Sun K.
asked 03/27/13Find the flux of F over Q where Q is bounded?
Find the flux of F over Q where Q is bounded by 3x+2y+z=6 and the coordinate planes, F=<y^2*x, 4x^2*sin(z), 3>. (Answer: 27/5).
The divergence of the vector field is y^2, which is what I've found. And z=6-3x-2y, so how should I set up the integral?
2 Answers By Expert Tutors
Inactive Tutor answered 03/28/13
div F = y^2
By the Divergence Theorem ∫∫F⋅N dS = ∫∫∫div F dV,
the flux of F over Q
= ∫∫∫(y^2) dx dy dz
= ∫∫∫{0, 6-3x-2y} (y^2)dz dx dy
= ∫∫{0, 2-(2/3)y}(6-3x-2y) (y^2) dx dy
= ∫{0, 3}y^2[(6-2y)(2-(2/3)y) - (3/2)(2-(2/3)y)^2] dy
= 27/5 <==Answer
Inactive Tutor answered 03/28/13
Inactive Tutor
You cannot have both x in [0,2], and y in [0,3]. One of the upper limit should be a function of either x or y.
03/28/13
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Inactive Tutor
whoops: Robert's comment on my answer right. I wrote my answer too quickly on m phone. To determine x in terms of y, look at the intersection of the plane in the xy plane, which is when z=0. Setting z=0 gives 3x+2y=6 so that y=3-1.5x. The correct limits are x in [0,2], y in [0,3-1.5x], and z in [0,6-3x-2y].
03/28/13