notes/education/physics/PHYS2220/Electric Charge.md

Electric Charge

• Charges come in two varieties, positive and negative.
• Net charge is the algebraic sum of an object's charges
• Protons and electrons have the same magnitude of charge (1e)
  • The SI Unit of charge is the Coulomb (abbreviated C)
  • The smallest discrete quantity of charge is \frac{1}{3}e.
• In an isolated system, the net charge will always remain constant.

Coulomb's Law

• Two charges will exert a force on each other along the line joining them.
  • The magnitude of this force is proportional to the product of the charges and inversely proportional to the to the \sqrt{dist}.
  • The equation to determine the force between two charges is as follows:
  vec{F}_{12} = \vec{r}k\frac{q_1q_2}{r^2} $$

    - $\vec{r}$ is a unit vector pointing from charge 1 to charge 2
    - $k$ is Coulomb's constant, or $8.99 * 10^9 \frac{Nm^2}{C^2}$ 
    - $q_1$ and $q_2$ are the charges
    - $r$ is the distance between those charges
    - The resulting force will push away if $q_1q_2$ is *positive*, and attract if $q_1q_2$ is negative. This is where the rule "opposites attract, like repels" comes from
  

• Coulomb's law only holds exactly true for point charges i.e a proton or electron.

The Superposition Principle

The superposition principle states that:

The net charge acting on a point charge is equal to the sum of all individual forces.

This means that to find the net force ac