The electric charge is a fundamental property of matter that causes forces of attraction and repulsion, and it is the real starting point for understanding what happens inside a wire.
To truly understand what happens inside a wire, we must stop looking at the wire and start looking at the atoms that compose it.
Electricity is not a strange fluid. It is the macroscopic manifestation of an intrinsic property of subatomic particles: Electric Charge.
In this article, we will establish the physical foundations upon which all electrical engineering is built.
Atomic Structure: The Stage
All ordinary matter is composed of atoms. Although the current quantum model is complex (probability clouds), to understand electricity the Bohr Model or planetary model is sufficient.
The atom consists of two distinct zones:
- The Nucleus: It is the dense and massive center. It contains:
- Protons (p⁺): Particles with a positive charge.
- Neutrons (n⁰): Particles with no charge (provide mass and nuclear stability).
- The Shell: It is the empty space around the nucleus where:
- Electrons (e⁻): Particles with a negative charge orbit.
The Mass Asymmetry
Here lies a fundamental fact for the engineer: The mass of a proton is approximately 1836 times greater than that of an electron.
Physical Implication: In solids (like a copper wire), the nuclei are too heavy and are trapped in a rigid crystalline structure. The electrons, being extremely light and located on the periphery, are the only ones with appreciable freedom of movement. Therefore, electric current in metals is always a flow of electrons, never protons.
Electric Charge (q): Fundamental Property
Electric charge is an intrinsic property of matter, just like mass. It determines the intensity with which particles interact within an electromagnetic field.
There are two types of charge, which Benjamin Franklin arbitrarily named:
- Positive (+)
- Negative (-)
The fundamental Law of Electrostatics states that:
- Charges of the same sign repel each other (Repulsive force).
- Charges of opposite sign attract each other (Attractive force).
Elementary Charge (e) and Quantization
For a long time, charge was thought to be a continuous fluid. Robert Millikan demonstrated that it is not. Charge is quantized. This means there exists a “minimum packet” of charge that cannot be divided (in the context of standard electricity, ignoring quarks).
This minimum unit is the Elementary Charge (e):
- The charge of the Proton is +e.
- The charge of the Electron is -e.
Any electric charge (Q) we encounter in the universe must necessarily be an integer multiple of this unit:
You cannot have a charge of 1.5 electrons. You either have 1 or 2.
The Macroscopic Unit: The Coulomb (C)
Since the charge of a single electron is absurdly small (10⁻¹⁹), in engineering we do not use the elementary charge. We use a more practical International System (SI) unit: the Coulomb (C).
A Coulomb is formally defined through the Ampere (1 Coulomb is the charge that passes in 1 second with a current of 1 Ampere), but physically it represents a huge “package” of particles.
How many electrons are needed to have -1 Coulomb of charge?
We are talking about 6.24 trillion electrons. When we say that “1 Ampere” passes through a wire, 6 trillion electrons are passing through every second.
The Neutral Atom and Ionization
In their natural state, an atom has the same number of protons and electrons.
The atom is electrically Neutral.
However, the electrons in the outermost shell (valence electrons) are very weakly bound to the nucleus. It is easy to remove them or add new ones with a minimal input of energy (friction, heat, electric field).
- Positive Ion (Cation): The atom has lost electrons. The positive charge of the nucleus predominates.
- Negative Ion (Anion): The atom has gained extra electrons. The negative charge predominates.
This ability of electrons to “jump” from one atom to another is what allows the existence of electric current.
