How do you calculate the electric force by object a exerted on object a (itself)?

I don't think an object can exert a force on itself.

Try it: Get up on a skateboard, and see if you can do anything to yourself that makes you start moving ... without touching anything else.

It'll be easy to tell if you succeed. If you actually do exert an unbalanced force on yourself, then you'll begin to accelerate.

god i hope this !