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A spherical ball of surface area 20 cm² is a perfect absorber of incident radiation. It hangs inside a closed box kept at 57 deg C. Taking the Stefan constant as 6.0 x 10⁻⁸ W/m²/K⁴, find the radiation energy falling on the ball each second.

1. 1.4 J
2. 1.8 J
3. 2.2 J
4. 2.4 J

Correct answer: 1.8 J

Solution

For a black ball in equilibrium with walls at temperature T, the radiation incident on it per second is sigma*A*T⁴ with T = 330 K, which gives about 1.8 J.

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