- #1
TerryW
Gold Member
- 191
- 13
Homework Statement
I have a couple of issues with this problem, but first of all, I want to find out what I am missing in my attempt to prove ##∇^2Φ = 4πρ##
Homework Equations
See below
The Attempt at a Solution
[/B]
If ##Φ = \frac {-M}{r}##
Then ##Φ_{,i} = \frac {Mx^i}{r^3}##
So ##Φ_{,ii} = \frac {-3Mx^ix^i}{r^5} + \frac {M}{r^3} = \frac {-2M}{r^3}##...(1)
Taking ##ρ = \frac{M}{V} = \frac{3M}{4πr^3}##
Gives ##Φ_{,ii} = \frac {-8πρ}{3}##
Not what I wanted at all!
I've looked through some of my solutions to previous exercises and found that I've used something like (1) before (actually ##\Big(\frac{1}{r}\Big)_{,ii} = \frac {-3x^ix^i}{r^5} + \frac {1}{r^3}##) and it produced the correct answer. Why doesn't it work here?TerryW