Electric charge is a fundamental property of matter — as basic as mass, but with a twist: it comes in two kinds.

• Positive charge (protons, or more loosely, the charge on a glass rod rubbed with silk)
• Negative charge (electrons, or a rubber rod rubbed with fur)

The rule is simple: like charges repel, opposite charges attract.

Coulomb’s law

The force between two point charges follows an inverse-square law, just like gravity:

$$F = k \frac{q_1 \, q_2}{r^2}$$

where $k \approx 8.99 \times 10^9$ N·m²/C², $q_1$ and $q_2$ are the charges, and $r$ is the distance between them.

• Same sign → positive force → repulsion
• Opposite signs → negative force → attraction
• Doubling the distance → one quarter the force

Charge is conserved

You can move charge around, but you can’t create or destroy it. The total charge of an isolated system never changes. This is conservation of charge — one of the most fundamental laws in physics.

When you rub a balloon on your hair, you don’t create charge. You transfer electrons from your hair to the balloon. The balloon becomes negative, your hair becomes positive, and the total stays zero.

Charge is quantized

All charge comes in integer multiples of the elementary charge $e \approx 1.602 \times 10^{-19}$ coulombs. A proton carries $+e$, an electron carries $-e$. You never find $0.7e$ floating around.

From force to field

Coulomb’s law describes the force between two specific charges. But there’s a more powerful way to think about it: one charge creates a field everywhere in space, and a second charge placed in that field feels a force. This is the step from “force between objects” to “electric field” — and it’s where Maxwell’s equations begin.