J = 1.45 x 10
6
x 200
Or J = 2.9 x 10
8
Am
2
Hence, A is the correct answer option.
Question 13
A solenoid of length 60 cm and radius 2.4 cm carries current of 3 A. If the number of turns is
400, what is the magnetic field at the center of the solenoid on its axis? (Permeability of free
space is 4π x 10
-7
WbA
-1
m
-1
)
A. 2.5 x 10
-3
T
B. 1.5 x 10
-3
T
C. 4.5 x 10
-4
T
D. 5.2 x 10
-4
T
E. 1.8 x 10
-4
T
Correct Answer: A
Explanation:
Magnetic field at the center of a solenoid is given as
B = (μ
o
nI) / l ------------- (1)
As length is given in cm we need to convert it into m.
1 cm = 10
-2
m
60 cm = 0.6 m
Putting the values in equation 1 we get
B = [(4π x 10
-7
* 400 * 3) / 0.6]
(π = 3.14 )
B = 2.5 x 10
-3
T
Hence, A is the correct answer option.
Question 14
What would be the minimum angle of incidence so that a ray of light travelling from medium 1
of refractive index 1.7 to medium 2 of refractive index 1.5 does not emerge out of medium 2?
A. sin
-1
(0.88)
B. cos
-1
(0.88)
C. tan
-1
(0.88)
D. tan
-1
(1.13)
E. sin
-1
(1.13)
Correct Answer: A
Explanation:
For a ray of light incident on a medium to not emerge from it is known as limiting case of total internal
reflection. For the minimum angle the refracted ray travels along the interface. For the incident ray
making an angle more than the minimum angle the ray will reflect back and this is known as total
internal reflection. From Snell’s law we have
μ
1
sin θ
i
= μ
2
sin θ
r
---------------- (1)