2000 - JAMB Physics Past Questions and Answers - page 3
21
A projection lantern is used to give the image of a slide on a screen. If the image is 24 times as large as the slide and the screen is 72.0m from the projecting lens, what is the position of the slide from the lens?
A
0.3m
B
3.0m
C
3.5m
D
4.0m
correct option: b
V = 72m
U = Magnification = 24
But Magnification = V/U
∴ 24 = 72/U
U = 72/24
U = 3.0m
Users' Answers & CommentsU = Magnification = 24
But Magnification = V/U
∴ 24 = 72/U
U = 72/24
U = 3.0m
22
An electron of charge 1.6 x 10-19C and mass 9.1 x 10-31kg is accelerated between two metal plates with a velocity of 4 x 107ms-1, the potential difference between the plates is
A
4.55 x 103V
B
4.55 x 102V
C
9.10 x 101V
D
4.55 x 101V
correct option: a
When a charged particle is accelerated across a region of p.d. V Volt, the work done is the product of the charge Q and the p.d. V
i.e. Work done = Q x V
But this work done is equal to the K.E. imparted to the charge given by K.E = 1/2MV2, where V is the velocity of the charge = Q x V = 1/2MV2
∴ p.d V = (1/2MV2)/Q
= 1/2 x 9.1-31 x [4 x 107]2/1.6 x 10-19
= 8 x 9.1 x 102/1.6
= 4550V
= 4.55 x 103V
Users' Answers & Commentsi.e. Work done = Q x V
But this work done is equal to the K.E. imparted to the charge given by K.E = 1/2MV2, where V is the velocity of the charge = Q x V = 1/2MV2
∴ p.d V = (1/2MV2)/Q
= 1/2 x 9.1-31 x [4 x 107]2/1.6 x 10-19
= 8 x 9.1 x 102/1.6
= 4550V
= 4.55 x 103V
23
A cell of internal resistance 0.01Ω can be measured accurately using the
A
Ohm meter
B
metre bridge
C
potentiometer
D
electroscope
correct option: c
Users' Answers & Comments24
What optical instrument can best be constructed with converging lenses of focal lengths 50cm and 5cm?
A
Compound microscope
B
Galileo's telescope
C
Terrestrial telescope
D
Astronomical telescope
correct option: d
Users' Answers & Comments25
in a pure semiconductor, the number of electrons in the conduction band is
A
less than the number of holes in the valence band
B
equal to the number of holes in the valence band
C
greater than the number of holes in the valence band
D
twice the number of holes in the valence band
correct option: b
Users' Answers & Comments26
In a thermonuclear reaction, the total initial mass is 5.02 x 10-27kg and the total final mass is 0.01 x 10-27kg. The energy released in the process is
[c = 3.0 x 108ms-1]
[c = 3.0 x 108ms-1]
A
9.0 x 10-13J
B
9.0 x 10-12J
C
9.0 x 10-11J
D
9.0 x 10-10J
correct option: a
In thermonuclear reactions,
Energy = ∆mC2
∆m = Change in Mass
C = Speed of light
∴ 5.02 x 10-27 - 5.01 x 10-27 x [3 x 108]2
0.01 x 10-27 x 9 x 1016
E = 9.0 x 10-13J
Users' Answers & CommentsEnergy = ∆mC2
∆m = Change in Mass
C = Speed of light
∴ 5.02 x 10-27 - 5.01 x 10-27 x [3 x 108]2
0.01 x 10-27 x 9 x 1016
E = 9.0 x 10-13J
27
A gas would serve as an electrical conductor under
A
reduced pressure and high current
B
increased magnetic field
C
reduced pressure and reduced potential
D
exposure to visible light
correct option: a
In general, gases conduct electricity under reduced pressure and high voltage/potential.
But from Ohms Law, current is directly proportional to voltage. Thus high current presupposes high voltage.
Users' Answers & CommentsBut from Ohms Law, current is directly proportional to voltage. Thus high current presupposes high voltage.
28
An electric generator with a power output of 3.0KW at a voltage of 1.5KV distributes power along cables of resistance 20.0Ω. The power loss in the cables is
A
80.0W
B
40.0W
C
10.0W
D
0.1W
correct option: a
Power input = 3KW = 3000W
Voltage = 1.5KV = 1500V
Current (I) = P/V = 3000/1500 = 2Amps.
∴ Power loss in the cables = I2R
= 2 x 2 x 20
= 80W
Users' Answers & CommentsVoltage = 1.5KV = 1500V
Current (I) = P/V = 3000/1500 = 2Amps.
∴ Power loss in the cables = I2R
= 2 x 2 x 20
= 80W
29
If the frequency of an emitted x-ray is 1.6 x 1016Hz, the accelerating potential is?
[e = 1.6 x 10-19C, h = 6.63 x 10-34Js]
[e = 1.6 x 10-19C, h = 6.63 x 10-34Js]
A
6630.0V
B
663.0V
C
66.3V
D
6.6V
correct option: c
Energy imparted by the accelerating system = e x V, where V is the accelerating potential; and this energy is equal to the energy of the X-ray, give by hf
e x V = hf
∴ V = hf/e
(6.63 x 10-34 x 1.6 x 10-16)/1.6 x 10-19
= 6.63 x 10 = 66.3V
Users' Answers & Commentse x V = hf
∴ V = hf/e
(6.63 x 10-34 x 1.6 x 10-16)/1.6 x 10-19
= 6.63 x 10 = 66.3V
30
If the fraction of the atoms of a radioactive material left after 120years is 1/64, what is the half-life of the material?
A
24 years
B
20 years
C
10 years
D
2 years
correct option: b
1/64 is the usual fraction left of any radio active material after 4 half-lives, since 1/64 = (1/2)6
This implies that 6 half lives have elapsed.
Thus 6 half lives = 120 years
∴ 1 half life = 120/6 = 20 years
Users' Answers & CommentsThis implies that 6 half lives have elapsed.
Thus 6 half lives = 120 years
∴ 1 half life = 120/6 = 20 years