The physics
Field lines start on + charges and end on − charges. Their number is proportional to the charge. Where they crowd, the field is strong.
An equipotential joins points at one voltage. The field crosses it at right angles, toward the lower voltage.
E = −dV/dx Ex = −∂V/∂x, Ey = −∂V/∂y
V(a) − V(b) = ∫ E·dr, from a to b (uniform E: ΔV = −E·Δr)
For two unlike charges, V = 0 where r₁/r₂ = |q₁/q₂|. That is a sphere round the smaller charge.