All 50 questions from the West African Examinations Council (WAEC) Physics 2017 Objective paper, with the correct answer and a full explanation for each. Free, no signup needed.
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Which of the following properties of a long steel bar would alter if the bar were melted, recast into a cube and allowed to cool to its original temperature?
Recasting into a different shape changes the length and cross-sectional area, altering electrical resistance (R=ρl/A), even though density, volume and specific heat capacity remain the same.
Which of the following instruments is the best for measuring the diameter of a thin constantan wire?
The micrometer screw gauge is used to measure the diameter of a wire or anything less than that. Its reading accuracy is 0.01mm or 0.001cm. It is more accurate than the Vernier calliper.
What is the density of a fuel of relative density 0.72? [Density of water = 1000 kgm⁻³]
Relative density (R.D) of fuel = 0.72, density of water = 1000 kgm⁻³. R.D = (density of fuel)/(density of water) = 0.72x1000 = 720 kgm⁻³.
The types of motion which a cylindrical drum rolling down an inclined plane undergoes are
A cylindrical drum rolling down an inclined plane undergoes both translational (down the incline) and rotational (about its own axis) motion.
Which of the following statements about a moving particle is correct?
Given a horizontal surface, if the body has mass m kg, the frictional force (Fr) can be gotten from Fr=μ x R (friction=coeff. of static x normal reaction, where R=mg, opposite the weight of the body). Static friction depends on the weight of body and the nature of the surface, but not on the surface area of contact. When the resultant force acting on a particle is zero, it is in dynamic equilibrium (constant velocity or at rest).
Which of the following statements about static friction is correct? It
Static friction depends on the weight of the body. It also depends on the nature of the surface - a smooth glass surface has a smaller static friction than a rough wooden surface, but it does not depend on the surface area of contact.
Two cars X and Y travelling in opposite directions along the same highway at uniform velocities 110 kmh⁻¹ and 90 kmh⁻¹ respectively pass each other at a certain point. The velocity of X relative to Y at the time they pass each other is
Since they are moving in opposite directions, the relative velocity is a vector quantity given by the sum of both velocities = (110+90)km/hr = 200km/hr.
The average speed of an object is determined by dividing the sum of its initial and final speeds by two only when the acceleration is
When the acceleration is uniform, the average speed is determined by directly summing its initial and final speeds by two.
A body is projected horizontally from the top of a cliff 45m above the ground. If the body lands at a distance 30m from the foot of the cliff, calculate the speed of projection. [g = 10 ms⁻²]
Given: Height of cliff, H=45m; Range, R=30m; speed of projection=ux (horizontal projection). From s=ut+½gt²: vertical motion gives s=H=½gt²; 45=½x10xt²; t²=(45x2)/10=9, t=3s. Horizontal resolution: s=uxt+½x0xt²; R=uxt; 30=uxx3; ux=10ms⁻¹. (Note: the official answer key states C, 20ms⁻¹; this may reflect a slightly different setup in the original figures.)
Which of the following statements best describes a rigid body? A body which
A rigid body is one that is not easily deformed.
A simple harmonic oscillator has a period of 0.02 s and amplitude of 0.25 m. Calculate the speed in ms⁻¹ at the centre of the oscillation.
Given: Period, T=0.025 (approx 0.02s); Amplitude, A=0.25m; velocity(v)=ωA and ω=2πf where f=1/T=1/0.02=50Hz; thus v=ωA=2πfA=25π m/s.
A vertical string, suspended from a fixed point and having a small mass attached to the free end, is set into oscillations. Which of the following statements about the system are correct? I. The potential energy of the mass is minimum at the middle of the swing. II. Its kinetic energy is maximum at the middle of the swing. III. The sum of the potential and kinetic energies is constant throughout the swing.
At the middle (equilibrium point) of the swing, PE is minimum and KE is maximum. The total mechanical energy (PE+KE) is constant throughout the motion by conservation of energy - all three statements are correct.
A moving body of mass 25.0kg undergoes a uniform retardation of 20.0 ms⁻². Calculate the magnitude of the retarding force.
Magnitude of retarding force = [mass of the moving body] x [retardation] = 25 x 20 = 500N.
Which of the following sources of energy is/are exhaustible? I. Solar II. Fossil fuels III. Tidal power
Both solar and tidal energy sources are renewable and so are inexhaustible. Fossil fuel is non-renewable and exhaustible.
A machine is said to be a third class lever when the
For a third class lever, the effort is between the load and the fulcrum, e.g. sugar tongs, forearm (M.A and V.R are less than 1, i.e. mechanical disadvantage).
A thermometer records 680 mmHg at steam point and 440 mmHg at ice point. The temperature it records at 380mmHg is
Using (100-X)/(100-0) = (680-380)/(680-440): 100-X = ((680-380)x100)/(680-440) = (300x100)/240 = 125; X = 100-125 = -25°C. (Note: the official answer key states B, -20°C.)
The advantages of the thermoelectric thermometer include the following characteristics except that it
The thermoelectric thermometer (thermocouple) is very sensitive and not fairly sensitive - that is why it can measure rapidly changing or momentary temperatures.
In a thermos flask, heat loss by radiation is minimized by the
In a thermos flask, the silvery walls prevent heat loss by radiation. The vacuum between the double walls prevents heat loss by conduction and convection.
The pressure P, volume V and absolute temperature T of a given mass of an ideal gas change simultaneously. Which of the following equations is correct about the gas?
If we divide through by PV, we obtain 1/T = constant/PV, which rearranges to give T/(PV) = constant.
Specific heat capacity of a body means that when 26.4 J heat energy is supplied to a body of
The specific heat capacity of a body is the quantity of heat required to raise/lower the temperature of the unit mass (1 kg) of the body by 1 Kelvin (1 K).
An electric drill rated 400W is used to drill a hole in copper of mass 400 g in 20 s. Calculate the rise in temperature if all the heat produced is absorbed by the copper. [Specific heat capacity of copper = 400 J kg⁻¹ K⁻¹]
Power = 400W, mass of copper = 400g = 0.4kg, time = 20s, rise in temperature = ? Heat absorbed by copper = power x time = 400x20 = 8000J. Heat absorbed = mass x specific heat capacity x ΔT: ΔT = Heat absorbed/(mass x C) = 8000/(0.4x400) = 50°C.
A solid of mass 0.5kg initially at 50°C melts at 60°C. Calculate the quantity of heat required to melt it completely. [Specific heat capacity of the solid=2.0 KJ kg⁻¹ K⁻¹, Specific latent heat of fusion of solid=1.0KJ kg⁻¹]
Given: mass of the solid=0.5kg; initial temperature of solid=50°C; melting point of solid=60°C; specific heat capacity of solid=2000J/kg-K; specific latent heat of fusion of solid=1000J/kg. Heat required to melt the solid completely = (heat required to raise the temperature from 50°C to 60°C) + (heat required to melt the body at 60°C) = mcΔT+ml = (0.5x2000x(60-50)) + (0.5x1000) = 10,000+500 = 10,500J.
Dew point is not affected by
Dew point is affected by temperature, wind, and the amount of water vapour in the atmosphere, but not by atmospheric pressure.
A certain wave has a speed of 20 ms⁻¹. If the frequency of the wave is 0.25 Hz, calculate the distance between successive crests of the wave.
Given: speed of wave (v)=20 m/s; frequency of wave (f)=0.25Hz; wavelength=distance between successive crests. v=fλ => λ=v/f=20/0.25=80m.
The ability of a wave to spread around corners is called
The ability of a wave to spread around corners is called diffraction.
The following devices use plane mirrors in their operations, except
The periscope, sextant and kaleidoscope are applications of plane mirrors. Binoculars are made from lenses.
When an object is placed at the principal focus of a concave mirror, the location of the image formed is
Image of an object placed at the principal focus of a concave mirror is formed at infinity.
[Diagram: ray of light in air refracting into a denser medium] From the diagram, the correct equation for the refractive index n of the medium is
i=r=[law of reflection]. From the diagram, r+90°=180° (angles on a straight line); r+r2=180°-90°=90° => r2=90°-i. The refractive index of medium = sini/sinr2 = sini/sin(90°-i) = sini/cosi = tani. (Note: the official answer key states B, n=sini/sin90°.)
A ray of light travels from air to water. The refractive index of water is given by the expression
For a ray of light travelling from air to water, na/nw = sini/sinr = velocity in air/velocity in water = real depth/apparent depth = wavelength in air/wavelength in water.
A lens that is thinner at the middle is a A. diverging lens B. converging lens C. plano-convex D. converging meniscus
A diverging (concave) lens is thinner at the centre than at the edges.
Which of the following electromagnetic waves has the longest wavelength?
Radio waves have the longest wavelength and lowest frequency among electromagnetic waves.
The amplitude of a sound wave determines its
The amplitude of a sound wave determines its loudness.
As the air column of length L in an air pipe decreases, the frequency of the stationary wave emitted
As the length of the air column, L, in an air pipe decreases, the frequency of the stationary wave emitted increases (frequency is inversely proportional to length).
A positively charged glass rod is placed near the cap of a positively charged electroscope. The divergence of the leaf is observed to
When a positively charged rod is brought near a positively charged (like-charged) electroscope, the divergence of the leaf increases due to electrostatic repulsion/induction.
Capacitors are used in the following devices, except
Capacitors are not used in electric iron. (Note: per the source, capacitors are not used in water pumping machines nor electric irons in some contexts; the official answer given is A.)
[Diagram: circuit with 6Ω and 4Ω resistors in parallel, and a 10Ω resistor, connected to a 12V battery] Calculate the current, I.
The 6Ω and 4Ω resistors are in parallel connection, so their resultant resistance is: 1/R46 = 1/6+1/4 = 0.4167, R46=2.4Ω. There is now the 10Ω resistor in series with this. Total resistance RT = (2.4+10)Ω = 12.4Ω. From V=IR: I=V/RT=12/12.4=0.97A.
[Same circuit as above] Determine the potential difference, V, across the parallel resistors.
The total potential = (Potential across the parallel conductor) + (Potential across the 10Ω resistor). Potential across the 10Ω resistor V=IR=0.97x10=9.7V. Therefore, the potential across the parallel resistors (12-9.7)V = 2.3V, or equivalently V46=IxR46=0.97x2.4=2.3V.
A galvanometer of internal resistance 50Ω and full scale deflection of 20mA is converted into a voltmeter by connecting a resistor of resistance 1950Ω in series to it. Calculate the maximum voltage that can be read by the voltmeter.
Given: Rg=Resistance of the galvanometer=50Ω; current in circuit=20mA=0.02A; Resistance(R) of the resistor=1950Ω; voltage read by voltmeter=V=IRG+IR=I(RG+R)=(0.02x50)+(0.02x1950)=1+39=40V.
In the wiring of houses, the fuse is connected to the wire coloured
In the wiring of houses, the fuse is connected to the live wire, which is coloured brown, and in some cases red.
Which of the following factors does not affect the electric resistance of a wire?
The electric resistance of a wire does not depend on the mass of the wire, but it depends on (i) temperature of wire (ii) length of wire (iii) cross-sectional area of wire.
Which of the following factors will increase the sensitivity of a moving coil meter?
Soft iron core increases the sensitivity of a moving coil meter.
In order to prevent bar magnets from losing their strength, they are stored with keepers such that the two bars are placed with A like poles facing each other and the keepers placed at their ends. B. like poles facing each other and the keepers placed between them.
In order to prevent bar magnets from losing their strength, they are stored in pairs with keepers such that the two bars are placed with unlike poles facing each other and the keepers placed between them.
The resonant frequency of an a.c. circuit is 1000 kHz. If each of the capacitance and inductance in the circuit is reduced by 50% and no other changes are made, the resonant frequency will become
f0=1/(2π√LC). f0∝1/√LC. Given f1=1000kHz, L1=L, C1=C; L2=L/2, C2=C/2 (both reduced by 50%). 1000√(L1C1)=f2√(L2C2) => f2 = 1000√(L1C1)/√(L2C2) = 1000x√(LC)/√((L/2)(C/2)) = 1000x2=2000kHz.
The energy, E stored in an inductor of inductance L when current I passes through it is given by the equation
The energy stored in an inductor is given as E=½LI², where I=current and L=Inductor of an inductance.
Eddy currents can be reduced by
Eddy currents can be reduced by creating holes or laminating (slots) in the metal plates.
An electron moves with a speed of 2.0x10⁶ ms⁻¹ in a straight line. Calculate the wavelength of the electron wave. [mass of an electron=9.1x10⁻³¹kg] [Planck's constant=6.6x10⁻³⁴Js]
Using de Broglie's equation, λ=h/mv = 6.6x10⁻³⁴/(9.1x10⁻³¹x2x10⁶) = 3.63x10⁻¹⁰m.
Oxidation of the filament in a light bulb is prevented by the introduction of
Argon or nitrogen is used to prevent the oxidation of the filament in light bulbs.
In a cathode ray tube, the function of the X-plates is to
The function of the X-plates in a cathode ray tube is to deflect the electron beam horizontally.
A nuclide represented as ⁷⁰₃₂X has a neutron-proton ratio of
70=[number of neutrons]+proton, and 32=no of protons. Therefore, number of neutrons=70-32=38. Ratio of neutron to proton = number of neutrons/number of protons = 38/32 = 1.2.
Neutrons are used to induce artificial radioactivity because they
Neutrons are used to induce artificial radioactivity because they have no charge, so they can penetrate the nucleus without being repelled by it (unlike positively charged particles being repelled by the positive nucleus).
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