Electric Charge and Coulomb's Law

Electric charge is a fundamental property of matter, as basic to the physical world as mass. Ordinary matter is built from atoms containing positively charged protons, negatively charged electrons, and neutral neutrons. The proton and electron carry equal but opposite amounts of charge, which is why bulk matter is normally electrically neutral: the number of protons balances the number of electrons. When this balance is disturbed, an object becomes charged. The SI unit of charge is the coulomb ($\text{C}$), and the smallest free charge observed in nature is the elementary charge, $e = 1.6 \times 10^{-19}\ \text{C}$. Because charge comes in whole-number multiples of $e$, we say charge is quantized: any charge $Q$ can be written as $Q = ne$, where $n$ is an integer.

Two rules govern how char