TF DerivationOfCoulombForce

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Poisson's Equation
2ϕ(ξ)=ρϵ0=eϵ0δ(ξ)

Fourier Transform of Poisson's Equation

1(2π)3/2eikξ2ϕ(ξ)dV=1(2π)3/2eϵ0eikξδ(ξ)dV
1(2π)3/2eikξ(ϕ(ξ))dV=e(2π)3/2ϵ0(1)

Product rule for dervatives

1(2π)3/2{(eikξϕ)(eikξ)(ϕ)}dV=e(2π)3/2ϵ0(1)


Gauss' Theorem:

(eikξϕ)dV=SeikξdA


Definition of derivative:

(eikξ)(ϕ)=(ϕeik)ϕ2eikξ


Substituting

1(2π)3/2{eikξϕdA(ϕeikξ)dV+ϕ2eikξdV}=e(2π)3/2ϵ0


Gauss' Low:

(ϕeikξ)dV=ϕeikξdA


1(2π)3/2{{eikξϕϕeikξ}dA+ϕ2eikξdV}=e(2π)3/2ϵ0


1(2π)3/2ϕ(ik)(ik)eikξdV=e(2π)3/2ϵ0


k21(2π)3/2ϕ(ξ)eikξdVxi=e(2π)3/2ϵ0

k2ϕ(k)=e(2π)3/2ϵ0

1.) Coulomb ϕ(k)=e(2π)3/2ϵ01k2 = potential in "k"(momentum) space

To find the potential in "coordinate" (ξ) space just inverse transform

ϕ(ξ)=1(2π)3/2e+ikξϕ(k)dVk
=1(2π)3/2eikξe2π)3/2ϵ01k2dVk
=e(2π)3ϵ0eikξk2dVk


dVk=k2sinθkdθkdϕkdk


=e(2π)3ϵ002πdϕk0πdθk0dk×k2sinθkeikξ
=e(2π)2ϵ00π0sinθkeikξcosθkk2dk

u=cosθ

du=sinθdθ