# using divergence theorem to prove Gauss's law

• Jun 2nd 2011, 08:39 AM
oblixps
using divergence theorem to prove Gauss's law
so if i have the vector field F = R/r^3 where R is (x,y,z) and r is the magnitude of the vector R, then div(F) = 0 except for at the origin where F is not defined.

i want to show that the volume integral of div(F) over a sphere centered at the origin is 4pi. so using the divergence theorem i have the surface integral of F over the surface of the sphere = the volume integral of div(F) over the solid sphere. it is easy to show that the flux of F through the surface of a sphere is 4pi. but the problem is that F is not continuously differentiable at the origin and the statement of the divergence theorem (at least the one i know of) requires that the vector field F be defined on a neighborhood containing the solid sphere. but that is not true because F is not defined at the origin so i shouldn't be able to apply the divergence theorem right?

however it seems my physics book has overlooked this and came to the conclusion that the volume integral of div(F) is indeed equal to 4pi by the divergence theorem which i am confused about.
• Jun 3rd 2011, 08:59 AM
Ruun
The point here is to use the Dirac's Delta function

$\displaystyle \vec{\nabla} \cdot \frac{\vec{R}}{r^3} = 4 \pi \delta(\vec{R})$
• Jun 3rd 2011, 02:17 PM
oblixps
so is it that we define the divergence of R/r^3 as 4 pi delta?

or that we somehow derive it using the divergence theorem? this does not seem possible however since you cannot apply the theorem in the first place. so i'm guessing my physics book is just "fudging" when they derive this result using the divergence theorem.
• Jun 3rd 2011, 04:48 PM
topsquark
Quote:

Originally Posted by oblixps
so is it that we define the divergence of R/r^3 as 4 pi delta?

or that we somehow derive it using the divergence theorem? this does not seem possible however since you cannot apply the theorem in the first place. so i'm guessing my physics book is just "fudging" when they derive this result using the divergence theorem.

I doubt many undergraduate texts mention the delta function (at least I haven't seen any), but the standard Electromagnetism text in graduate school, Jackson's "Classical Electrodynamics", takes great pains to do the derivation and mention the use of the delta function in the manner that Ruun mentioned. The details are skipped in the undergraduate texts presumably to make it easier for the student to work with, with the understanding that the theorems are going to be made rigorous later on. This is, perhaps, unfortunate but typical practice for undergraduate Physics texts.

-Dan
• Jun 4th 2011, 12:51 AM
Ruun
In fact, we discuss this topic in my Electromagnetism course, very naive and physics style. We followed Wagness "Electromagnetic Fields" (by the way the book with most typos I have ever seen, at least the spanish edition), but Dirac's Delta was discussed in the classroom, I think that the book does not mention it.

I'm afraid I don't know the mathematical subtelties.