Some particles in the universe have a fundamental property called electric charge. Much like mass is the property that causes gravity, electric charge is the property that causes particles to feel the electromagnetic force. This force is responsible for everything from the static shock you get from a doorknob to the light coming from your screen.

There are two kinds of electric charge: positive and negative. Protons, found in the nucleus of an atom, have a positive charge. Electrons, which orbit the nucleus, have a negative charge. Neutrons, the third main particle in an atom, have no charge at all.

An object's overall charge depends on the balance of its protons and electrons. If an object has more electrons than protons, it has a net negative charge. If it has fewer electrons than protons, it's positively charged. If the numbers are equal, the object is electrically neutral.

The type of charge a particle has determines how it interacts with other charged particles. The rule is simple and absolute.

This means two positive charges will push each other away, as will two negative charges. A positive charge and a negative charge, however, will pull toward each other. This is the force that holds electrons in orbit around the atomic nucleus.

You can see this effect when you rub a balloon on your hair. The balloon strips electrons from your hair, giving the balloon a net negative charge and your hair a net positive charge. Since they now have opposite charges, they attract, and your hair stands on end, reaching for the balloon.

Charge isn't a continuous fluid. It's quantized, which means it exists in discrete, indivisible packets. The smallest possible unit of charge is called the elementary charge, denoted by the symbol $e$. Every proton has a charge of exactly $+e$, and every electron has a charge of exactly $-e$.

Any observable amount of charge, whether positive or negative, will always be a whole-number multiple of this elementary charge. You can have a charge of $2e$ or $-10e$, but you can't have a charge of $0.5e$.

The official SI unit for charge is the coulomb (C). One coulomb is the total charge of approximately $6.24 \times 10^{18}$ electrons. It's a huge amount of charge, so in everyday situations, we often deal with microcoulombs (μC) or nanocoulombs (nC).

Materials behave differently when it comes to electric charge. Their properties are determined by how freely electrons can move within them. We can group materials into three main categories.

In conductors, some electrons (called conduction electrons) are not bound to any single atom and can wander throughout the material. This mobility is why metals are so good at conducting electricity.

In insulators, electrons are held firmly to their atoms and cannot move around easily. This is why electrical wires are coated in rubber or plastic—to prevent the charge from escaping.

Semiconductors are special. Their ability to conduct can be precisely controlled by adding impurities or applying an electric field. This property is the foundation of modern electronics, from computer chips to solar panels.

Understanding these basic properties of charge is the first step toward exploring the powerful and fascinating world of electricity and magnetism.