A radiation of energy E falls normally on a perfectly reflecting surface. The momentum transferred to the surface is
E/c
2E/c
B.
2E/c
Initial momentum of surface
p_{i} = E/c
where c = velocity of light (constant)
Since, the surface is perfectly so, the same momentum will be reflected completely
Final momentum
p_{f}= E/c
therefore
Change in momentum
∆p = p_{f} − pi
= -E/c - E/c = -2E/c
Thus, momentum transferred to the surface is
∆p' = |∆p| = 2E/c
Two rigid boxes containing different ideal gases are placed on a table. Box A contains one mole of nitrogen at temperature T_{0}, while Box B contains one mole of helium at temperature (7/3) T_{0}. The boxes are then put into thermal contact with each other and heat flows between them until the gases reach a common final temperature. (Ignore the heat capacity of boxes). Then, the final temperature of the gases, T_{f}, in terms of T_{0} is
D.
The temperature of two outer surfaces of a composite slab, consisting of two materials having coefficients of thermal conductivity K and 2K and thickness x and 4x, respectively are T_{2} and T_{1} (T_{2} > T_{1}). The rate of heat transfer through the slab, in a steady state is with f, equal to
1
1/2
2/3
2/3
D.
2/3
Assuming the sun to be a spherical body of radius R at a temperature of T K, evaluate the total radiant power, incident on Earth, at a distance r from the Sun.
where r_{0} is the radius of the Earth and σ is Stefan’s constant
C.
A light ray is incident perpendicular to one face of a 90° prism and is totally internally reflected at the glass-air interface. If the angle of reflection is 45°, we conclude that the refractive index n
n<2
B.
Angle of incidence i > C for total internal reflection.
Here i = 45° inside the medium. ∴ 45° > sin−1 (1/n)
⇒ n > √2.
If the temperature of the sun were to increase from T to 2T and its radius from R to 2R, then the ratio of the radiant energy received on earth to what it was previously will be
4
16
32
32
D.
32
The figure shows a system of two concentric spheres of radii r_{1} and r_{2} and kept at temperatures T_{1} and T_{2} respectively. The radial rate of flow of heat in a substance between the two concentric sphere is proportional to
C.
One end of a thermally insulated rod is kept at a temperature T_{1} and the other at T_{2}. The rod is composed of two sections of lengths
(k_{2}
(k_{2}
(k_{1}
(k_{1}
C.
(k_{1}
A heater coil is cut into two equal parts and only one part is now used in the heater. The heat generated will now be
doubled
four times
one fourth
one fourth
A.
doubled
Time taken by a 836 W heater to heat one litre of water from 10°C to 40°C is
50 s
100 s
150 s
150 s
C.
150 s
Let time taken in boiling the water by the heater is t sec. Then
Q = ms ∆ T