- #1
Haths
- 33
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Two problems one that I have some idea about solving, the other I have no idea at all about where to start.
1. Find the surface integral of E . dS where E is a vector field given;
E = yi - xj + 1/3 z3 and S is the surface x2 + z2 < r2 and 0 < y < b
Well Gauss' theorum would be the place to start to make this easy for me...Already having calculated the divergance of the field to be;
div E = z2
But my problems came to the paramaterisation of the integral. Assuming that my co-ord. transfer is;
x = r cos(a)
y = y
z = r sin(a)
z2 = r2sin2(a)
The jacobian for the cylindrical co-ords I believe being r2
Hence my integral becomes;
[tex]
$ int int int r^{4} sin^{2}(a) dr da dy$
[/tex]
The limits being b -> 0 / 2PI -> 0 / r -> 0 respectfully
...and I end up with;
1/5 PI b r5
I have no idea if this is right or not, it 'feels' right, but I'm not confident in the answer, nor if I have made some mistake along the way.
2. Finding the curl of;
[tex]
A(r,t) = a e^{(ip \dot r - i \omega t)}
[/tex]
where a and p are constant vectors...
The only idea I had was to convert it into sines and cosines, but that didn't help me that much.
Cheers,
Haths
1. Find the surface integral of E . dS where E is a vector field given;
E = yi - xj + 1/3 z3 and S is the surface x2 + z2 < r2 and 0 < y < b
Well Gauss' theorum would be the place to start to make this easy for me...Already having calculated the divergance of the field to be;
div E = z2
But my problems came to the paramaterisation of the integral. Assuming that my co-ord. transfer is;
x = r cos(a)
y = y
z = r sin(a)
z2 = r2sin2(a)
The jacobian for the cylindrical co-ords I believe being r2
Hence my integral becomes;
[tex]
$ int int int r^{4} sin^{2}(a) dr da dy$
[/tex]
The limits being b -> 0 / 2PI -> 0 / r -> 0 respectfully
...and I end up with;
1/5 PI b r5
I have no idea if this is right or not, it 'feels' right, but I'm not confident in the answer, nor if I have made some mistake along the way.
2. Finding the curl of;
[tex]
A(r,t) = a e^{(ip \dot r - i \omega t)}
[/tex]
where a and p are constant vectors...
The only idea I had was to convert it into sines and cosines, but that didn't help me that much.
Cheers,
Haths