## hosein 3 years ago Hi A question?

1. hosein

can explain how this two sides are equal?$\cos \theta da=r^2d \Omega$

2. hosein

where da is surface element in spherical coordinate & omeg is solid angle

3. JamesJ

It looks wrong. If you integrate over the entire sphere, the right hand side is $4 \pi r^2$ but the left hand side is $8 \pi r^2$

4. hosein

it's in classical electrodynamics (by Jackson)page28

5. TuringTest

I think this is the same formula we are finding in figure 4.2.4 on this page http://ocw.mit.edu/courses/physics/8-02sc-physics-ii-electricity-and-magnetism-fall-2010/conductors-and-insulators-conductors-as-shields/MIT8_02SC_notes9.pdf though the area is not represented as a differential here. I trust that James' math is right though, so that must have some change in the problem.

6. quarkine

r square* d(sigma) = r*d(theta) *r*d(phi) =projection of area element=dA cos(theta) ....in azimuthal coordinates..|dw:1334694350771:dw|