A infinite plane of charge of uniform surface charge density is along the x - z plane. What is the potential energy of a charge q at a distance y from it? What reference point would you choose for zero of potential energy?
Answers
These are imaginary surfaces surrounding a charge distribution. In particular, if the charge distribution is spherical (point charge, or uniformly charged sphere), the surfaces are spherical, concentric with the center of the charge distribution. Electric field lines are always perpendicular to the equipotential surfaces. The equation  implies that due to the negative sign, the direction of E is opposite to the direction in which V increases; E is directed from higher to lower levels of V (from higher potential to lower potential). Another words, the gradient of a scalar (in this case E field) is normal to a surface of constant value (equipotential surface) of the scalar and in the direction of maximum rate of change of constant scalar. Remember this statement when we perform the experiment.