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Thermal Physics

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2 Jan 2008 00:59:18 IST

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H.C VERMA HEAT PROBLEM ON RADIATION

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Q.A copper sphere is suspended in an evacuated chamber maintained at a temperature of 300K.the sphere is maintained at a constant temperature of 500K by heating it electrically.A total of 210W of electric power is needed to do it.When the surface of the copper sphere is completely blackened,700W is neede to maintain the same temperatre of the sphere.Calculate emissivity of copper.

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svj 29

Blazing goIITian

Joined: 27 Dec 2007

Posts: 822

2 Jan 2008 02:38:57 IST

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dekh bhai...........
u knw power=sigma A e delta T to the power 4
in the 1st case......
u know the value of stefens cnst.. and temp change.....only A and e r unknown...u have d power to given as 210watts....
yaane ki jitni heat wo radiate kar ra hai....210 watt power dene se utni hi heat use mil rai hai........
u'll get the eqn as:
Ae=210/3048.48

nw in 2nd case when the body acts as blckbody.....yaane ki jyada radiate karega....e=1
nw u knw sigma, e, delta T only A is unknown .....
form the eqn u'll get A=700/3048.48

put the value of A in 1st eqn....u'll get d ans.......
as 0.3

Krishna Gopal Singh

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Joined: 29 Dec 2006

Posts: 5153

2 Jan 2008 08:14:31 IST

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Perfect answer svj, but answer is straight forward. As you have to maintain sphere at constant temp, it must be radiating all the power you are giving it. In first case it was radiating 210W and when it became a black body it was radiating 700W. So emmisivity is 210/700 =0.3

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