Point Charge Potential

The electric potential (voltage) at any point in space produced by a point charge Q is given by the expression below. It is the electric potential energy per unit charge and as such is a characteristic of the electric influence at that point in space. Since it is a scalar quantity, the potential from multiple point charges is just the sum of the point charge potentials of the individual charges and can be extended to calculate the potential from a continuous charge distribution.

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k=Coulomb's constant
Voltage = k x charge/radius
At a distance r = x10^ m
from a charge Q = x10^ C
the voltage is V = x10^ volts.

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Potentials for other charge geometries
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Point Charge Potential

The potential (voltage) of a point charge can be evaluated by calculating the work necessary to bring a test charge q in from an infinite distance to some distance r. The zero of potential is chosen at infinity.

Considering a radial path from a to b, the work done by the Coulomb force is obtained from a line integral which in this case becomes just a polynomial integral since we are moving along a straight radial line:
The change in potential is the work done per unit charge against the Coulomb force, so

For a positive charge q, the Coulomb force does positive work in moving it from a to b, so that represents a drop in potential energy.

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Voltage concepts
 
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