TA radiometer has two square vanes (1.0 cm by 1.0 cm), attached to a light horiz
ID: 1415201 • Letter: T
Question
TA radiometer has two square vanes (1.0 cm by 1.0 cm), attached to a light horizontal cross arm, and pivoted about a vertical axis through the center, as shown in the figure. The center of each vane is 6.0 cm from the axis.
One vane is silvered and it reflects all radiant energy incident upon it. The other vane is blackened and it absorbs all incident radiant energy. An electromagnetic wave with an intensity of 300 W/m2 is incident normally upon the vanes.
What is the force on the blackened vane due to the light?
Explanation / Answer
Power P = Intensity*Area of cross section
Intensity I = 300 W/m^2
area of cross section is A = 1*1*10^-4 = 10^-4 m^2
Power P = 300*10^-4 m^2
P = Force*Velocity
velocity of Em waves is v = 3*10^8 m/s
force F = P/v= (300*10^-4)/(3*10^8) = 1*10^-10 N
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