All 20 questions from the Post-UTME Screening (Post-UTME) Physics 2017 Objective paper, with the correct answer and a full explanation for each. Free, no signup needed.
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The boiling point of water exceeds 100C0 when
Boiling occurs when vapour pressure equals atmospheric pressure; a higher boiling point corresponds to needing a higher (greater than standard atmospheric) vapour pressure to boil, i.e. at increased external pressure.
The main function of the steam-trap incorporated in the apparatus for determining the specific latent heat of vaporization of water is to
The steam trap prevents already-condensed water (rather than dry steam) from passing through and being wrongly counted in the measurement.
Which of the following expressions correctly defines relative humidity?
Relative humidity is the ratio of the actual density (or partial pressure) of water vapour present to the density (or pressure) of saturated water vapour at the same temperature, expressed as a percentage.
The coils and fins at the back of a refrigerator are painted black so that the refrigerator
Black surfaces are better emitters of heat by radiation, helping the coils/fins dissipate heat more effectively.
Use the diagram above (a stationary wave pattern showing displacement against distance, with a marked span of 21m) to answer this question. The diagram above represents a stationary wave. Determine the wavelength
Based on the number of loops shown across the marked 21m span, the wavelength works out to 12m. (Note: this question depends on a value that was not fully legible in the source scan; the answer given is a best estimate.)
The frequency of a tuning fork is 400Hz and the velocity of sound in air is 320ms-1. How far does the sound travel when the fork executes 30 cycles?
Time for 30 cycles = 30/400 = 0.075s. Distance = v x t = 320 x 0.075 = 24m.
In which of the following media will the speed of sound be least?
Sound travels slowest through gases (like air) compared to solids such as lead, copper or wood.
The wave theory of light CANNOT be used to explain
The photoelectric effect could not be explained by the wave theory of light and required the particle (photon/quantum) theory.
The pitch of a musical note depends on its
The pitch of a musical note is determined by its frequency.
The image formed by a pinhole camera is
A pinhole camera forms a real, inverted image on the screen.
An object is placed at a distance of 20cm in front of a concave mirror. If the magnification is I (unity), what is the focal length of the mirror?
For unit magnification with a real, inverted image, the object is at the centre of curvature (u=2f=20cm), giving f=10cm.
An object at the bottom of a sheet of glass of thickness 6.6mm appears to be displaced by 2.1mm when viewed from above. What is the refractive index of the glass?
Apparent displacement = t(1-1/n). 2.1 = 6.6(1-1/n). Solving gives n approximately 1.47.
Which of the following is NOT true of total internal reflection?
Total internal reflection only occurs when the angle of incidence exceeds the critical angle, not at all angles of incidence, making this statement false.
The image formed on the retina of a normal eye is
The eye's lens forms a real, inverted image on the retina, which the brain then interprets as upright.
Which of the following electromagnetic radiations has the longest wavelength?
Radio waves have by far the longest wavelength in the electromagnetic spectrum among those listed.
The force between two equal point charges of magnitude 2.0uC is equal to 0.4N. Calculate the distance between the charges.
F = kq1q2/r^2. 0.4 = 9x10^9 x (2x10^-6)^2/r^2. r^2 = 0.036/0.4 = 0.09, so r = 0.3m = 30cm.
Use the diagram above (three capacitors C1, C2 and C3 in series with a 24V battery) to answer this question. In the diagram, three capacitors C1, C2 and C3 are connected in series to a 24V battery. If the capacitances are respectively equal to 2F, 3F and 6F, calculate the potential difference across the capacitor labelled C2.
Series capacitance: 1/C=1/2+1/3+1/6=1, so C=1F. Charge Q=CV=1x24=24C. V2=Q/C2=24/3=8V.
The angle of dip is equal to
The angle of dip is 90 degrees at the magnetic poles (where the field is vertical) and 0 degrees at the magnetic equator.
Use the diagram above (a magnetic field pattern between two poles, with points labelled m, n, p, t) to answer this question. In the diagram above, neutral points occur at
Based on the symmetry of the field pattern shown, the neutral points (where the resultant field is zero) occur at m and p.
A step-down transformer is connected to a mains of 240V. If the secondary coil is 10 000 turns, what is the number of turns in the primary coil when the output voltage is 24V?
Np/Ns = Vp/Vs. Np = Ns x Vp/Vs = 10000 x 240/24 = 100000.
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