Working efficiency of a 800 W bulb is 3% and it is kept at the centre of a spherical surface having diameter 20 m. The force acting on the surface by the electromagnetic waves is .

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(2) 8×10⁻⁸ N. Given: a 800 W bulb that turns 3% of that into radiation, at the centre of a sphere of diameter 20 m. Radiated power: P = 0.03×800 = 24 W. Idea: an electromagnetic wave carries momentum as well as energy. Energy U absorbed brings momentum U/c with it, so a surface absorbing power P receives momentum at the rate P/c, and that rate of change of momentum is the force. The sphere surrounds the bulb, so every watt the bulb radiates lands on it. The radius does not enter: the intensity falls as 1/(r²) but the area grows as r², and the two cancel. F = P/c = (24)/(3×10⁸) = 8×10⁻⁸ N. So the force is 8×10⁻⁸ N. It is the total of the pushes on all parts of the sphere; as vectors those pushes point outwards in every direction and their sum is zero, which is why the question asks only for the magnitude.

Working efficiency of a 800 W bulb is 3% and it is kept at the centre of a spherical surface having diameter 20 m. The force acting on the surface by the electromagnetic waves is ______.

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