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 that falls upon it.The other vane is blackened and it absorbs all incident radiant energy.Radiant energy,having an intensity of 300 W/m²,is incident normally upon the front surfaces of both vanes.What is the radiant power absorbed by the blackened vane? A)0.030 W B)0.040 W C)0.050 W D)0.060 W E)0.090 W

Question

Quizplus · Accepted Answer

The radiometer works on the principle of the radiation pressure, where the radiant energy incident on the vanes exerts a force causing them to rotate. In this case, the silvered vane reflects all the radiant energy, leading to a net pressure difference, causing the vanes to rotate. The blackened vane absorbs all the radiant energy, leading to no net pressure difference, causing no rotation.

Using the formula for radiant flux (power per unit area), the radiant power incident on the vanes is:

P = I*A = 300 W/m^2 * (1 cm / 100)^2 = 0.03 W

Thus, the radiant power absorbed by the blackened vane is also 0.03 W.

Therefore, the answer is A.

A Radiometer Has Two Square Vanes (1