2008 - JAMB Physics Past Questions and Answers - page 4
31
The d.c generator has essentially the same components as the a.c generator except the presence of?
A
slip-ring
B
carbon brushes
C
split ring
D
armature
correct option: c
To correct and A.C generator with a D.C generator the slip ring is replaced with split-ring communicator
Users' Answers & Comments32
A step-down transformer has a power output of 50 W and efficiency of 80%. if the mains supply voltage is 200 v, calculate the primary current of the transformer.
A
0.31 A
B
3.20 A
C
3.40 A
D
5.00 A
correct option: a
Efficiency = (power output)/power input x 100%
= 1s VS x 100% 1p
Vp
∴ (80)/100 = (50)/IP x 200
I P = (50 x 100)/80 x 200 = 3.125 A
Users' Answers & Comments= 1s VS x 100% 1p
Vp
∴ (80)/100 = (50)/IP x 200
I P = (50 x 100)/80 x 200 = 3.125 A
33
caesium has a work function of 3 x 10-19J. the maximum energy of liberated electrons when it is illuminated by light of frequency 6.7 x 1014
HZ is?
[h = 6.6 x 1010-34Js]
HZ is?
[h = 6.6 x 1010-34Js]
A
1.42 x 10-19J
B
3.00 x 10-19J
C
4.42 x 1010-19J
D
7.42 x 10-19J
correct option: a
Max kinetic energy = Energy of incident electron - work function. EK = hf - w
= 6.6 x 10-34 x 6.7 x 1014 - 3 x 10-19
= 4.422 x 10-19 - 3 x 10-19
= 1.422 x 10-19J
Users' Answers & Comments= 6.6 x 10-34 x 6.7 x 1014 - 3 x 10-19
= 4.422 x 10-19 - 3 x 10-19
= 1.422 x 10-19J
34
zener diode is used for?
A
current amplification
B
power amplification
C
voltage regulation
D
energy conversion
correct option: c
Zener diode is useful in stabilized power supply i.e to keep steady the voltage of a power supply.
Users' Answers & Comments35
When a pure semiconductor is heated, its resistance
A
increases
B
decreases
C
remains the same
D
increases and then decreases
correct option: b
Heating a pure semiconductor increases the mobility of electrons and holes and hence the conductivity. thus, the resistance decreases on heating
Users' Answers & Comments36
I. for current amplification.
II. for voltage stabilization.
III. for power amplification.
IV. as a switch.
which of the above are the uses of transistor?
II. for voltage stabilization.
III. for power amplification.
IV. as a switch.
which of the above are the uses of transistor?
A
I,II,III and IV
B
I,III, and IV only
C
I, II and III only
D
I,II and IV only
correct option: b
Transistor is used for amplification of current, power and as a switch in logic gates.
Users' Answers & Comments37
The dimension of electromotive force are `
A
ML2T-3I-1
B
ML2T-3I-2
C
M2LT-2I-1
D
M2L2T-1I-1
correct option: a
Electrical work W = QE,
Where Q is the quantity of change and E is the electromotive force.
∴ E = W = FxDQ_It
Where D is the displacement,I = current and t = time
But dimension of force,
= unit of mass x unit of Acceleration
= unit of mass xunit of velocityunit of time
= unit of mass x unit of displacement__unit of time x unit of time
ML/T2 = MLT-2
∴ Dimension of emf E
= MLT-2L = ML2T-3I-1__IT
Users' Answers & CommentsWhere Q is the quantity of change and E is the electromotive force.
∴ E = W = FxDQ_It
Where D is the displacement,I = current and t = time
But dimension of force,
= unit of mass x unit of Acceleration
= unit of mass xunit of velocityunit of time
= unit of mass x unit of displacement__unit of time x unit of time
ML/T2 = MLT-2
∴ Dimension of emf E
= MLT-2L = ML2T-3I-1__IT
38
The resultant of two forces 12N and 5N is 13N. What is the angle between the two forces?
A
0o
B
45o
C
90o
D
180o
correct option: c
R2 - P2 + Q2 = 122 + 52
= 144 + 25 = 169
∴R = √169 = 13N
∴ angle between the forces is 90o
Users' Answers & Comments= 144 + 25 = 169
∴R = √169 = 13N
∴ angle between the forces is 90o
39
A force F required to keep a 5 kg mass moving round a cycle of radius 3.5 m at a speed of 7 ms-1. What is the speed, if the force is tripled?
A
4.0 ms-1
B
6.6 ms-1
C
12.1 ms-1
D
21.0 ms-1
correct option: c
The force which is centripetal
F = MV2_R
=5 x 723.5
= 70
V = √(Fr) and if F is tripped, then
V = √(3Fr)/m
= √(3x70x3.5)/5
= 12.1ms-1
Users' Answers & CommentsF = MV2_R
=5 x 723.5
= 70
V = √(Fr) and if F is tripped, then
V = √(3Fr)/m
= √(3x70x3.5)/5
= 12.1ms-1
40
A body of mass 4 kg resting on a smooth horizontal plane is simultaneously acted upon by two perpendicular forces 6N and 8N. Calculate the acceleration of the motion
A
2.5 ms-1
B
3.0 ms-1
C
4.0 ms-1
D
4.5 ms-1
correct option: a
Smooth plane has negligible friction
∴ only the resultant of the perpendicular forces is acting on the body, which is given as
F =
√ 82 + 62
=
√ 64 + 36
=
√ 100
= 10N
But F = Ma
∴ a = F/M = 10/4 = 2.5 ms-1
Users' Answers & Comments∴ only the resultant of the perpendicular forces is acting on the body, which is given as
F =
√ 82 + 62
=
√ 64 + 36
=
√ 100
= 10N
But F = Ma
∴ a = F/M = 10/4 = 2.5 ms-1