1.3 Electrical Terminology and Flow Direction

1.3 Electrical Terminology & Electron Flow

To analyze, build, or troubleshoot aircraft electrical circuits, a technician must speak the language of electrical physics. This section defines the fundamental terms of electricity—potential difference, electromotive force (EMF), voltage, current, resistance, and conductance—and resolves the historical discrepancy between conventional current flow and actual electron flow.

Potential Difference and Electromotive Force (EMF)

Electricity is the movement of charge. However, charges will not flow without an external force driving them.

• Potential Difference: The difference in electrical charge between two points in a circuit. It is defined as the work done in moving a unit charge from one point to another. $V = \frac{W}{Q}$ Where:
  • $V$ is potential difference in Volts.
  • $W$ is work done or energy transferred in Joules (J).
  • $Q$ is charge in Coulombs (C).
• Electromotive Force (EMF): The maximum potential difference that an electrical source (like a battery or generator) can produce when no current is flowing (open-circuit condition). EMF represents the energy per unit charge supplied by the source.
• Voltage: The common term for potential difference and EMF, measured in Volts (V). One Volt is defined as the potential difference across a conductor when one Joule of work is required to move one Coulomb of charge through it ($1\text{ V} = 1\text{ J/C}$).

Current ($I$)

Electric current is the rate at which electrical charge flows past a specific point in a circuit. $I = \frac{Q}{t}$

Where:

• $I$ is the current in Amperes (A).
• $Q$ is the charge in Coulombs (C).
• $t$ is the time in seconds (s).

One Ampere (often shortened to "Amp") is equal to one Coulomb of charge passing a point in one second ($1\text{ A} = 1\text{ C/s}$). In terms of subatomic particles, if $6.242 \times 10^{18}$ electrons flow past a cross-section of a wire in one second, the current is exactly one Ampere.

Resistance ($R$) and Conductance ($G$)

• Resistance: The physical opposition to the flow of electric current. The unit of resistance is the Ohm ($\Omega$). One Ohm is defined as the resistance that allows a current of one Ampere to flow when a potential difference of one Volt is applied across it. Resistance in a wire depends on four physical factors: $R = \rho \cdot \frac{L}{A}$ Where:
  • $\rho$ is the resistivity of the material in Ohm-meters ($\Omega\cdot\text{m}$).
  • $L$ is the length of the conductor in meters.
  • $A$ is the cross-sectional area of the conductor in square meters ($m^2$).
  • Temperature: In most metals (positive temperature coefficient), resistance increases as temperature rises because thermal vibration of the crystal lattice blocks electron movement.
• Conductance: The ease with which electric current flows through a material. Conductance ($G$) is the reciprocal of resistance: $G = \frac{1}{R}$ The SI unit of conductance is the Siemens (S) (historically referred to as the mho, which is "ohm" spelled backward, denoted by the symbol $\mho$).

Conventional Flow vs. Electron Flow

One of the most persistent sources of confusion for aviation technicians is the difference between conventional current flow and electron flow.

1. Conventional Flow: Established by Benjamin Franklin in the 18th century, before the discovery of the electron. Franklin assumed that electricity was a fluid that flowed from a point of excess (positive, $+$) to a point of deficiency (negative, $-$).
   • Direction: Positive ($+$) terminal to Negative ($-$) terminal.
   • Usage: Used in almost all engineering schematics, aircraft maintenance manuals (AMMs), wiring diagrams, and standard electrical formulas (such as Fleming's rules).
2. Electron Flow: Reflects the physical reality discovered by modern physics. Since electrons are negatively charged particles, they are repelled by the negative terminal of a voltage source and attracted to the positive terminal.
   • Direction: Negative ($-$) terminal to Positive ($+$) terminal.
   • Usage: Used when describing the internal operation of semiconductor devices (diodes, transistors, vacuum tubes) and cathode ray tubes (CRTs).

[!IMPORTANT] Exam Strategy: Always read the question carefully. If the question asks for the direction of electron flow, the answer is negative to positive. If the question asks for the direction of current flow (or conventional current), the answer is positive to negative.

Worked Exam Calculations

Calculation 1: Potential Difference (Work and Charge)

Problem: An aircraft emergency battery does $48\text{ Joules}$ of work to move a charge of $4\text{ Coulombs}$ through an emergency bus light. What is the potential difference across the light? Solution: $V = \frac{W}{Q} = \frac{48\text{ J}}{4\text{ C}} = 12\text{ Volts}$

Calculation 2: Current (Charge and Time)

Problem: A landing gear position switch contacts are closed, and a charge of $150\text{ Coulombs}$ flows through the circuit indicator in $30\text{ seconds}$. What is the current in Amperes? Solution: $I = \frac{Q}{t} = \frac{150\text{ C}}{30\text{ s}} = 5\text{ Amperes}$

Calculation 3: Conductance

Problem: An aircraft heated pitot tube has a measured heater element resistance of $8\ \Omega$. Calculate the conductance of the heating element. Solution: $G = \frac{1}{R} = \frac{1}{8\ \Omega} = 0.125\text{ Siemens (S)}$