Question

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 temperature) 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 circulation of steam are more efficient in warming a building than heating system based on circulation of hot water.

Answer 1

(a) a body whose reflectivity is large would naturally absorb less heat. So, a body with large reflectivity is a poor.

(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 tumbler. Consequently, the tumbler appears colder. On the other hand, wood is a bad conductor, so, heat does not flow from the human body to the wooden tray in this case. Thus, it appears comparatively hotter.

(c) Let T the temperature of the hot iron in the furnace. Heat radiated per second per unit area, E = σT⁴