Coulomb's Law - The law of force

Any two charged objects will create a force on each other.   Opposite charges will produce an attractive force while similar charges will produce a repulsive force.  The greater the charges, the greater the force.  The greater the distance between them, the smaller the force.  For two spherically shaped charges the formula would look like:

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  • F  the force on each charge, + indicates repulsion, - indicates attraction
  • k the electrostatic constant 
  • q1 the quantity of charge 1 measured in coulombs
  • q2 the quantity of charge 2 measured in coulombs
  • r the radius of separation from center of one charge to the center of the other.

The forces are force pairs of each other so they will always be equal in size and opposite in direction. 

Quantity of charge can be measured in either elementary charges (an elementary charge is the amount of charge on one electron or proton) or in Coulombs.  An elementary charge is a very tiny unit of charge.  Since it is so small it is not usually a convenient unit to measure typical amounts of charge.  It would be similar to measuring distances from one town to the next, in millimeters.  On the other hand, a coulomb is an incredibly large unit of charge.  It is actually too large a unit of charge for talking about electrostatics (stationary charges) but it is an appropriately sized unit as we begin describing the quantity of charge moved in an electric circuit.   Unfortunately, we are stuck with either one unit or the other.  Here is a comparison of the two units:

1Coulomb = 6.3x1018 elementary charges
1elementary charge = 1.6x10-19 Coulomb

If you use the electrostatic constant (k) given above you will notice that it is given in terms of Coulombs so that forces us to have to measure the quantities of charge in Coulombs.

You should notice that the formula for Coulomb's law is very similar to Newton's Universal Law of Gravitation

fkq1q2.gif (1673 bytes)

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