1.1 DC Basic Terms, Atomic Structure, and Charge

1.1 DC Basic Terms, Atomic Structure, and Charge

Quick Takeaway: Direct Current (DC) electricity relies on the controlled movement of free valence electrons through conductive materials driven by Electromotive Force (EMF). One Coulomb represents $6.242 \times 10^{18}$ electrons, and a current of one Ampere corresponds to the transfer of one Coulomb per second ($1\text{ A} = 1\text{ C/s}$). In aircraft 28 VDC power systems, understanding electron energy states, valence shell mobility, and charge mechanics forms the essential groundwork for troubleshooting avionics equipment and airframe wiring.

---

Atomic Structure and Electrical Charge

To master aircraft electrical systems, an avionics technician must first understand matter at its subatomic level. All physical matter in an airframe—from copper wiring to aluminum skin and silicon microchips—is composed of atoms. The classic Bohr model of the atom illustrates a central nucleus containing positively charged protons and uncharged neutrons, surrounded by negatively charged electrons orbiting in discrete energy shells designated $K, L, M, N, O, P,$ and $Q$.

       [ Electron Orbit (K Shell) ]
                e-  (-1.602 x 10^-19 C)
               /  \
             /  +  \
            | (P)(N)|  Nucleus (Protons + Neutrons)
             \  +  /
               \  /
                e-

The fundamental properties of these subatomic particles dictate electrical behavior:

• Proton: Positively charged particle residing in the nucleus ($+1.602 \times 10^{-19}\text{ Coulombs}$).
• Neutron: Electrically neutral particle residing in the nucleus, providing atomic mass.
• Electron: Negatively charged particle orbiting the nucleus ($-1.602 \times 10^{-19}\text{ Coulombs}$) with an extremely light mass ($9.109 \times 10^{-31}\text{ kg}$).

Valence Electrons and Energy Band Theory

The outermost electron shell of an atom is known as the valence shell, and the electrons residing within it are valence electrons. The number of valence electrons determines the chemical valence and, crucial to avionics, the electrical conductivity of the element:

1. Conductors (1 to 3 valence electrons): Materials such as copper ($Cu$), silver ($Ag$), gold ($Au$), and aluminum ($Al$) possess loosely bound valence electrons. In copper, which has an atomic number of 29, the single electron in its outermost $N$ shell experiences minimal electrostatic attraction from the nucleus. At room temperature, thermal energy easily liberates this valence electron into the conduction band, transforming it into a "free electron" that drifts through the metallic crystal lattice.
2. Insulators (5 to 8 valence electrons): Materials such as Teflon (PTFE), Kapton (polyimide), rubber, and glass have filled or nearly filled valence shells. These atoms hold their valence electrons tightly. The energy gap (forbidden band) between the valence shell and the conduction band is wide, requiring extreme energy to dislodge electrons under normal operating conditions.
3. Semiconductors (4 valence electrons): Elements like silicon ($Si$) and germanium ($Ge$) contain four valence electrons, forming covalent crystalline bonds. Their conductivity falls between conductors and insulators and can be precisely modified through chemical doping to build diodes, transistors, and integrated circuits used in modern flight control computers.

---

Fundamental Units of Electrostatics and Electrodynamics

In aircraft DC systems, electrical phenomena are quantified using precise international standards (SI units).

1. Electrostatic Charge: The Coulomb ($C$)

An individual electron possesses a tiny electrostatic charge ($1.602 \times 10^{-19}\text{ C}$). Because measuring charge in terms of individual electrons is impractically small for engineering work, the standard unit of quantity of electricity is the Coulomb (C). One Coulomb is defined as the total charge accumulated by $6.242 \times 10^{18}$ electrons:

$1\text{ Coulomb (C)} = 6.242 \times 10^{18}\text{ electrons}$

When an object gains excess electrons, it acquires a negative electrostatic charge; when it loses electrons, it acquires a positive electrostatic charge. Opposite charges attract, and like charges repel, according to Coulomb's Law:

$F = k \cdot \frac{|q_1 \cdot q_2|}{r^2}$

where $F$ is electrostatic force, $q_1$ and $q_2$ are charges, $r$ is separation distance, and $k$ is Coulomb's constant ($8.988 \times 10^9\text{ N}\cdot\text{m}^2/\text{C}^2$). Static charge accumulation on an aircraft skin during flight through precipitation (precipitation static or P-static) must be safely discharged via static wicks to prevent severe RF interference with navigation radios.

2. Electrical Current: The Ampere ($A$)

Electrical current ($I$) is defined as the rate of flow of electrical charge past a given point in a conductor per unit time. The standard unit of current is the Ampere (A), named after André-Marie Ampère:

$I = \frac{Q}{t}$

where $I$ is current in Amperes, $Q$ is charge in Coulombs, and $t$ is time in seconds. One Ampere represents one Coulomb of charge passing a reference point in one second ($1\text{ A} = 1\text{ C/s}$).

Electron Flow vs. Conventional Current: In the historical convention established before the electron was discovered, current was assumed to flow from positive ($+$) to negative ($-$). However, physical reality in solid conductors dictates Electron Flow Theory, in which free electrons move from the negative terminal (high electron density) toward the positive terminal (electron deficit). The ASTM NCATT AET curriculum emphasizes electron flow for circuit physical analysis, while noting that schematic symbols (such as diode arrows) point in the direction of conventional current flow.

3. Electromotive Force (EMF) and Voltage ($E$)

Electrons will not drift in a unified direction unless acted upon by an external force. Electromotive Force (EMF), represented by the letter $E$ or $V$ and measured in Volts (V), is the electrical pressure or potential difference that drives free electrons through a circuit. One Volt is defined as the potential difference required to expend one Joule of energy in moving one Coulomb of charge between two points ($1\text{ V} = 1\text{ J/C}$):

$E = \frac{W}{Q}$

In aircraft systems, EMF is generated chemically by batteries or electromagnetically by DC generators and alternators.

4. Electrical Resistance: The Ohm ($\Omega$)

Resistance ($R$) is the opposition offered by a material to the flow of free electrons, measured in Ohms ($\Omega$). One Ohm is the amount of resistance that limits current to one Ampere when an EMF of one Volt is applied. Conductor resistance depends on four physical factors:

$R = \rho \cdot \frac{L}{A}$

• Material Resistivity ($\rho$): Intrinsic property of the conductor (e.g., copper has lower resistivity than aluminum).
• Length ($L$): Resistance is directly proportional to wire length.
• Cross-Sectional Area ($A$): Resistance is inversely proportional to cross-sectional area (thicker wire has lower resistance).
• Temperature Coefficient ($\alpha$): Most metallic conductors exhibit a positive temperature coefficient; as wire temperature rises, atomic vibrations increase, impeding electron flow and raising resistance.

---

Avionics Technical Context & Wire Material Selection

Modern aircraft wiring systems must balance high electrical conductivity, mechanical strength, weight limitations, and thermal tolerance. The standard military/aerospace wire specification AS22759 (formerly MIL-W-22759) mandates silver-plated or nickel-plated stranded copper conductors insulated with fluoropolymer materials like ETFE (Tefzel) or PTFE (Teflon).

---

Avionics Trap: High-Resistance Ground Returns

In 28 VDC aircraft electrical systems, the aluminum or composite airframe structure serves as the common negative return path (ground). Because aluminum has low resistivity and large cross-sectional area, airframe resistance is negligible under ideal conditions.

However, corrosion, loose bonding jumpers, or paint on contact surfaces create high localized resistance ($R_{\text{ground}}$). Applying electron flow theory, electrons leaving the negative battery terminal enter the airframe ground structure. If a grounding stud accumulates surface oxidation, $R_{\text{ground}}$ increases. Under load, this high resistance produces an unwanted voltage drop across the ground joint ($V_{\text{drop}} = I \cdot R_{\text{ground}}$), reducing available voltage to Line Replaceable Units (LRUs) and causing mysterious intermittent avionics reboots during high-current engine starting.