Question

please answer it correctly.......with reason... .

Answer 1

When there is a uniform intensity of electric field with respect to distance ' r',  over a given space, we can use   E= V/r . But if intensity of electric field is non uniform with respect to distance 'r' , over a given space, we have to use E= -dv/dr for small segment in which considaring uniformality of electric field intensity in that segment.
    By integrating all E= -dv/dr , for entire length we had an average value of intensity of electric field.