Question

why the force due to -q charge in opposite direction of electric field

Answer 1

Hello!
Here the electric field (E, due to some outside influence) exerts force on both the charges q and -q.

The force on charge q is F1 = qE

and the force on charge -q is F2 = -qE = -F1

this negative sign means that the direction of the force is in opposite direction to both the force on +q and the electric field E.

So, this comes as a consequence of the polarity of the charge.