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Thermal Physics

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 Joined: 5 Feb 2012 Post: 3
18 Feb 2012 13:55:56 IST
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a copper sphere is suspened in an evacuatd chamber at 300K. the sphere is maintained at 900 k by heating electrically. a total of 300w is needed to do dis. wen haf of d surface of copper is completely blackened to make dis portion completely absorbtive 600 w is needed to maintain same temp of sphere. the emissivity of copper is

1/4

1/3

1/2

1

#### Comments (2)

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Joined: 27 Dec 2011
Posts: 121
21 Feb 2012 16:51:35 IST
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energy radiated is initially u1=e(sigma)AT^4....A is area of the sphere and sigma is a constant....A1=4(pi)R^2....u1=300w.....wen half of the sphere is blackened energy radiated is u2=(1)(sigma)(A/2)T^4+e(sigma)(A/2)T^4...the emissivity of the half part that is darkened is 1 and the other half is e......divide the two equations and get the value of e

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Joined: 29 Dec 2006
Posts: 5535
29 Feb 2012 21:56:16 IST
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Initially both the halves were radiatting 150W each (300/2)

On blackening one side, the other side will keep on iradiating 150W while blackened part will iradiate 450W (600-150).

So the emmisivity of copper = 150/450 = 1/3

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