I'm struggling a little bit to interpret the diagram. The top and bottom edges of the patch have length $r\sin\theta$ and the left and right edges have length $r$, right? And this leads to the equation for dA?
FeiFeiFei
Yes. I think when the area is tiny, it can be treated as a rectangle, where the top length is rsin(theta)d_phi and the right length is r*d_theta.
I'm struggling a little bit to interpret the diagram. The top and bottom edges of the patch have length $r\sin\theta$ and the left and right edges have length $r$, right? And this leads to the equation for dA?
Yes. I think when the area is tiny, it can be treated as a rectangle, where the top length is rsin(theta)d_phi and the right length is r*d_theta.