I have spent too long thinking about a problem and have gotten myself confused. Rather than spouting all the different ways I've gone about doing this calculation, I will just present the problem:

Imagine a flexible molecular chain that has a positively charged end (+1 electron unit, 1.62e–19 C) and one negatively charged end (–1 electron unit). For the sake of simplicity, let's say that the chain is neutral and non-polarizeable. If there is no applied external electric field, the chain will fold over to bring the charges as close together as possible, let's say 5Å (5e–10 m), but given an applied external field of sufficient strength, the chain will extend completely such that the charges are now separated by a distance of 15 Å. What is the minimum field strength required to accomplish this (for the moment assuming the solvent has no significant dielectric constant or polarizeability itself...)

[diagram=758_2]

Thank you!