P18_017.PDF

17. The intensity is the rate of energy ﬂow per unit area perpendicular to the ﬂow. The rate at which energy

ﬂows across every sphere centered at the source is the same, regardless of the sphere radius, and is the

same as the power output of the source. If P is the power output and I is the intensity a distance r

from the source, then P = IA =4 πr 2 I ,where A (= 4 πr 2 ) is the surface area of a sphere of radius r .

Thus P =4 π (2 . 50 m) 2 (1 . 91

10 − 4 W / m 2 )=1 . 50

10 − 2 W.