Assuming we have a charge existing in an electric field, then we have a work done on this charge called the internal work, if we apply an external electric field why does it have to be in the opposite direction of the internal one? (did we just assume it this way or what?), and why does the external electric field equal to the internal one (in magnitude)? in other words, i'm having trouble with the following laws :
Why does the electric field in the case of internal work and external work has the same magnitude?