Rigid body with field won't move

Hi, everyone. The Blender Artists forum has helped me a great deal in coming up to speed with the software, so thanks to everyone here.

I’d like to use charge force fields on rigid body objects to simulate charged particles (e.g. protons and electrons) being repelled and/or attracted to one another. I’m finding that a rigid body that has any force field won’t move unless hit by another rigid body. That would work for situations where one object has a field and the others are just active rigid bodies, but in my case all of the objects need to have a field. Here’s an example file with two objects, each with a force field, that don’t move (removing the field from one of the objects allows that object to move):
objectsWithFieldsDontMove.blend (494 KB)
I’m using version 2.71 and cycles render (as opposed to game engine).
Can anyone suggest a way to fix this problem so I can run the simulation? Thanks.