Guillaume Apollinaire

- The magnitude of the force of attraction (or repulsion), F
_{12}between two point charges q_{1}and q_{ 2 }is given by Coulomb's Law. - The direction of this force is along the line joining the two charges with the sense determined by the relative signs of the charges
- Note that the force on each charge has the same magnitude (as required by Newton's third law of motion).
- For two 1 Coulomb charges separated by 1 metre the
magnitude
of the force is given by,

F = (9 x 10^{9}x 1 x 1 )/ 1 = 9 x 10^{9}NewtonsThis is an

force (sufficient to move Mt. Everest with an acceleration of 1cm/s*extremely large*^{2}). The Coulomb is aunit. Typical macroscopic charges are measured in micro-coulombs (10*very large*^{-6}C). - To handle situations with more than one charge, the charges must
be treated in pairs, so that the overall force on one charge will be
the vector sum of the force
due to each of the other charges. For example the force on q
_{1}due to all other charges q_{2}, q_{3}, q_{4}... would be given by,

where R_{12} is the distance between the
charges. k is a constant of proportionality known as the Coulomb
constant, having the value 9 x
10^{9} N.m^{2} / C^{2} in a
vacuum.

Note that the Coulomb constant, k, is
often replaced with (1/4π ε_{0}), where
ε_{0}is the permittivity of the vacuum (more later).

F_{1} = F_{21} +
F_{31} + F_{41} + ...

both are "inverse square" laws. Substitute charge for mass and "k" for "G" and you have Coulomb's law.

The relative magnitudes of the Coulomb constant, k = 9 x 10^{9} and the gravitational constant, G =
6.67 x 10^{-11}, is an indication of the relative strengths of
the two forces. The electrical force of attraction is much, much
stronger than the gravitational force of attraction.

- Notice the similarity of Coulomb's Law to Newton's Law of Gravitation

both are "inverse square" laws. Substitute charge for mass and "k" for "G" and you have Coulomb's law.

The relative magnitudes of the Coulomb constant, k = 9 x 10

Albert Einstein

*Dr. C. L. Davis*

*Physics Department*

*University of Louisville*

*email*: c.l.davis@louisville.edu