When two electrically charged bodies come closer, appears a force that attracts or repels them, depending on the sign of the charges of this two bodies.
This is stated by Coulomb's Law:
"The electrostatic force [tex]F_{E}[/tex] between two point charges [tex]q_{1}[/tex] and [tex]q_{2}[/tex] is proportional to the product of the charges and inversely proportional to the square of the distance [tex]d[/tex] that separates them, and has the direction of the line that joins them"
Mathematically this law is written as:
[tex]F_{E}= K\frac{q_{1}.q_{2}}{d^{2}}[/tex]
Where [tex]K[/tex] is a proportionality constant.
Now, if [tex]q_{1}[/tex] and [tex]q_{2}[/tex] have the same sign charge (both positive or both negative), a repulsive force will act on these charges.
