2003 - JAMB Physics Past Questions and Answers - page 4
31
Which of the following pairs of light rays show the widest separation in the spectrum of white light
A
Blue and red
B
Green and yellow
C
Indigo and violet
D
Orange and red
correct option: a
From the above diagram the widest separation is given by Blue and red
Users' Answers & Comments32
The most suitable cell used for short interval switches in electric bells is a
A
lead- acid accumulator
B
Daniel cell
C
Leclanche cell
D
nickel iron accumulator
correct option: c
Users' Answers & Comments33
A ray incident on a glass prism undergoes minimum deviation when the
A
incident angle is equal to the angle of refraction
B
refraction angle equals 90o
C
incident angle equals 90o
D
incident angle is equal to the angle of emergence
correct option: d
At minimum deviation, it is observed that light passes symmetrically through, the prism such that the angle of incidence (io) is angle to the angle of emergency (eo).(i/e Io = eo)
Users' Answers & Comments34
The count rate of a radioactive material is 800 count/min. If the half-life of the material is 4 days, what would the count rate be in 16 days later
A
200 count / min
B
100 count / min
C
50 count / min
D
25 count / min
correct option: c
116 days = 4 half - lives
Thus, after 4 half lives the fraction left of any radioactive materials is given by (1/2)4 = 1/16
∴ Count rate / amount left = 1/16 x 800/1
= 50 count / min
Users' Answers & CommentsThus, after 4 half lives the fraction left of any radioactive materials is given by (1/2)4 = 1/16
∴ Count rate / amount left = 1/16 x 800/1
= 50 count / min
35
A circuit has an area of 0.4m2 and consists of 50 loops of wire. If the loops are twisted and allowed to rotate at a constant angular velocity of 10 rad s-1 in a uniform magnetic field of 0.4T, the amplitude of the induced voltage is
A
8V
B
16V
C
20V
D
80V
correct option: d
The induced voltage = time rate of change of magnetic flux
But ∅B = Magnetic Flux
= Magnetic Induction (B) x (A)
= 0.4T x 0.4m2
= 0.16 volt second
= 0.16 x 10 x 50
= 80 volts
Users' Answers & Comments| i/e E = - | NΔ∅B |
| Δt |
But ∅B = Magnetic Flux
= Magnetic Induction (B) x (A)
= 0.4T x 0.4m2
= 0.16 volt second
| i/e E = - | NΔ∅B |
| Δt |
= 0.16 x 10 x 50
= 80 volts
36
The force on a charge moving with velocity v in a magnetic field B is half of the maximum force when the angle between v and B is
A
90o
B
45o
C
30o
D
0o
correct option: c
the force on a moving charge in a magnetic field is given by: F = q V β sin θ
and this force is maximum when θ = 90o, during which sin 90o = 1: and for sinθ = 0.5, θ = 30o
Users' Answers & Commentsand this force is maximum when θ = 90o, during which sin 90o = 1: and for sinθ = 0.5, θ = 30o
37
The maximum kinetic energy of the photoelectron emitted from a metal surface is 0.34eV. If the work function of the metal surface is 1.83eV,find the stopping potential.
A
2.17V
B
1.49V
C
1.09V
D
0.34V
correct option: d
The stopping potential is the potential required to stop the most energetic photo-emitted electron. From the relation e Vs = K.E,(max) where e is the electron charge, and Vs is the stopping potential => e x Vs = 0.34eV
∴ Vs = (0.34eV) / e = 0.34V
∴ stopping potential = 0.34V
Users' Answers & Comments∴ Vs = (0.34eV) / e = 0.34V
∴ stopping potential = 0.34V
38
When an alternating current given by I = 10sin (120π)t passes through a 12Ω resistor, the power dissipated in the resistor is
A
1200W
B
600W
C
120W
D
30W
correct option: a
An a.c current is usually expressed as I = Io sin wt; and comparing this with the given current I = 10sin (120π)t, => Io = 10.
and power in an a.c circuit is given as:
P = I2R
= (102 x 12)
= 1200W
Users' Answers & Commentsand power in an a.c circuit is given as:
P = I2R
= (102 x 12)
= 1200W
39
In a semi conductor junction diode, as the depletion or barrier layer is forward-biased, the layer
A
widens
B
narrows
C
remains constant
D
widens then narrows
correct option: c
Users' Answers & Comments40
The major difference between a pure semiconductor and a pure metal is that
A
metals are harder than semiconductor
B
while resistance of metals decrease with temperature , the reverse is the case for semiconductor
C
the resistance of metals increases with temperature, while for semiconductors, it is the reverse
D
metals have forbidden gaps while semiconductors have not
correct option: c
Experiment shows that increase in temperature increases the resistance of a pure metal but decreases the resistance of pure semiconductor
Users' Answers & Comments