Hi It is known that a surface integral of something around a volume can be equal to a volume integral. I wonder if someone has worked with the volume outside. Imagine a ball in air. The airdrag on it is typically calculated by integrating the force over its area. An equivalent would be to integrate the power losses in the fluid in all space outside the ball. There would also be an equivalent for two dimensions.
Maybe the divergence theorem could be used to show this? Maybe by just applying it two times and showing that the outer surface integral goes to zero as the radius increases? Or maybe by having a small tube connecting the two surfaces and showing that the area integral of the tube reaches zero as the tube radius becomes smaller? David David Jonsson, Sweden, phone callto:+46703000370
