# Jackson Question

Can someone explain to me how Jackson, on page 42 of the 3rd ed. of Classical Electrodynamics, when he is deriving the interaction energy $$W_{int}$$ in his example involving two point charges, gets from equation (1.57) to (1.58). I thought about typing up the TeX but I'm sure most of you have this book. I know he makes that substitution he mentions but I'm not sure how to go about doing that. Help!

## Answers and Replies

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Born2bwire
Gold Member
I keep my copy of Jackson at the office and not at home. I do not need him to pervade my life any more than is possible.

Redbelly98
Staff Emeritus
It's just a straight substitution: Solve for $\mathbf{x}$ in terms of $\mathbf{\rho}$, and substitute. If you get stuck, post your work and we'll be able to help you better.