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Question
A 100 W light bulb is placed at the centre of a spherical chamber of radius 20 cm. Assume that 60% of the energy supplied to the bulb is converted into light and that the surface of the chamber is perfectly absorbing. Find the pressure exerted by the light on the surface of the chamber.
(Use h = 6.63 × 10-34J-s = 4.14 × 10-15 eV-s, c = 3 × 108 m/s and me = 9.1 × 10-31kg)
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Solution
Given :-
Power of the light bulb, P = 100 W
Radius of the spherical chamber, R = 20 cm = 0.2 m
It is given that 60% of the energy supplied to the bulb is converted to light.
Therefore, power of light emitted by the bulb, P' = 60 W
Force,
`F = P/c`
where c is the speed of light
`F = 60/(3 xx 10^8) = 2 xx 10^-7 "N"`
`"Pressure" = "Force"/"Area"`
`= (2 xx 10^-7)/(4 xx 3.14 xx (0.2)^2` (`A = 4pir^2`)
`= 1/(8 xx 3.14) xx 10^-5`
`= 0.039 xx 10^-5`
`= 3.9 xx 10^-7`
`= 4 xx 10^-7 "N/m"^2`
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