# Need help finding flux across a surface

1. Dec 1, 2011

### nprince

1. The problem statement, all variables and given/known data

Find the upward flux across the surface z = x of the vector field F=(y,-x,0).

2. Relevant equations

∫∫(F$\bullet$n)ds

3. The attempt at a solution

I know what to do once I parametrize z = x but I am stuck on the parametrization. Would it just be: r(x,y) = (zi + j + xk) ?

2. Dec 1, 2011

### nprince

or could I say f(x,y,z) = z - x

so the normal vector would be : $\nabla$f / ||$\nabla$f||

= -i + k / $\sqrt{}$-12+12

3. Dec 2, 2011

### HallsofIvy

Staff Emeritus
The first thing you are going to have to do is put bounds on your surface. You don't want to integrate across the entire infinite plane, do you?

As far as the parameterization of the surface is concerned, it is r(x, y)= xi+ yj+ xk. You are missing the "y" but I suspect that was a typo.

The best way to find n, or better, $\vec{n} dS= d\vec{S}$, is to find the "fundamental vector product" of the surface. That is the cross product $\vec{r}_x\times\vec{r}_y$.

$$d\vec{S}= (\vec{r}_x\times\vec{r}_y)dxdy$$

I personally don't like the notation $\vec{n}dS$ because if you take that literally, as you have here, you have to divide by the length of the normal vector to find $\vec{n}$ and then multiply by it to find dS- and, of course, those will cancel!

4. Dec 2, 2011

### nprince

So then it's just the double integral of the vector field dot that cross product and I just need to find the limits of integration. Thanks!