Some equipotential surfaces are shown in the figure . Determine electric field at points `A , B,C` in `(a)` and in `(b)`.

Explanation

In the direction of field , potential decreases. In the above cases , electric field is perpendicular to equipotential surfaces.
(a) `E_(A) = (40 - 30)/(20) = (1)/(2) V//cm` , along `+x`-axis
`E_(B) = (30 - 20)/(10) = 1 V//cm` , along `+x`-axis
`E_(C ) = (20 - 10)/(5) = 2 V//cm` , along `+x`-axis
(b)
Take perpendicular distance between equipotential lines , here electric field is uniform.
`E = (20 - 10)/(5 sin 30^(@)) = 4 V//cm`