Is a plane taken a equipotential surface?
Why Electric field is taken in right direction?
Is that E vector(Electric field)?
Dear Student, Which plane you are taking about? If it is the plane of the paper then it shouldn't be an equipotential surface because electric field exists on it and electric field is zero for an equipotential surface. What I can make out is that it is the derivation of electric field and potential relation. E vector is the electric field vector and Work done per unit charge on the charge in moving it from B to A against the electric field is determined in an electric field acting in the opposite direction which is equal to the difference in potential between the final and the initial point is V_A - V_B. This increases the potential energy of the system amount of energy stored per unit charge is the positive potential difference. You can take E along the left direction but then there will be a decrease in potential energy of the system and V_A - V_B will be negative. Regards