Things that are electrically charged behave very similarly to magnets. Using this analogy, we can look at how objects with charges are either attracted or repelled. Most everyone has played with magnets as a kid, and you may have noticed that magnet usually have ends which are labeled (N)orth (S)outh. With very little effort, you can figure out that north attracts south, north repels north and south repels south. Electrical charges are exactly the same. Positive charges attract negative charges, positive charges repel positive charges, and negative charges repel negative charges.
It can be useful (as in trying to complete a homework assignment, but I am sure there are other uses which I just can’t think of at the moment) to be able to calculate the forces present between two charged objects. In order to do this, all you need to know is the charge on each of the objects, and the distance between the objects. In the above solution video, I use Coulomb’s Law to calculate the repulsive force of two balloons which have become negatively charged.