The electric charge is a fundamental property of matter that allows particles to exert electrical forces.
If you ask someone what electricity is, they’ll probably tell you it’s “the flow of electrons.” And they’d be right. But why do they flow? Where do they come from? And why do some materials allow them to flow while others don’t?
To answer this, we have to stop thinking about wires and plugs for a moment and look inside matter, on a very small scale. Let’s talk about the structure of the atom.
The Structure of the Atom
All matter we know is made up of atoms. Although the current quantum model is complex (probability clouds), to understand electricity, the Bohr model (the classic miniature solar system) is enough for us.
An atom is composed of two main parts:
- The Nucleus: Located at the center. It contains Protons (with a positive charge) and Neutrons (with no electric charge). Almost all the mass of the atom resides here, but it does not “directly” participate in ordinary electrical conduction.
- The Electron Cloud: The region where we find electrons (with a negative charge) distributed in orbitals. They do not follow defined orbits like planets around the Sun, although we often draw them that way for simplicity.
Electrical Equilibrium
In its natural state, an atom is electrically neutral. This means it has the same number of protons as electrons.
The force that keeps the electrons orbiting around the nucleus is the electrostatic force: opposite charges attract. The positive nucleus pulls on the negative electrons.
This attractive force is strong near the nucleus, but it weakens as we move away towards the outermost layers of the atom.
Electric Charge (Q)
Electric charge is an intrinsic property of matter, as fundamental as mass. It manifests through forces of attraction (opposite charges) or repulsion (like charges).
The unit of measurement for electric charge in the International System is the coulomb (C).
However, the coulomb is a macroscopic unit. The magnitude of the elementary charge (
The charge of the electron is negative, so it is written as
This has an impressive numerical implication. If we want to gather a charge whose magnitude is 1 coulomb, how many electrons do we need to group together?
We are talking about more than 6 quintillion electrons to reach the magnitude of a single coulomb.
Remember this: when we say that 1 ampere flows through a wire, we are saying that 1 coulomb per second passes through a section. That is, a huge stream of electrons per second.
The Valence Electron and Conduction Electrons
Here we arrive at the most important concept of this article. Not all electrons are equal for practical purposes.
Electrons are organized in shells or energy levels around the nucleus.
- Electrons in the inner shells are very strongly bound to the nucleus. It is very difficult to remove them.
- Electrons in the outermost shell are called valence electrons.
Since these valence electrons are farther from the nucleus, the attractive force they feel is much weaker. Additionally, the inner electrons create a “shielding” effect, further reducing the nucleus’s influence on the outer electrons.
Delocalized Electrons in a Metal
In metals, such as copper, gold, or aluminum, the orbitals of many atoms combine. Consequently, some of their valence electrons become delocalized throughout the entire crystal lattice instead of belonging to a specific atom.
These electrons are already available to conduct. Without an electric field, their thermal motion is random and produces no net current; upon applying a field, they acquire a small, ordered drift velocity.
We often call them free electrons, although conduction electrons is a more precise name.
In a metallic conductor, current appears when the conduction electrons acquire a net ordered motion.
Ions: When the Balance is Broken
When an atom loses a free electron, it ceases to be neutral. It now has one more proton than the number of electrons.
- The atom that loses electrons becomes positively charged (cation).
- The atom that gains electrons becomes negatively charged (anion).
Although in copper wires it is the electrons that move, in other media (such as in batteries or electrolysis), ions also move and transport charge.
For an electric current to exist, we need charges with freedom of movement. The electronic structure of the material determines why copper conducts easily and plastic does not (we will see this in its own entry).
