00:01
Okay, so this one asks you to find the flux through a face of a cube, given that the charge is a distance d over to above it.
00:17
So you might look at this, and if you try to approach it in the way that you approach like the earlier ones, you would say, okay, well, here's my surface, and then if i draw the electric field at the surface, it's kind of kind of flare outward.
00:33
Because the electric field of a charge is radially symmetric, so it goes something like this.
00:40
And it varies along the surface.
00:43
So like if you're on the outside, for example, you're like furthest from the charge.
00:48
So the electric field's going to be smallest, and then in the middle it's going to be biggest.
00:51
And so in general, it's not like an impossible integral to do, but you would hope they weren't make, the book wasn't making the jump so fast and there's a more clever way.
01:01
And indeed there is.
01:02
So this is a great example of galseslaw.
01:06
So galseslaw tells us that if we want to get the electric field, like the total flux through a closed surface.
01:15
So now i'll use this like circle on my integral.
01:19
Then that simplifies to whatever the charge enclosed is divided by epsilon not, the fundamental constant...