Grayham1990
New member
- Joined
- Nov 1, 2011
- Messages
- 1
Hey,
I'm currently preparing for my finals and I've found a bunch of questions I'm having trouble with,
This one is just a using Stoke's Theorem to determine a surface integral and I get an answer of zero on the RHS,
When I parametrize each surface of S I get a non zero result on the LHS of the equation.
Here is the question:
They gave a big hint to what G is and that F is actually the curl of H from the previous question,
H = (3y, -xz, yz^2 )
So S is made up of two surfaces and has 2 boundaries,
One boundary a unit circle in the x-y plane, the other a circle of radius 1/2
So I turned the boundary integral into two, one for each circle
I made my parameters thi(t) = (rcost, rsint, z), where r is either 1 when z is 1 or 1/2 when z is 2 and 0<=t<=2Pi
then just did the usual line integral, int H(thi) dot thi(t)' dt and got -4Pi for the unit circle and -5Pi/4 for the other
So together -21Pi/4
Which seems wrong to me, so I tried to check by doing the LHS surface integral
The cylinder param. was thi(u,v)=(ucosv,usinv,u) 0<=v<=2Pi, 1<=u<=2
did the usual thing, dot product of F(thi) with the cross product between the partials of thi wrt u,v
then for that part I got 16Pi,
The second part was the weird thing,
This is where I'm not sure at all, I did thi(u,v) = (u,v,u^2 + v^2)
did the usual stuff,
then after the dot product I let u=rcosq v=rsinq (q was theta)
and integrated for 0<=r<=1 and 0<=q<=2Pi
and got 7Pi
so total 23Pi no where near what I got on the RHS,
Does anyone know what I've done wrong?
Thanks alot
I'm currently preparing for my finals and I've found a bunch of questions I'm having trouble with,
This one is just a using Stoke's Theorem to determine a surface integral and I get an answer of zero on the RHS,
When I parametrize each surface of S I get a non zero result on the LHS of the equation.
Here is the question:
They gave a big hint to what G is and that F is actually the curl of H from the previous question,
H = (3y, -xz, yz^2 )
So S is made up of two surfaces and has 2 boundaries,
One boundary a unit circle in the x-y plane, the other a circle of radius 1/2
So I turned the boundary integral into two, one for each circle
I made my parameters thi(t) = (rcost, rsint, z), where r is either 1 when z is 1 or 1/2 when z is 2 and 0<=t<=2Pi
then just did the usual line integral, int H(thi) dot thi(t)' dt and got -4Pi for the unit circle and -5Pi/4 for the other
So together -21Pi/4
Which seems wrong to me, so I tried to check by doing the LHS surface integral
The cylinder param. was thi(u,v)=(ucosv,usinv,u) 0<=v<=2Pi, 1<=u<=2
did the usual thing, dot product of F(thi) with the cross product between the partials of thi wrt u,v
then for that part I got 16Pi,
The second part was the weird thing,
This is where I'm not sure at all, I did thi(u,v) = (u,v,u^2 + v^2)
did the usual stuff,
then after the dot product I let u=rcosq v=rsinq (q was theta)
and integrated for 0<=r<=1 and 0<=q<=2Pi
and got 7Pi
so total 23Pi no where near what I got on the RHS,
Does anyone know what I've done wrong?
Thanks alot
Last edited: