# Homework Help: Dirac Delta function question

1. Feb 2, 2009

### yungman

I cannot get the answer as from the solution manuel.

Please tell me what am I assuming wrong.
Thanks

2. Feb 2, 2009

### Dick

Just showing that the value of a function is '-infinity' at a point and zero otherwise doesn't tell you it's a delta function. Apply the divergence theorem to grad(1/r) on a sphere around the origin to figure out how much 'charge' is there.

3. Feb 2, 2009

### yungman

I understand that and I showed in my own answer. What I don't get is where the 4$$\pi$$ come from!!
Thanks

4. Feb 2, 2009

### Dick

No, all your answer said was the value at zero is '-infinity' (I'm putting that in quotes because I'm not even sure it makes sense). I said that DOESN'T mean it's a delta function. The 4*pi comes from checking the flux through a sphere around the origin. It comes from the 4*pi in the formula for the area of a sphere.