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 circulation of steam are more efficient in warming a building than those based on circulation of hot water
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(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) .
(d) Generally , Gases are insulators .The earth atmosphere acts like an insulating blanket around it and doesn't allow heat escape out but reflect it back to the earth . if eath without atmosphere then all its heat will escape out. Therefore the earth would naturally be colder .
(e) This is because , steam has much higher heat capacity than the heat capacity of water at the same temperature . Thus , eventually steam will heat building better than water .
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Q21 : 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 circulation of steam are more efficient in warming a building than those based on circulation of hot water.
Answer :
(a) A body with a large reflectivity is a poor absorber of light radiations. A poor absorber will in turn be a poor emitter of radiations. Hence, a body with a large reflectivity is a poor emitter.
(b) Brass is a good conductor of heat. When one touches a brass tumbler, heat is conducted from the body to the brass tumbler easily. Hence, the temperature of the body reduces to a lower value and one feels cooler.Wood is a poor conductor of heat. When one touches a wooden tray, very little heat is conducted from the body to the wooden tray. Hence, there is only a negligible drop in the temperature of the body and one does not feel cool.Thus, a brass tumbler feels colder than a wooden tray on a chilly day.
(c) An optical pyrometer calibrated for an ideal black body radiation gives too low a value for temperature of a red hot iron piece kept in the open.Black body radiation equation is given by:Where,E = Energy radiationT = Temperature of optical pyrometerTo = Temperature of open spaceÃÆ’ = ConstantHence, an increase in the temperature of open space reduces the radiation energy.When the same piece of iron is placed in a furnace, the radiation energy, E = ÃÆ’T4
(d) Without its atmosphere, earth would be inhospitably cold. In the absence of atmospheric gases, no extra heat will be trapped. All the heat would be radiated back from earth’s surface.
(e) A heating system based on the circulation of steam is more efficient in warming a building than that based on the circulation of hot water. This is because steam contains surplus heat in the form of latent heat (540 cal/g).
Answer :
(a) A body with a large reflectivity is a poor absorber of light radiations. A poor absorber will in turn be a poor emitter of radiations. Hence, a body with a large reflectivity is a poor emitter.
(b) Brass is a good conductor of heat. When one touches a brass tumbler, heat is conducted from the body to the brass tumbler easily. Hence, the temperature of the body reduces to a lower value and one feels cooler.Wood is a poor conductor of heat. When one touches a wooden tray, very little heat is conducted from the body to the wooden tray. Hence, there is only a negligible drop in the temperature of the body and one does not feel cool.Thus, a brass tumbler feels colder than a wooden tray on a chilly day.
(c) An optical pyrometer calibrated for an ideal black body radiation gives too low a value for temperature of a red hot iron piece kept in the open.Black body radiation equation is given by:Where,E = Energy radiationT = Temperature of optical pyrometerTo = Temperature of open spaceÃÆ’ = ConstantHence, an increase in the temperature of open space reduces the radiation energy.When the same piece of iron is placed in a furnace, the radiation energy, E = ÃÆ’T4
(d) Without its atmosphere, earth would be inhospitably cold. In the absence of atmospheric gases, no extra heat will be trapped. All the heat would be radiated back from earth’s surface.
(e) A heating system based on the circulation of steam is more efficient in warming a building than that based on the circulation of hot water. This is because steam contains surplus heat in the form of latent heat (540 cal/g).
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