Electrostatic Force
Other than the force of gravity, the force that you are likely most familiar with is the electrostatic, or coulombic, force. This force causes electrically charged particles to be repelled or attracted to each other. Positively charged objects are attracted to negatively charged objects, and vice versa. Positively charged objects repel other positively charged objects, and negatively charged objects repel other negatively charged objects.
When someone rubs a balloon on your hair, your hair becomes positively charged and the balloon becomes negatively charged. You may see the balloon seem to disobey gravity by sticking to your hair. This is because the negatively charged balloon is attracted to your positively charged hair. You may also see your hair become frizzy and large – this is because individual positively charged strands of your hair are repelled by other positively charged strands of your hair. These forces are collectively referred to as the electrostatic, or coulombic, force. The magnitude of these forces is given by the following equation:
… where k is Coulomb’s constant (9 * 109 N m2 / C2 ), q refers to the charge of one of two particles (measured in the unit Coulombs, or C), and r is the distance between the two particles, in meters. For example, if two particles were one meter away from each other, and both had charges of +1 coulomb, then each particle would experience a force of 9*109 Newtons. Since both of these particles are positive, then the forces will be directed away from the other particles.
If two particles were one meter away from each other, and one particle had a charge of –1 coulomb and the other a charge of +1 coulomb, then this equation would yield a force magnitude of -9 * 109 Newtons. The negative sign in this quantity refers to the fact that that the forces would then be directed towards one another instead of away from one another. Both particles still experience a force of 9 * 109 Newtons, but the change of sign refers to a change in the direction of the forces.