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Two separated charges

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Two separated charges diagram
The electrostatic force between two point charges with a given separation is given by Coulomb's law (F=Q1*Q2/4π*r²*ε0). ε0 is the vacuum permittivity. The integration of this force over distance gives the potential energy of the system.
Charge 1:
Charge 2:

Potential energy:
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The electrostatic attraction (for opposite charges) or repulsion (for like charges) is denoted by a positive or negative force, respectively. The potential energy is the amount of energy that would be released by allowing the charges to separate to infinity (likewise, for opposite charges it is negative - the energy input required to separate them to infinity).
The factor 1/4π*ε0 is also known as Coulomb's constant. And remember to choose convenient units from the pull-down menus!