The current generated by the second particle is J = q 2 v 2 δ δ ( r − − r 2 ) , which has a Fourier transform J ( k ) ≡ ≡ ∫ ∫ d 3 r exp ⁡ ⁡ ( − − i k ⋅ ⋅ r ) J ( r ) = q 2 v 2 exp ⁡ ⁡ ( − − i k ⋅ ⋅ r 2 ) .