Why electric field is always at right angle to the equipotential surface ?

Explanation

Work moving a charge from one point to other on an equipotential surface is zero.
`W_(AB)=0`
But `W_(AB)=intvecE.vec(dl)=Edlcostheta`
`implies` For equipotential surface, `E dl cos theta=0`
`impliescostheta=0=cos90^(@)`
`impliestheta=90^(@)`
Equipotential surfaces for a uniform electric field are equidistance parallel planes and perpendicular to field lines.