Internal and external work on a charge?

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?

• > "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?" This is odd, where did your read it? What is the "internal electric field"? – Ján Lalinský Nov 21 '17 at 15:10
• The internal electric field is the one causing repulsion or attraction between charges and produces internal work, the external electric field is caused by any source outside the system of charges and produces external work on the charge in the system, my question is why are the internal and external fields are equal in magnitude and why are they in opposite directions ? – Mohamed A. Nov 21 '17 at 17:17