I am talking about pbl 2.46 pag 107.
The potential Arexp(−λr)
The book says the solution for the charge density is: [δ3(r)−λ2rexp(−λr)]ϵ0A
The first part comes from ∇21r , the second comes from ∇2exp(−λr).
I think this part is missing: 2∇1r⋅∇exp(−λr).
Or am I misunderstanding something?
Answer
This is but the screened Poisson equation. Since the functions are radial, only the radial part of the Laplacian survives, ∇2(e−λr/r)=1r2∂r(r2∂r(e−λr/r))=1r2∂r(−λre−λr+e−λrr2∂r1r)=1r2∂r(r2∂r1r) e−λr−λe−λr(1r2+∂r1r)+λ2e−λrr.
Since
∇21r=−4πδ(3)(→r)=−δ(r)r2,
So your text is just fine. You probably misunderstand, that, in fact, ∇2e−λr=(λ2−2λ/r)e−λr, as evident from above. This additional O(λ) term here exactly cancels the cross term you wrote down. (Try evaluating ∇2r to see the point.)
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