Sun K.

asked • 03/27/13

Find 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?

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].

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03/28/13

Inactive Tutor

one more comment: in my comment above "x in terms of y" should have been "y in terms of x" -- because I wound up solving for y as a function of x. Robert solved for x in terms of y, getting x=2-2/3y. To parametrize the region this way, you would st y in [0,3], x in [0,2-2/3y], and z in [0,6-3x-2y]. Both parametrizations describe the same vlume, and it is a deep fact about integrals that the parametrization you use does not change the integral. You should check this by doing the integral with mparametrization above to see that you get the same answer!

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03/28/13

Sun K.

But I didn't get the correct answer, I got -54. I set up the integral like this: integral from 0 to 3, integral from 0 to 2, integral from 0 to 6-3x-2y of y^2 dz dx dy. The next step was integral from 0 to 3, integral from 0 to 2, y^2(6-3x-2y) dx dy, integral from 0 to 3 [xy^2(6x-3x^2/2-2yx)] from 0 to 2 dy=integral from 0 to 3 (12y^2-8y^3) dy=-54.

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03/28/13

Inactive Tutor

no, I said my original answer was wrong. Look at my comments above. don't use the parametrization I gave below.

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03/28/13

Sun K.

Okay, thanks, sorry that I was clueless.

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03/28/13

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