• The Electric Force originates from the charge on one particle and acts on the charge of another particle.
  
  
• Both charges exert a force on each other that have the same magnitude but are in opposite directions.
  
  
• These two force are not the same force. They have to have the same magnitude or else Newton's 3rd law would be vilotated.

F	= Magnitude of the Electric Force that charge q1 exerts on charge q2. (SI: N)
	or (and this is a separate force with the same size but opposite direction)
	= Magnitude of the Electric Force that charge q2 exerts on charge q1. (SI: N)
qi	= Amount of charge on the ith charge. (SI: C = Coul = Coulomb)
	1 e = 1.602x10-19 C (Charge on an electron) 1 C = 6.242x1018 electrons
r	= Positive distance between the center of mass of the charges. (SI: m)
k	= 8.9875x109 N.m2/C2 (Coulomb's Force Constant)
	~ 9.0x109 N.m2/C2 (Approximate value used in this course)
	= 1/4peo (eo = Permittivity of empty space) Alternate expression often used.
eo	= 8.8542x10-12 C 2/N.m2 (Permittivity of free space)

Coulomb's Law QTMovie (144K)
Two particles with different size positive charges and different masses are observer to move apart do to the electric force action upon each other. Coulomb's Law IP Simulation

• The Force is along a line joining the two charges

F is attractive if q1 & q2 have opposite signs.

F is repulsive if q1 & q2 have the same signs.

• You can not rely on the sign you get using Coulomb's Law to determine the direction of the force if you leave off the absolute sign around the product of the charges.
  
  
• The symbol F represent the magnitude of the Electric Force and the magnitude is always a positive number.

• Calculate the electric force and make it a positive number. Then add the sign term depending upon the location of the origin you have chosen.
  
  
• You could use vector notation to express the force that q₁ exerts on q₂. If we let the force that charge 1 exerts on charge 2 be denoted as F₁₂, the Coulumb force be expressed by:

where r₁₂ with a little hat on top is a unit vector point from location of q₁ towards q₂. I find this notation only useful for complex problems that involve computer programing or for problems were you place the origin at some point other than the location of one of the charges.