Electric charge is a fundamental property of matter that determines how particles create and interact with electric and magnetic fields. It comes in two types, positive and negative, and it’s the reason lightning strikes, your phone turns on, and your nerves can send signals. Every electrical phenomenon you encounter traces back to this single property.
Charge at the Atomic Level
Inside every atom, protons carry positive charge and electrons carry negative charge. Neutrons, as the name suggests, are neutral. The amount of charge on a single proton or electron is called the elementary charge, and it’s incredibly small: 1.602 176 634 × 10⁻¹⁹ coulombs (the coulomb being the standard unit for measuring charge). That number is exact, not rounded. Since 2019, the international system of measurement units actually defines the ampere, the unit of electric current, by fixing the elementary charge to that precise value.
A proton’s charge is +e and an electron’s charge is −e. They’re equal in size but opposite in sign. In a normal atom, the number of protons equals the number of electrons, so the whole atom is electrically neutral. When an object gains or loses electrons, it ends up with a net charge. Every charged object you encounter, from a staticky sock to a Van de Graaff generator, has its charge because electrons have been added to it or stripped away from it.
How Positive and Negative Charges Interact
Opposite charges attract each other. Like charges repel. This is the most basic rule of electrostatics, and it scales predictably. The force between two charged objects depends on two things: how much charge each one carries and how far apart they are. Doubling the charge on one object doubles the force. Doubling the distance between them cuts the force to one quarter. This relationship, known as Coulomb’s law, is one of the most important equations in physics.
The force acts equally on both objects. If a positive charge pulls on a negative charge with a certain force, the negative charge pulls back on the positive one with exactly the same force in the opposite direction. This holds whether you’re talking about two protons inside a nucleus or two charged balloons across a room.
Charge Is Always Conserved
One of the iron rules of physics is that electric charge cannot be created or destroyed. In any closed system, the total charge stays constant. You can move charge around, separate positive from negative, or transfer electrons between objects, but the total always adds up to the same number. This is the law of conservation of charge, and no experiment has ever found a violation of it. It arises from deep symmetries in the equations governing electric and magnetic fields.
This means that when you rub a balloon on your hair and the balloon becomes negatively charged, your hair becomes equally positively charged. Electrons moved from your hair to the balloon, but none were created or lost in the process.
How Charge Moves Through Materials
Whether a material conducts electricity depends on how tightly its atoms hold onto their outermost electrons. In metals like copper, the outermost electron shell contains just one or two electrons that are far from the nucleus and weakly bound. These electrons easily break free and drift through the material, which is why metals are good conductors. Copper, for example, has a single electron in its outermost shell that readily wanders away from its parent atom.
Insulators like rubber and glass have atoms whose outer electrons are tightly held. Very few electrons break free, so charge has no easy path to flow through. Semiconductors, the materials that power computer chips, fall somewhere in between and can be engineered to conduct under specific conditions.
The carriers of charge also differ depending on the medium. In a metal wire, free electrons do all the work. In a saltwater solution or a battery’s electrolyte, the current is carried by ions, which are atoms or molecules that have gained or lost electrons. These ions are chemically active, which is why electrolysis can break water into hydrogen and oxygen or plate metal onto a surface.
Static Charge and the Triboelectric Effect
When two different materials rub together, electrons transfer from one surface to the other. This is called the triboelectric effect, and it’s the reason you get shocked after shuffling across carpet in socks. The transfer happens through several mechanisms: electrons jumping between surfaces, ions migrating, and in some cases tiny bits of material physically moving from one object to another.
Different materials have different tendencies to give up or grab electrons. Glass tends to lose electrons (becoming positive), while rubber tends to gain them (becoming negative). The buildup of charge on a surface is what we call static electricity. It stays put until it finds a path to discharge, which is the spark you feel when you touch a doorknob.
Charge in Your Body
Your nervous system runs on carefully managed charge differences. Every nerve cell maintains a voltage of about −70 millivolts across its membrane at rest, with the inside of the cell more negative than the outside. This voltage exists because the cell actively pumps charged ions, primarily potassium and sodium, to maintain an uneven distribution across its membrane.
When a nerve signal fires, channels in the membrane open and allow sodium ions to rush in, briefly flipping the voltage to positive. This wave of charge reversal travels down the nerve fiber at high speed, carrying signals from your brain to your muscles or from your fingertips to your brain. The entire process depends on the same fundamental property: charged particles interacting through electric fields.
Measuring Charge
The standard unit of charge is the coulomb, named after the French physicist Charles-Augustin de Coulomb. One coulomb equals the amount of charge that flows past a point when one ampere of current runs for one second. In terms of actual particles, one coulomb is roughly 6.24 × 10¹⁸ electrons, an enormous number that reflects just how tiny the charge on a single electron is.
In everyday life, you rarely encounter charges measured in full coulombs. A lightning bolt transfers about 5 coulombs. The static shock from a doorknob involves a few microcoulombs at most. The charges that matter in electronics are typically measured in millionths or billionths of a coulomb, yet they’re enough to power the device you’re reading this on.