1998 - JAMB Physics Past Questions and Answers - page 4
31
A man wears convex lens glasses of focal length 40cm in order to correct his eye defect. instead of the optimum 25cm, his least distance of distinct vision is
A
14cm
B
28cm
C
75cm
D
150cm
correct option: d
F = \(\frac{uv}{u + v}\)
F = -ve
v = -ve
- 30 = \(\frac {25 {\times} (-v)}{25 + (-v)}\)
v = 150cm
Users' Answers & CommentsF = -ve
v = -ve
- 30 = \(\frac {25 {\times} (-v)}{25 + (-v)}\)
v = 150cm
32
Which of the following are true about the infra-red radiation? i. it is invisible ii. it is called heat ray iii. its frequency is higher than that of blue light iv. it travels as a transverse wave
A
i, ii, iii and iv
B
i, ii, iv only
C
i, iii, iv only
D
ii, iii, iv only
correct option: b
Users' Answers & Comments33
The force of repulsion between two point positive charges 5\(\mu\)C and 8\(\mu\)C separated at a distance of 0.02 map art is. [\(\frac{1}{4{\pi}{\varepsilon}_o}\) = 9 x 109Nm2C-2]
A
1.8 x 10-10N
B
9.0 x 10-8N
C
9.0 x 102N
D
4.5 x 103N
correct option: c
F =\(\frac{1}{4{\pi}{\varepsilon}_o}\) x \(\frac {q_1q_2}{r^2}\)
F =\(\frac {5 {\times} 10 {\times} 8 {\times} 10}{0.002 {\times} 0.002}\) x 9 x 10
F = 900N
Users' Answers & CommentsF =\(\frac {5 {\times} 10 {\times} 8 {\times} 10}{0.002 {\times} 0.002}\) x 9 x 10
F = 900N
34
An ebonite rod rubbed with fur attracts a glass rod rubbed with silk because
A
ebonite has a negative charge while glass has a positive charge
B
ebonite has a positive charge while glass has a negative charge
C
both have negative charges
D
both have positive charges
correct option: a
Users' Answers & Comments35
Which of the following can be used to reduce local action in a Laclanche cell?
A
A carbon rod as the positive pole
B
Pure zinc as the negative pole
C
Potassium permanganate solution in contact with the positive pole
D
common salt solution
correct option: c
Users' Answers & Comments36
Find the work done in moving a 2 C charge between two points X and Y in an electric field if the potential difference is 100 volts
A
50J
B
100J
C
200J
D
400J
37
A parallel plate capacitor has a common plate area of 5 x 10-8m2 and plate separation of 2 x 103m, Assuming free space, What is the capacitance? [\(\varepsilon\)o = 9.0 X 10-12C2N-1m-2]
A
2.25 x 10-17F
B
4.50 x 10-17F
C
2.25 x 10-16F
D
4.50 x 10-16F
correct option: c
C = \(\frac{{\varepsilon}_oA}{d}\)
= \(\frac{5{\times} 10^{-8} {\times} 10^{-12}{\times} 9}{2 {\times}10^{-3}}\)
= 22.5 x 10-17F
Users' Answers & Comments= \(\frac{5{\times} 10^{-8} {\times} 10^{-12}{\times} 9}{2 {\times}10^{-3}}\)
= 22.5 x 10-17F
38
The iron core of an induction coil is made from bundles of wire so as to
A
minimize eddy-currents
B
generate eddy-current
C
prevent sparking at the contact breaker
D
get the greatest posible secondary voltage
correct option: a
Users' Answers & Comments39
Electricity is supplied to a school along a cable of total resistance 0.5\(\Omega\) with the maximum current drawn from the mains as 100A. The maximum energy dissipated as heat for 1 hr is
A
3.6 x 103J
B
5.0 x 103J
C
3.0 x 105J
D
1.8 x 107J
correct option: d
Elect. energy = I2Rt
= 100 x 100 x 0.5 x 3600
= 1.8 x 107J
Users' Answers & Comments= 100 x 100 x 0.5 x 3600
= 1.8 x 107J
40
A transformer which can produce 8V from a 240v a.c. supply, has an efficiency of 80%. If the current in the secondary coil is 15A, calculate the current in the primary coil.
A
0.625A
B
1.600A
C
2.500A
D
6.250A