A copper sphere is suspended in an evacuated chamber maintained at 300 K . The sphere is mantained at a constant temperature of 500 K by heating it electrically. A total of 300 W of electric power is

Question

A copper sphere is suspended in an evacuated chamber maintained at 300 K . The sphere is mantained at a constant temperature of 500 K by heating it electrically. A total of 300 W of electric power is needed to do it. When half of the surface of the copper sphere is completely blackened, 600W is needed to maintain the same temperature of the sphere. Calculate the emissivity of copper.

TestHub · Accepted Answer

Initially ​ T = 300 T s ​ = 500 ​ 300 = σ e A [ 50 0 4 − 30 0 4 ] ​ afterwards 600 = 2 σ eA ​ [ 50 0 4 − 30 0 4 ] + 2 σ A ​ [ 50 0 4 − 30 0 4 ] dividing (2) by (1) 2 = e ( 2 e ​ + 2 1 ​ ) ​ ⇒ 2 e = 2 e ​ + 2 1 ​ ⇒ e = 3 1 ​

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