3.1 — Electron Theory

Electron theory explains the fundamental nature of electricity by examining atomic structure and the behaviour of electrons. This section lays the groundwork for understanding how electrical current flows and why different materials conduct electricity to different degrees — essential knowledge for every aircraft maintenance engineer working with electrical systems.

Structure of Matter

All matter is composed of atoms, the smallest particles of an element that retain its chemical identity. Each atom consists of a central nucleus containing positively charged protons and electrically neutral neutrons, surrounded by orbiting negatively charged electrons.

Particle	Location	Charge	Relative Mass
Proton	Nucleus	Positive (+1)	1
Neutron	Nucleus	Neutral (0)	1
Electron	Shells (orbits)	Negative (−1)	≈ 1/1836

In a neutral atom the number of electrons equals the number of protons, so the overall charge is zero. The atomic number (Z) equals the number of protons and defines the element. The mass number (A) is the total of protons plus neutrons.

Electron Shells and Energy Levels

Electrons occupy discrete energy levels called shells, labelled K, L, M, N outward from the nucleus. Each shell has a maximum capacity:

Shell	Number (n)	Max Electrons
K	1	2
L	2	8
M	3	18
N	4	32

The outermost shell is called the valence shell, and the electrons in it are valence electrons. These determine the electrical and chemical properties of the element.

Valence and Free Electrons

When a valence electron gains enough energy, it can break free from its parent atom and move through the material. Such electrons are called free electrons. The ease with which this happens determines whether a material is a conductor, semiconductor, or insulator.

The energy band model explains this:

• Valence band — the energy range occupied by valence electrons bound to atoms.
• Conduction band — the energy range where electrons are free to move and carry current.
• Band gap (forbidden gap) — the energy difference between the valence and conduction bands.

Conductors, Semiconductors, and Insulators

Property	Conductor	Semiconductor	Insulator
Valence electrons	1–3	4	5–8 (or full shell)
Band gap	None (bands overlap)	Small (≈ 0.7–1.1 eV)	Large (> 5 eV)
Conductivity	Very high	Moderate (variable)	Extremely low
Free electrons	Abundant at room temp	Few at room temp; increases with temp	Almost none
Temp effect on resistance	Increases with temp	Decreases with temp	Decreases slightly
Examples	Copper, aluminium, silver, gold	Silicon, germanium	Glass, rubber, PVC, mica, ceramic

Conductors in Detail

Good conductors have atoms with only 1 or 2 valence electrons loosely bound to the nucleus. In metals, these electrons form a "sea" of free electrons that can drift through the material when a voltage is applied. Copper has one valence electron and is the most commonly used conductor in aviation.

Semiconductors in Detail

Silicon and germanium have 4 valence electrons and form covalent bonds with neighbouring atoms. At absolute zero they behave as insulators, but at room temperature some bonds break, releasing electron-hole pairs. Their conductivity can be dramatically altered by doping — adding tiny amounts of impurity atoms:

• N-type — doped with a pentavalent element (e.g. phosphorus), providing extra free electrons.
• P-type — doped with a trivalent element (e.g. boron), creating "holes" (positive charge carriers).

Insulators in Detail

Insulators have a full or nearly full valence shell, so electrons are tightly bound and the band gap is very large. Virtually no free electrons are available for conduction. Insulators are essential for preventing unwanted current flow and protecting personnel from electric shock.

Conventional Flow vs. Electron Flow

There are two conventions for describing current direction:

Convention	Direction	Used By
Conventional flow	Positive (+) to negative (−)	Circuit diagrams, engineering standards, EASA exams
Electron flow	Negative (−) to positive (+)	Physics, understanding actual particle movement

In reality, electrons (the actual charge carriers in metals) flow from negative to positive. However, before the electron was discovered, current was defined as flowing from positive to negative. This conventional current direction is used in virtually all circuit analysis and is the standard in EASA Part 66 examinations.