Impedance is the overall (total) opposition offered to the passage of a.c. current by a mixed circuit containing resistance and one or both of inductance and capacitance, also measured in ohms.

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(i) As the coil rotates in the magnetic field, it continuously cuts (changes) the magnetic flux linking it, and by electromagnetic induction this changing flux induces an e.m.f. in the coil. (ii) The slip rings transfer the alternating induced current from the rotating coil to the external circuit continuously. (iii) Lenz's Law: the direction of the induced current is always such as to oppose the change in magnetic flux producing it. (iv) Increase the number of turns on the coil; increase the speed of rotation of the coil (also: increase the area of the coil or use a stronger magnet).

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Energy = Power × time = 6W × (5×60)s = 6×300 = 1800J.

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Binding energy is the energy released when the individual constituent particles (nucleons) come together to form an atomic nucleus; equivalently, it is the energy required to separate a nucleus into its individual constituent nucleons.

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(i) X-rays travel through space/vacuum with the speed of light. (ii) X-rays can be polarised. (iii) X-rays are not deflected by electric or magnetic fields.

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(i) Generation of electrical power. (ii) Medical diagnosis and treatment (e.g. radiotherapy). (iii) Preservation of food and sterilization of equipment (or industrial radiography/tracing).

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(i) f = c/λ = (3.0×10⁸)/(4.5×10⁻⁷) ≈ 6.67×10¹⁴Hz. (ii) E = hf = 6.6×10⁻³⁴×6.67×10¹⁴ ≈ 4.4×10⁻¹⁹J. (iii) Energy of photoelectrons = E − work function = 4.4×10⁻¹⁹ − 3.0×10⁻¹⁹ = 1.4×10⁻¹⁹J.