# Fourier Tranform of electric dipole charge density

1. Jul 5, 2011

### naftali

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

This is supposed to be simple, so I guess I miss something..
We have charge q at x1=d*cos (w*t), y=0, z=0. and charge -q at
x2=-d*cos (w*t), y=0, z=0. I need to do fourier transform to the charge density.

2. Relevant equations
The fourier transform is : $\rho_{\omega} = \int \rho (r,t)e^{i\omega t} dt$

3. The attempt at a solution
I first try only for the first charge. We have $\rho (r,t) = \delta(x-d\cdot cos (\omega t))\cdot\delta(y)\cdot\delta(z)$
But I don't know how to do the intergral :
$\rho_{\omega} = \int \delta(x-d\cdot cos (\omega t))\cdot\delta(y)\cdot\delta(z)e^{i\omega t} dt$
Any ideas? Do I really need to do this integral?

2. Jul 5, 2011

### Mute

Generally, if you have an integral over a delta function of another function, you can use the relation

$$\delta(f(t)) = \sum_{j}\frac{\delta(t-t^\ast_j)}{|f'(t^\ast_j)|},$$
where the $t^\ast_j$ are the solutions of $f(t^\ast) = 0$. Note that this relation doesn't work if the derivative is zero at a solution.

3. Jul 6, 2011

Hi,