Friday, 8 March 2019

homework and exercises - Is this problem from Griffiths' "Classical electromagnetism" correct?


I am talking about pbl 2.46 pag 107.


The potential Arexp(λr)

is given I have to find the charge density and the electric field.


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: 21rexp(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(eλr/r)=1r2r(r2r(eλr/r))=1r2r(λreλr+eλrr2r1r)=1r2r(r2r1r) eλrλeλr(1r2+r1r)+λ2eλrr.

Note the cross O(λ) term vanishes.


Since
21r=4πδ(3)(r)=δ(r)r2,

one has (2λ2)(eλrr)=δ(r)r2.


So your text is just fine. You probably misunderstand, that, in fact, 2eλr=(λ22λ/r)eλr, 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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