The Electric Force: Coulomb's Law

Remember, UNLIKE charges attract,

or

and LIKE charges repel,

or

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 thesquareof the distance r between the charges,

F 1 / r^{2}

^{}

F 1 / r^{2}

^{}

F 1 / r^{2}

We can combine these asF Q q / r^{2}

Whileproportionalitiesare good for qualitative discussions and comparisons,equationsare much easier to use for calculations. We can change this to anequationwith aconstant of proportionality, k.F = k Q q / r^{2}

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 ofk = 9 x 10^{9}N m^{2}/C^{2}

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?

(c) Doug Davis, 2002; all rights reserved

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