Saved Bookmarks
| 1. |
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 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 those based on circulation of hot water. |
|
Answer» Solution :(a) A body with large reflectivity is a poor ABSORBER of heat and poor absorbers of heat are poor emitters. (b) Because brass tumbler is a good conductor of heat, when we touch a brass tumbler on a chilly day, heat quickly flows from our body to the tumbler and it appears colder. On the other hand, wood is a bad conductor of heat so on touching, heat does not FLOW from our body to the wood and it appears comparatively hotter, (C) LET T be temperature of red-hot iron piece. When red-hot iron piece is in open having temperature of surroundings `T_0`, Heat RADIATED per second per unit area `E_1 = sigma (T^4 - T_0^4) `....(i) When body is in the furnace having temperature T, then heat radiated per second per unit area `E_2 = sigma T^4`.....(ii) From (i) and (ii),we have `E_1 lt E_2`So the optical pyrometer gives low value of the temperature of a body placed in open. (d) The heat radiated by the earth is reflected back by the atmosphere. If atmosphere is not present then all the heat will escape from the earth surface and the earth will be too cold to live. (e) Because steam at `100^@C` contains more heat than boiling water at `100^@C` . Secondly 1 gm of steam possess 540 calories more heat than 1 gm of water at `100^@C` . Thus heating systems based on circulation of steam are more efficient in warming a building than those based on circulation of hot water. |
|