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surface area of a sphere is 4(r 2 , the total solid angle subtended by the sphere is Now consider a point charge q surrounded by a closed surface of arbitrary shape (Fig. 24.22). The total electric flux through this surface can be obtained by evaluating where r is the distance from the charge to the area element, ) is the angle between the E and +A for the element, and E # k e q/r 2 for a point charge. In Figure 24.23, we see that the projection of the area element perpendicular to the radius 2 is equal to the solid angle +5 that the surface element +A subtends at the charge q. We also see that +5 is equal to the Thus we have derived Gauss’s law, Equation 24.6. Note that this result is independent ! E # k e
q
$
dA cos ) r
2 # k e
q
$
d
5 # 4(k e
q # q / 0 +! E # E"+
A # (E cos ))+A # k e
q
+ A cos ) r
2 5 # 4(r
2 r
2 # 4( steradians Summary 753 Electric flux is proportional to the number of electric field lines that penetrate a (24.2) In general, the electric flux through a surface is (24.3) ! E # " surface E"d
A ! E # EA cos ) S U M M A R Y Figure 24.23 The area element +A subtends a solid angle +5 # (+A cos ) )/r 2 at the charge q. ∆Ω q r ∆A ∆A cos θ ∆A θ E θ Figure 24.22 A closed surface of arbitrary shape surrounds a point charge q. The net electric flux through the surface is independent of the shape of the surface. θ ∆A ∆Ω q E Take a practice test for this chapter by clicking on |