James Stewart-Calculus_ Early Transcendentals-Cengage Learning (2015)

Section 16.9 The Divergence Theorem 1141
In Section 16.5 we rewrote Green’s Theorem in a vector version as
y F n ds −
C
yy div Fsx, yd dA
D
where C is the positively oriented boundary curve of the plane region D. If we were seeking
to extend this theorem to vector fields on R 3 , we might make the guess that
1 yy F n dS − y yy div Fsx, y, zd dV
S
E
where S is the boundary surface of the solid region E. It turns out that Equation 1 is true,
under appropriate hypotheses, and is called the Divergence Theorem. Notice its similarity
to Green’s Theorem and Stokes’ Theorem in that it relates the integral of a derivative
of a function (div F in this case) over a region to the integral of the original function F
over the boundary of the region.
At this stage you may wish to review the various types of regions over which we were
able to evaluate triple integrals in Section 15.6. We state and prove the Diver gence Theorem
for regions E that are simultaneously of types 1, 2, and 3 and we call such regions
simple solid regions. (For instance, regions bounded by ellipsoids or rectangular boxes
are simple solid regions.) The boundary of E is a closed surface, and we use the convention,
introduced in Section 16.7, that the positive orientation is outward; that is, the unit
normal vector n is directed outward from E.
The Divergence Theorem is sometimes
called Gauss’s Theorem after the great
German mathe matician Karl Friedrich
Gauss (1777–1855), who discovered
this theorem during his investigation
of electrostatics. In Eastern Europe
the Divergence Theorem is known
as Ostrogradsky’s Theorem after
the Russian mathe matician Mikhail
Ostrogradsky (1801–1862), who published
this result in 1826.
The Divergence Theorem Let E be a simple solid region and let S be the
boundary surface of E, given with positive (outward) orientation. Let F be a vector
field whose component functions have continuous partial derivatives on an
open region that contains E. Then
y
S
y F ? dS − y y
E
y div F dV
Thus the Divergence Theorem states that, under the given conditions, the flux of F
across the boundary surface of E is equal to the triple integral of the divergence of F
over E.
Proof Let F − P i 1 Q j 1 R k. Then
so
y yy div F dV − y y
E
E
div F − −P
−x 1 −Q
−y 1 −R
−z
y −P
−x dV 1 y y
E
y −Q
−y dV 1 y y
E
y −R
−z dV
If n is the unit outward normal of S, then the surface integral on the left side of the
Divergence Theorem is
y
S
y F dS − yy F n dS − y
S
S
y sP i 1 Q j 1 R kd n dS
− yy P i n dS 1 yy Q j n dS 1 yy R k n dS
S
S
S
Therefore, to prove the Divergence Theorem, it suffices to prove the following three
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