A spherical tungsten piece of radius `(1)/(sqrt(pi)) cm` is suspended in an evacuated chamber maintained at `200 K`. The piece is maintained at `400 K` by heating it electrically. Find the rate at which the electrial energy must be supplied. The emissivity of tungsten is `0.50`, the Steafan constant `sigma` is `6.0 xx 10^(-8) W//m^(2)-K^(4)`

Explanation

Net rate of loss of energy
`Delta u = e sigma A (T^(4) - T_(0)^(4))`
`= 0.5 xx 6 10^(-8) xx 4 pi ((1)/(sqrt(pi))) xx 10^(-4) [(400)^(4) - (200)^(4)]`
`12 xx 10^(-12) xx (256 - 16) xx 10^(8)`
Rate at which the electrical energy supplied
`= 0.228 W`