Question

Explain why

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

(ii) Heating systems based on circulation of steam are more efficient in

warming a building than those based on circulation of hot water.​

Answer 1

Answer:

(a) yes , it's true . we know, r + a + t = 1

Where r is reflectivity,

a is absorbance .

t is transmittance or emittance (e) respectively of the surface of the body .

A/C to Kirchoff's law ,

e directly proportional to a .

It means good absorbers are good emitters and hence poor reflectors and vice-versa . I.e. if r is large ( I.e. large reflectivity ) a is smaller and hence e is smaller I.e., poor emitter.

(b) The thermal conductivity of brass is high I.e., brass is a good conductor of heat. So, when a brass tumbler is touched , heat quickly flows from human body to the tumbler . consequently, the tumbler appears colder . on the other hand , wood is a bad conductor of heat.

So, heat doesn't flow from the human body to the wooden tray, thus it appears relatively hotter.

(c) Let T is the temperature of the hot iron in the furnace .

A/C to Stefan's law ,

heat radiated per second per unit area(E ) is given by E = ßT⁴. Where , ß is Stefan's constant .when the body is placed in open at temperature To, then the heat radiated /sec/area ( E') is given by E' = ß( T⁴ - To⁴) .

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