Explain why
(i) a body with large reflectivity is a poor emitter.
(ii) a brass tumbler feels much colder than a wooden
tray on a chilly day. .
(iii) 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.
(iv) the earth without its atmosphere would be inhospitably cold.
(v) heating systems based on circulation of steam are
more efficient in warming ft building than those based on circulation of hot water.
(i) A body with large reflectivity is a poor absorber of heat.
According to Kirchhoff’s law, a poor absorber of heat is a poor emitter. Hence, a body with large reflectivity is a poor emitter.
(ii) Brass is a good conductor of heat. When a brass tumbler is touched, heat quickly flows from human body to tumbler. Consequently, the tumbler appears colder. Wood is a bad conductor. So, heat does not flow from the human body to the tray in this case. Thus, it appears comparatively hotter
(iii) Let T be the temperature of the hot iron in the furnace.
Heat radiated per second per unit area,
When the body is placed in the open at temperature the
heat radiated/second/area,
Clearly, E’ < E. So, the optical pyrometer gives too low a value for the temperature in the open.
(iv) Heat radiated out by the earth is reflected back by the atmosphere. In the absence of atmosphere, at night all heat would escape from the earth’s surface and thereby the earth’s surface would be inhospitably cold. Also, atmosphere helps in maintaining the temperature through convection current.
(v) Though steam and boiling water are at the same temperature but each unit mass of steam contains a larger amount of additional heat called the latent heat, e.g., each gram of steam has 540 cal of more heat than each gram of boiling water. Hence, steam loses more heat than boiling water.