Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Surface integrals

  1. Feb 8, 2007 #1
    [Can someone help with the following? I am supposed to evaluate the line integral of ∫F∙dr. The curve is oriented counterclockwise as viewed from above. So suppose that F(x,y,z) = (x+y^2)I + (y+z^2)j + (z+x^2)k, and C is the triangle formed by (1,00), (0,1,0), (0,0,1).

    I know that the integral ∫F∙dr over some surface S is equal to ∫∫curlF∙dS and is equal to ∫∫curlF∙n/|n|ds or ∫∫curlF∙ndA, where n is the normal vector. So, curl F is -2zi -2xj -2yk. And since the region has the expression x+y+z = 1, so suppose the region z can be written as (x, y, 1-x-y), and the normal vector to this region has the expression <dz/dx, -dz/dy, 1> or <-1,-1,1>. So wouldn’t curl F dot n be 2z+2x-2y, and since z = 1-x-y, curl F n should be 2-2x-2y+2x-2y? So I am supposed to integrate the integral ∫∫2-4y dydx where 0≤y≤1-x and 0≤x≤1? Can someone point out what I’m doing wrong here? Thanks.
     
  2. jcsd
  3. Feb 8, 2007 #2

    mathwonk

    User Avatar
    Science Advisor
    Homework Helper

    i cant read all that, but couldn't you just check your work by simply parametrizing the triangle and just doing a one variable integral?

    it is especially easy on the vertical and horizontal dies where, on the y axis side say, both dx and dz are zero.
     
  4. Feb 8, 2007 #3
    But I'm supposed to evaluate it using the double integral, so.
     
  5. Feb 8, 2007 #4

    arildno

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    Dearly Missed

    The plane equation is, indeed, x+y+z=1.
    Hence, the normal vector is parallell to (1,1,1)
     
  6. Feb 8, 2007 #5
    Oh I see where I went wrong! Thanks!!!
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook