# Stokes Theorem :( help me

• Sep 20th 2013, 11:49 PM
Yoon
Stokes Theorem :( help me
Attachment 29217
Attachment 29218
First is the problem and next one is my sols.

but it doesn't satisfy Stokes' Theorem S curl(F)*dA= Sc F*dl

surface integral = 0, Line integral (with x^2+y^2=1) = 2pi

what's wrong with me
• Sep 21st 2013, 09:23 AM
HallsofIvy
Re: Stokes Theorem :( help me
Well, I can't speak to what's wrong with you! But here, you have a sign error.
$\nabla\times F= \left|\begin{array}{ccc}\vec{ix} & \vec{j} & \vec{k} \\ \frac{\partial}{\partial x} & \frac{\partial}{\partial y} & \frac{\partial}{\partial z} \\ \frac{-y}{x^2+y^2} & \frac{x}{x^2+y^2} & 0 \end{array}\right|$
$= \left(\frac{\partial}{\partial x} \left(\frac{x}{x^2+ y^2}\right)- \frac{\partial}{\partial y}\frac{-y}{x^2+y^2}\right)\vec{k}$
With those two "-" signs, the partial derivatives add:
$\left(\frac{y^2- x^2}{(x^2+ y^2)}+ \frac{y^2- x^2}{(x^2+ y^2)}\right)\vec{k}= 2\frac{y^2- x^2}{(x^2+ y^2)}\vec{k}$