The Electric Force: Coulomb's Law

Remember, UNLIKE charges attract,


and LIKE charges repel,


What determines the magnitude of this electric force F?

The force F is directly proportional to the charges Q and q,

F Q q

The force F is inversely proportional to the square of the distance r between the charges,

F 1 / r2

F 1 / r2

F 1 / r2

We can combine these as

F Q q / r2

While proportionalities are good for qualitative discussions and comparisons, equations are much easier to use for calculations. We can change this to an equation with a constant of proportionality, k.

F = k Q q / r2

This constant of proportionality, k, depends upon the units we use. If we measure force F in newtons (N), distance r in meters (m), and charges Q and q in coulombs (C), then k has a value of

k = 9 x 109 N m2/C2

Now we must ask, just what is a coulomb of charge, anyway?

The electric charge of an electron or a proton is labeled e and is equal to . . .

Coulomb's Law describes the force F between two electric charges, Q and q, a distance r apart,


What is the force between two positive charges, 1 C and 2 C, when they are separated by a distance of 1 m?

Conductors & Insulators

Force is a Vector

Return to Ch 17, Electrostatics & Electric Energy
(c) Doug Davis, 2002; all rights reserved