Explain why :

(a) a body with large reflectivity is a poor emitter

(b) a brass tumbler feels much colder than a wooden tray on a chilly day

(c) an optical pyrometer (for measuring high temperatures) calibrated for an ideal black body radiation gives too low a value for the temperature of a red hot iron piece in the open, but gives a correct value for the temperature when the same piece is in the furnace

(d) the earth without its atmosphere would be inhospitably cold

(e) heating systems based on the circulation of steam are more efficient in warming a building than those based on the circulation of hot water

Asked by Abhisek | 1 year ago |  184

##### Solution :-

(a) A body with a large reflectivity is a poor absorber of heat radiation. A poor absorber will be a poor emitter of radiations. Therefore, a body with a large reflectivity is a poor emitter.

(b) Brass is a good conductor of heat and wood is a poor conductor of heat. When we touch a brass tumbler, heat is conducted from our hand to the tumbler and there is a drop in the body temperature Therefore, we feel cold.  When a wooden tray is touched on a chilly day, very less heat is conducted from the hand to the wooden tray and body temperature is not decreased much. So, we do not feel cold.

(c) The radiation energy from a red hot iron piece place in a furnace is given by the relation

E=σT4

When the iron piece is placed in the open, the radiation energy is given by the relation

E=σ(T4−T04)

Here, E is the energy radiation

T is the temperature of optical pyrometer

σ is a constant

T0 is the temperature of the open space.

The increase in the temperature of the open space reduces the radiation energy.

(d)  If there is no atmosphere, extra heat will not be trapped. All the heat from the sun will be radiated back from the surface of the earth. So without the atmosphere, the earth would be inhospitably cold.

(e) Steam at 100°C is much hotter than water at 100°C. This is because steam will have a lot of heat in the form of latent heat (540 cal/g).  Therefore, heating systems based on the circulation of steam are more efficient in warming a building than those based on the circulation of hot water.

Answered by Pragya Singh | 1 year ago

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