2017 - JAMB Physics Past Questions and Answers - page 3
21
Which of the following operations can be used to convert an alternative current dynamo into a direct current dynamo
A
number of turns in the coil is increased
B
strength of the field magnet is increased
C
slip rings are replaced with split ring cummutator
D
coil is wound on a soft iron armature
correct option: c
A d.c generator is one in which its current is allowed to flow in one direction even through it may vary in value an a.c generator can only be made to produce a d.c by replacing the two slip rings with a single split ring or commutator.
Users' Answers & Comments22
A
80µF
B
6.0µF
C
4.0µF
D
2.0µF
correct option: d
For capacitance i n parallel, 2µF and 2µF are in parallel,
their equivalence is 2µF and 2µF = 4µF
The 4µF generated is now in series with the remaining 4µF.
The net capacitance for series connection is
\(\frac{1}{C}\) = \(\frac{1}{4}\) + \(\frac{1}{4}\) = 1 + \(\frac{1}{4}\) = \(\frac{2}{4}\)
C = \(\frac{4}{2}\)
= 2µF
Users' Answers & Commentstheir equivalence is 2µF and 2µF = 4µF
The 4µF generated is now in series with the remaining 4µF.
The net capacitance for series connection is
\(\frac{1}{C}\) = \(\frac{1}{4}\) + \(\frac{1}{4}\) = 1 + \(\frac{1}{4}\) = \(\frac{2}{4}\)
C = \(\frac{4}{2}\)
= 2µF
23
The instrument used to measure the pressure of a gas is
A
barometer
B
thermometer
C
hygrometer
D
manometer
correct option: d
Barometer is used to measure atmosphere pressure
Thermometer is used to measure temperature
Hygrometer is used to measure relative density of a liquid
Manometer is used to measure the pressure of a gas
Users' Answers & CommentsThermometer is used to measure temperature
Hygrometer is used to measure relative density of a liquid
Manometer is used to measure the pressure of a gas
24
A man of mass 50kg ascends a flight of stains 5m high in 5 seconds. If acceleration due to gravity is 10ms\(^{-2}\) the power expended is
A
100w
B
200w
C
400w
D
500w
correct option: d
Power is the time rate of doing work
i.e power = \(\frac{\text{energy}}{\text{time}}\)
Since energy = mgH
∴ power = \(\frac{mgH}{t}\)
= \(\frac{50 \times 10 \times 5}{t}\)
= 500W
Users' Answers & Commentsi.e power = \(\frac{\text{energy}}{\text{time}}\)
Since energy = mgH
∴ power = \(\frac{mgH}{t}\)
= \(\frac{50 \times 10 \times 5}{t}\)
= 500W
25
A radio station broadcast at a frequency of 600KHZ. If the speed of light in air is 3 × 108ms\(^{−1}\). Calculate the wavelength of the radio wave
A
2.20 × 10\(^3\)m
B
5.0 × 10\(^2\)m
C
5.0 × 10\(^5\)m
D
11.8 × 10\(^{11}\)m
correct option: b
V = fλ
λ = \(\frac{v}{f}\)
= \(\frac{3 \times 10^8}{600 \times 10^3}\) = 500m
= 5 × 102m
N.B 600KHz Frequency = 600 × 103Hz
Users' Answers & Commentsλ = \(\frac{v}{f}\)
= \(\frac{3 \times 10^8}{600 \times 10^3}\) = 500m
= 5 × 102m
N.B 600KHz Frequency = 600 × 103Hz
26
Which of the following is a scalar quantity?
A
Tension
B
Weight
C
Impulse
D
Mass
correct option: d
Scalar quantity is a quantity with only magnitude, it has no direction
Tension, weight, impulse are vector quantities because they have direction. Mass is the only scalar quantity there
Users' Answers & CommentsTension, weight, impulse are vector quantities because they have direction. Mass is the only scalar quantity there
27
Determine the focal length of a thin converging lens if the power is 5.0 dioptres
A
0.1 m
B
0.2 m
C
2.0 m
D
2.5 m
correct option: b
Power of a lens = \(\frac{1}{f}\)
f = focal length
f = \(\frac{1}{p}\)
= \(\frac{1}{5}\)
= 0.2m
Users' Answers & Commentsf = focal length
f = \(\frac{1}{p}\)
= \(\frac{1}{5}\)
= 0.2m
28
A string of length 5cm is extended by 0.04m when a load of 0.8kg is suspended at the end. How far will it extend if a force of 16N is applied? [g = 10ms\(^{-2}\)]
A
0.04m
B
0.12m
C
0.01m
D
0.08m
correct option: d
From Hoke's Law, F = ke, K = Constant of Force
For he first case, Force = mg,
= 0.8 × 10 = 8N
∴ 8 = k × 0.04
k = \(\frac{8}{0.04}\)
= \(\frac{200N}{m}\)
Since K is constant
For the second case, F = ke
F = 200 × e
∴ e = \(\frac{F}{200}\)
= \(\frac{16}{200}\)
= 0.08m
Users' Answers & CommentsFor he first case, Force = mg,
= 0.8 × 10 = 8N
∴ 8 = k × 0.04
k = \(\frac{8}{0.04}\)
= \(\frac{200N}{m}\)
Since K is constant
For the second case, F = ke
F = 200 × e
∴ e = \(\frac{F}{200}\)
= \(\frac{16}{200}\)
= 0.08m
29
How can energy loss be minimized through Eddy-current?
A
By using high resistance wire
B
By using insulated soft iron wires
C
By using low resistance wires
D
Bu using turns of wires
correct option: a
Eddy currents are produced by the varying flux cathode, the iron core of an equipment thus reducing efficiency due to power consumption. It can be reduced by laminating the core by breaking up the path of eddy current or by increasing the resistance of the core
Usage of insulated soft iron wire is to reduce hysteresis loss
Usage of low resistance wire (thick wire) is to reduce I2R loss
Usage of thick wire is to reduce leakage heat loss due to leakage of magnetic flux
Thus the correct answer is to use high resistance wire or thin wire
Users' Answers & CommentsUsage of insulated soft iron wire is to reduce hysteresis loss
Usage of low resistance wire (thick wire) is to reduce I2R loss
Usage of thick wire is to reduce leakage heat loss due to leakage of magnetic flux
Thus the correct answer is to use high resistance wire or thin wire
30
A
4.0 x 10\(^{-2}\)J
B
4.0 x 10\(^{-4}\)J
C
8.0 x 10\(^{-4}\)J
D
8.0 x 10\(^{-2}\)J
correct option: b
c = 8µF, v = 10v
Energy stored = \(\frac{1}{2}\) cv2
= \(\frac{1}{2}\) × (8 × 10− 6) × 102
= 4 × 10\(^{-4}\)
Users' Answers & CommentsEnergy stored = \(\frac{1}{2}\) cv2
= \(\frac{1}{2}\) × (8 × 10− 6) × 102
= 4 × 10\(^{-4}\)