00:03
We want to find the flux of this field, vector field through this surface.
00:10
So i'm going to work in cylindrical coordinates.
00:15
The reason is that the whole thing is cylindrical and it just lends itself to that.
00:25
We know a equals two and b equals seven.
00:30
So our flux is given by this integral.
00:33
We need to know da.
00:34
So da always points outward from the surface.
00:38
And so it's going to point in the row hat direction, the direction of the row unit vector.
00:47
And the area element is a times d phi times dz, in that pointing out in that direction.
01:02
The function f is four times the radius times the row unit vector.
01:15
So we're going to have to dot product those together, but row hat dot row hat is just one.
01:22
So on phi, we integrate zero to two pi because we're going all the way around...