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A copper sphere is suspended in an evacu...

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

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Verified by Experts

The correct Answer is:
C
`Q=eA rho(T_(1)^(4)-T_(2)^(4))`
`For any body ,`
`210= eA rho[(500)^(4)-(300)^(4)]`
`For black body,`
`700=1xxAxx rho [(500)^(4)-(300)^(4)]`
`dividing `
`(210)/(700)=e/1`
`e=0.3`.
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