The Potential of a Localized Charge Distribution

Setting the reference point an infinity, the potential of a point charge q at the origin is
    V(r) = -1/4πε₀  ∫ q/r² dr’ = 1/4πε₀ q/r’ | = 1/4πε₀ q/r’ q/r

The potential of a point charge q is V(P) = 1/4πε₀ q/r

Suppose that we have some electric charges, q₁, q₂, q₃, …. (Call them source charges)
By the principle of superposition, the potential at any point is the sum of the potentials due to all the source charges separately. So the potential of a collection of charges is

V(P) = 1/4πε₀ ∑_(i=1)^nqi/ri

Suppose charge is distributed throughout some finite volume with a known charge density ρ (C/m3). For a continuous distribution,
    V(P) = 1/4πε₀ ∫ ρ/r dτ

Eq. (3) tells us how to compute V when we know ρ.
The integrals analogous to Eq (3) for line and surface charges are
    1/4πε₀ ∫ λ/r dl and 1/4πε₀ ∫ σ/r da