A solid at temperature T₁, is kept in an evacuated chamber at temperature T₂ > T₁ . The rate of increase of temperature of the body is proportional to

1. T₂ – T₁

2.   \(T^2_2 -T^2_1\)n

3.   \(T^3_2 -T^3_1\)

4.   \(T^4_2 -T^4_1\)

The thermal radiation emitted by a body is proportional to Tⁿ where T is its absolute temperature. The value of n is exactly 4 for

1.  a blackbody

2.  all bodies

3.  bodies painted black only

4.  polished bodies only.

Two bodies A and B having equal surface areas are maintained at temperatures 10°C and 20°C. The thermal radiation emitted in a given time by A and B are in the ratio

1.   1 : 1.15

2.  1 : 2

3.   1 : 4

4.  1 : 16.

A solid sphere and a hollow sphere of the same material and of equal radii are heated to the same temperature.

(a) Both will emit equal amount of radiation per unit time in the biginning.

(b) Both will absorb equal amount of radiation from the surrounding in the biginning.

(c) The initial rate of cooling (dT/dt) will be the same for the two spheres.

(d) The two spheres will have equal temperatures at any instant

Choose the correct option 

1.  (a) , (b) 

2. (b) , (c) 

3.  (d) only 

4. (a) , (d)