A radiometer has two square vanes (1.0 cm by 1.0 cm), attached to a light horizo

Question

A 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 0.30 kW/m2 is incident normally upon the vanes. What is the torque due to radiation pressure on the vane assembly about the vertical axis? (c=3*10^8, mu = 4pi*10^-7 T*m/A, epsilon not = 8.85*10^-12 C^2/n*m^2 )

Explanation / Answer

Intensity = power/area

radiated power P = (300*0.01)^2 = 0.03 Watts

radiation pressure Pr = I/c = 300/3*10^8 = 1*10^-6 pa

torque T = FX r

RADIATION PRESSUE =F/A

fORCE F = Pr*A = 10^-6 *0.01)^2

T = 1*10^-6 * 0.01*0.01* 0.06

T = 6*10^-12 Nm

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