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A tungsten sphere of radius 1.0 cm hangs inside an evacuated chamber held at 300 K and is kept at 1000 K by electrical heating. Find the electrical power that must be supplied. Take emissivity of tungsten = 0.30 and Stefan constant sigma = 6.0 x 10⁻⁸ W/m²-K⁴.

1. 18 W
2. 22 W
3. 26 W
4. 30 W

Correct answer: 18 W

Solution

P = e*sigma*(4 pi r²)*(T⁴ - T0⁴) = 0.30*6.0e-8*(4 pi*1e-4)*(1000⁴ - 300⁴) approx 18 W.

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