Electric Charge

What is electric charge?

Electric charge is a property that objects can have. Charged objects push or pull on each other, and that force can either attract or repel.^[1] Everything electrical, from a static spark to a power station, comes down to charges and how they move.

Two kinds of charge

There are two types of charge. Benjamin Franklin proposed calling them positive and negative.^[1] The rule for how they behave is simple: if two objects carry the same sign of charge, they repel; if the charges are of opposite sign, they attract.^[1]

Inside atoms, electrons carry negative charge and protons carry positive charge of exactly the same size: about 1.602 × 10⁻¹⁹ coulombs each. This amount, called the elementary charge, is the smallest possible amount of charge an object can have.^[1] It’s so small that one coulomb, the SI unit of charge, is about 6.24 billion billion elementary charges.^[4] The coulomb is named after the French physicist Charles-Augustin de Coulomb.^[1] Coulomb had a long career in the French military engineering corps. He invented a device called a torsion balance that let him measure very small charges, and published a series of important papers on electricity and magnetism in the late 1780s.^[5]

How things get charged

Most objects are neutral: they hold equal amounts of positive and negative charge. An object becomes charged when electrons are added to it or taken away from it.^[1] Rubbing two materials together is a classic way to move electrons from one to the other. More than 2,500 years ago, Thales of Miletus recorded that amber rubbed with fur attracts the fur.^[1]

Charge is conserved. It can be transferred from place to place and from one object to another, but the net charge of the universe stays constant.^[1] When a balloon picks up extra electrons from your hair, your hair is left with exactly that much positive charge.

Conductors and insulators

Whether charge can move through a material depends on its electrons. In copper, each atom has one outer electron that wanders from atom to atom. These conduction electrons are the ones that move when electricity flows.^[2] Materials like this, including most metals, are conductors.^[2]

Insulators have no conduction electrons, so charge flows through them only with great difficulty, if at all. Amber, fur, wood, glass and plastic are all insulators.^[2]

Earth is a good conductor. It can easily supply or accept extra charge, so connecting a charged object to the ground with a wire lets electrons flow to or from it. This is called grounding.^[2]

A charged object can also affect a conductor without touching it. Bring it close, and the conductor’s free electrons shift towards one end or away from it, leaving opposite charges at the two ends. This is charging by induction.^[2]

Coulomb’s law: how strong is the force?

The electric force between two charged objects is proportional to the charge on each, and inversely proportional to the square of the distance between them.^[3] Coulomb’s experiments with his torsion balance led him to this inverse square law.^[5] So if you double the distance, the force drops to a quarter; triple it, and the force falls to a ninth.

The math (optional)

Coulomb’s law is usually written as:

F = k × q₁ × q₂ / r²

Here q₁ and q₂ are the two charges in coulombs, r is the distance between them in metres, and k is Coulomb’s constant, about 8.99 × 10⁹ N·m²/C².^[3] The huge value of k is why even tiny amounts of charge can produce noticeable forces.

Static electricity in everyday life

Static electricity is all around us: clothes clinging after the dryer, a wool sweater crackling, a balloon stuck to a wall, a small shock from a doorknob, and lightning in a thunderstorm.^[1] A comb run through your hair can even attract a thin stream of water or small scraps of paper.^[1]

Going further

Charge on the move is an electric current, covered in Electricity & Magnetism and in Electric Circuits. The free simulation below lets you rub a balloon on a sweater and watch the charges move.