Coulomb's law states that the force between two charges and separated by a distance is proportional to the signed magnitudes of the charges and inversely proportional to . In SI units, charge is measured in coulombs (C) and distance in meters (m). The constant equals , giving the force in newtons (N):

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This Demonstration describes the interaction of three point charges. Charge 1 is fixed in space; you can vary the distances from charge 1 to charge 2 and charge 1 to charge 3, as well as the angle between charges 2 and 3. The two plots at the bottom show the forces and between charges 1 and 2 and between charges 1 and 3 as a function of distance. The top-right plot is the total force on charge 1, which is the sum of the two vectors and (so it depends on the angle between two charges):