All 20 questions from the Post-UTME Screening (Post-UTME) Physics 2019 Objective paper, with the correct answer and a full explanation for each. Free, no signup needed.
Circuit diagram: two parallel branches (2 ohm with 2 ohm, and 4 ohm with 4 ohm) in series with a 1 ohm resistor, connected to a 4V battery. In the figure, the current through the 1-ohm resistor is
A. 4/9 A
B. 2/3 A
C. 1 ACorrect
D. 4 A
Explanation
Two 2-ohm resistors in parallel give 1 ohm; two 4-ohm resistors in parallel give 2 ohm. Total circuit resistance = 1+2+1 = 4 ohm. Current = V/R = 4/4 = 1A, and this full current flows through the series 1-ohm resistor.
The material of an electric wire of length 1.0m has a resistivity of 6.0x10^-6 ohm.m. If the cross-sectional area of the wire is 4.0x10^-3 cm^2, the resistance of the wire is
A. 1.50 x 10^1 ohmCorrect
B. 0.60 x 10^3 ohm
C. 0.15 x 10^2 ohm
D. 1.50 x 10^-3 ohm
Explanation
Convert area: 4.0x10^-3 cm^2 = 4.0x10^-7 m^2. R = (resistivity x length)/area = (6.0x10^-6 x 1.0)/(4.0x10^-7) = 15 ohm = 1.50x10^1 ohm.
Two equal capacitances are connected first in series and then in parallel. The ratio of the effective series capacitance to the effective parallel capacitance is
A. 1/4Correct
B. 1/2
C. 1
D. 4
Explanation
For two equal capacitors C: series capacitance = C/2; parallel capacitance = 2C. Ratio = (C/2)/(2C) = 1/4.
For long distance transmission of electrical power, the voltage is stepped up in order to (reduce the current and hence minimize I^2R power loss in the transmission wires)
A. reduce current and power loss in the transmission wiresCorrect
B. increase the resistance in the wire proportionately to the potential difference
C. increase the rate of electrical consumption
D. increase the current for brighter lights
Explanation
Stepping up the voltage for a given power reduces the current needed, which reduces I^2R (resistive/heat) power losses in the long transmission wires. (Note: option A's exact wording was cut off in the scan but is the physically correct completion; options B-D are incorrect.)
A pin at the bottom of a beaker filled with water appears to be at a depth of 9 cm when viewed vertically from above. If the refractive index of water is 4/3, calculate the real depth of the water in the beaker.
A. 6.75cm
B. 10.33cm
C. 12.00cmCorrect
D. 15.75cm
Explanation
Real depth = apparent depth x refractive index = 9 x 4/3 = 12.00cm.
Use the diagram above (a diamond/bridge arrangement of four resistors, each 100 ohm, between points P and Q) to answer this question. In the figure above, calculate the total resistance between P and Q if each of the resistors is 100 ohm
A. 50 ohm
B. 100 ohmCorrect
C. 250 ohm
D. 500 ohm
Explanation
In this symmetric bridge-like network of equal resistors, the equivalent resistance between P and Q works out to 100 ohm.
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