2010 - JAMB Physics Past Questions and Answers - page 4
31
In a discharge tube, most of the gas is pumped out so that electricity is conducted at
A
low voltage
B
steady voltage
C
high pressure
D
low pressure
correct option: d
pumping out of the gas in the discharge tube reduces the pressure inside the tube, therefore, electricity is conducted at low pressure
Users' Answers & Comments32
The ground stated energy for the hydrogen atom is 5.44 x 10-19J . If an electron drops from zero energy to ground state, calculate the frequency of the emitted radiation.
[h = 6.6 x 10-34 Js]
[h = 6.6 x 10-34 Js]
A
5.0 x 1016 Hz
B
2.0 x 1016 Hz
C
5.0 x 1015 Hz
D
2.0 x 1015 Hz
correct option: e
Correct Answer: Option E
∆E = hf
f = (∆E)/h = (5.44 x 10-19)/6.6 x 10-34 = 8.24 x 1014
Users' Answers & Commentsf = (∆E)/h = (5.44 x 10-19)/6.6 x 10-34 = 8.24 x 1014
33
Transistors are used for the
A
rectification of signals
B
conversion of a.c to d.c
C
amplification of signals
D
conversion of d.c to a.c
correct option: c
transistor serve the same purpose as triode valve, which is amplification of signals.
Users' Answers & Comments34
Which of the following is a pure semiconductor?
A
carbon
B
silicon
C
transistor
D
phosphorus
correct option: b
A semiconductor is a material whose conductivity is between good conductor and insulator. Example of pure semi conductors are silicon and germanium
Users' Answers & Comments35
A
has no acceleration between point F and G
B
decelerates between F and H
C
has a constant soeed between points E and F
D
Accelerates between points F and G
correct option: a
Above are the V-t graphs for
I Uniform acceleration
II uniform retardation
III Uniform velocity
Thus the car has no acceleration between point F and G since that portion of the graph represents uniform velocity
Users' Answers & CommentsI Uniform acceleration
II uniform retardation
III Uniform velocity
Thus the car has no acceleration between point F and G since that portion of the graph represents uniform velocity
36
A
III, IV and VIII
B
II, V and VIII
C
III, VI and IX
D
I, IV and VII
correct option: c
A bod y is said to be in stable equilibrium if after a slight displacement, the body returns to its former position. Thus, position III, VI and XI represents position of stable equilibrium
Users' Answers & Comments37
A bob of weight 0.1 N hangs from a massless string of length 50 cm. A variable horizontal force which increase from zero is applied to pull the bob until the string makes an angle of 60o with the vertical. The work done is
A
0.500 J
B
0.250 J
C
0.050 J
D
0.025 J
correct option: d
Length of the string
L = y + x = 0.5m
∴ x = L - y
= distance moved in the direction of force (which is the weight) when the body is released But cos 60o = y/0.5
∴ y = 0.5cos60 = 0.5 x 1/2 = 0.25
∴ x = l - y = 0.5 - 0.25 = 0.25m
Work done = force x distance moved in the direction of force.
= 0.1 x 0.25 = 0.0252J
Users' Answers & CommentsL = y + x = 0.5m
∴ x = L - y
= distance moved in the direction of force (which is the weight) when the body is released But cos 60o = y/0.5
∴ y = 0.5cos60 = 0.5 x 1/2 = 0.25
∴ x = l - y = 0.5 - 0.25 = 0.25m
Work done = force x distance moved in the direction of force.
= 0.1 x 0.25 = 0.0252J
38
Two metals P and Q of length l1 and l2 are heated through the same temperature difference. If the ratio of the linear expansivities of P to Q is 2:3 and the ratio of lengths is 3:4. What is the ratio of increase in length of P to Q?
A
7:5
B
5:7
C
1:2
D
2:1
correct option: c
Metals P, Q
original length lP, lQ
Increase in length ΔlP, ΔlQ
Linear expansivities αP, αQ
∴ αP/αQ = 2/3 ; and lP/lQ = 3/4
But increase in length, Δl = αlΔθ
∴ΔlP = αPlPΔθ
ΔlQ = αQlQΔθ
∴ Ratio of increase in length of P to Q = 1:2
Users' Answers & Commentsoriginal length lP, lQ
Increase in length ΔlP, ΔlQ
Linear expansivities αP, αQ
∴ αP/αQ = 2/3 ; and lP/lQ = 3/4
But increase in length, Δl = αlΔθ
∴ΔlP = αPlPΔθ
ΔlQ = αQlQΔθ
| ∴ | ΔlP | = | αPlPΔθ | = | 2 | x | 3 | = | 1 |
| ΔlQ | αQlQΔθ | 3 | 4 | 2 |
∴ Ratio of increase in length of P to Q = 1:2
39
THe velocity of sound in air at 16oC is 340ms-1. What will it be when the pressure is doubled and its temperature raised to 127oC?
A
400 ms-1
B
4,000 ms-1
C
8,000 ms-1
D
160,000 ms-1
correct option: a
The velocity of sound in air, V∝√T, where T is absolute temperature
Thus V = K√T or K = V/T
V is independent of pressure and density at a fixed temperature.
∴ V1/√T1 = V2/√T2
T1 = 16 + 273 = 289K, V1 = 340ms-1
T2 = 127 + 273 = 400K, V2 = ?
= 400m/s
Users' Answers & CommentsThus V = K√T or K = V/T
V is independent of pressure and density at a fixed temperature.
∴ V1/√T1 = V2/√T2
T1 = 16 + 273 = 289K, V1 = 340ms-1
T2 = 127 + 273 = 400K, V2 = ?
| 340 | = | V2 |
| √289 | √400 |
| ∴ V2 = | 340√400 |
| _√289 |
= 400m/s
40
Three capacitors of capacitance 2μF, 4μFand 8μF are connected in parallel and a P.D of 6V is maintained across each capacitor, the total energy stored is
A
2.52 x 10-6J
B
6.90 x 10-6J
C
2.52 x 10-4J
D
6.90 x 10-4J
correct option: c
C = C1 + C2 + C3
= 2 + 4 + 8 = 14μF
V = 6V
Energy = 1/2CV2
1/2 x 14 x 10-6 x 62
= 2.52 x 10-4 J
Users' Answers & Comments= 2 + 4 + 8 = 14μF
V = 6V
Energy = 1/2CV2
1/2 x 14 x 10-6 x 62
= 2.52 x 10-4 J