1. gerryliyana

2. Algebraic!

what are r and sigma representing.

3. Carl_Pham

r is probably the vector from the origin to any point on the surface, and sigma is probably the unit vector normal to the surface at that point.

4. gerryliyana

@CliffSedge can you help me please

5. CliffSedge

Been too long since I've done integrals like these. I'll have to refresh my memory first.

6. Jemurray3

|dw:1350687107096:dw|

7. Jemurray3

|dw:1350687145672:dw|

8. Jemurray3

So when it says that $\vec{r}\cdot d\vec{\sigma}=0$ for three of the surfaces, can you see which three this pertains to and why?

9. gerryliyana

why?

10. Jemurray3

when, in general, is a dot product zero?