# Determine charge at origin, based on charge density function

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1. Sep 23, 2015

### xSilja

1. The problem statement, all variables and given/known data

a) and b) are no problem.

I need help to solve c) and d)

2. Relevant equations
c) Delta dirac function
Gauss' law

d) Gauss' law
$\int_V {\rho \, d\tau} = Q_{enclosed}$

3. The attempt at a solution
By taking laplace on the potential I get:

$\rho(\mathbf{r}) = \frac{q_0}{4 \, \pi \, r} \, e^{-r/\lambda} \, \left( \frac{cos^2(\theta)}{\lambda^2} + \frac{2}{r^2} (1-3 \, cos^2(\theta)) \right)$

c) I got a hint that it was a good idea to use the dirac delta function along with the charge distribution.

But I'm not exactly sure why. As I understand it the dirac delta function "picks out" the value of a function at zero. So I'd get:

$\int {\rho(\mathbf{r}) \, \delta(r) \, dr} = \rho(0)$

I realize that there must be a dimensional problem here, but I'm not sure how to use a delta function in 3D and spherical coordinates.
Also how will it help me to find the density at the origin? Can I apply Gauss' law here and let the radius go towards zero to get the charge in the origin?

d) I want to solve the integral

$Q = \int_0^\pi \int_0^{2 \, \pi} \int_a^{\infty} \, \rho(\mathbf{r}) \, r^2 \, \sin(\theta) \, dr \, d\theta \, d\phi$

I tried evaluating this with Maple.
By assuming a>0 I get a complex function multiplied by infinity, which is not of much use.
If I also assume lambda>0 (as it says in the problem) I get rid of the infinity, but get exponential integrals instead.
I'm not sure how to move on from here. I suspect I need to modify my function for charge distribution by assumptions, to make it simpler.

2. Sep 23, 2015

### Staff: Mentor

This assumes your charge density formula doesn't have problems with r=0. That might be, but I wouldn't rely on it.

Using the electric field, you can determine the total charge up to a radius a and then let a go to zero.
The opposite limit also works for (d) and does not need evaluating any actual integrals.

3. Sep 24, 2015

### xSilja

By best idea was also to use Gauss' law. $\int_V \mathbf{\nabla} \cdot \mathbf{E} \, d\tau = \frac{1}{\epsilon_0} \, Q_{enclosed}$
How can I avoid evaluating any actual integrals?
The charge density/ elctric field depends on both r and theta.

4. Sep 24, 2015

### Staff: Mentor

Set them up, then find useful bounds on them.