3.1 Electrical Terminology

CAAS SAR-66 Module 3 topic 3.3 Electrical Terminology is the vocabulary layer for every later DC and AC calculation. If you confuse EMF with a voltage drop, or mix conventional current with electron flow, you will lose marks on otherwise simple questions. Treat this section as a working toolkit: define each quantity, name its SI unit, know what factors change it, and connect the two exam relationships G = 1/R and Q = It.

Potential Difference, EMF, and Voltage

Potential difference is the difference in electric potential between two points in a circuit. It is the “electrical pressure” available to move charge through a load. The SI unit is the volt (V). One volt equals one joule of energy transferred per coulomb of charge (1 V = 1 J/C).

Electromotive force (EMF) is the potential difference produced by an energy source (battery, generator, thermocouple, and so on) when the source is open-circuit, or more precisely the work done per unit charge by the source as charge travels around a complete circuit. EMF is also measured in volts, but conceptually it is the source voltage, not a drop across a resistor.

Voltage in maintenance language often means either EMF or a measured potential difference. On the exam, read the stem carefully:

• “EMF of the battery” → source voltage (ideally open-circuit).
• “Voltage across the lamp” → potential difference (voltage drop) at that load.
• “Supply voltage” → usually the source terminal voltage under the stated load.

Factors affecting potential difference / EMF

Worked idea: A 28 V aircraft bus with 0.05 Ω feeder resistance and 40 A load current drops V = IR = 40 × 0.05 = 2.0 V in the feeder. The load may see about 26 V even though the generator EMF is higher. Potential difference between two points is always local to those points.

Current

Current is the rate of flow of electric charge. The SI unit is the ampere (A). One ampere equals one coulomb of charge passing a point every second (1 A = 1 C/s).

Current requires a closed path and a driving potential difference. In a simple series loop, the same current passes every series element. In parallel branches, currents divide according to branch resistances.

Factors affecting current

• Applied voltage (higher V tends to raise I if R is fixed).
• Circuit resistance (higher R lowers I for a given V — Ohm’s law territory in topic 3.6).
• Number and arrangement of parallel paths.
• Temperature (many conductors increase resistance as they heat, reducing current slightly for a fixed voltage).
• Source ability to deliver current (capacity, state of charge, generator regulation).

Unit conversion drill: Convert 350 mA to amperes: 350 mA = 350 × 10⁻³ A = 0.35 A. Convert 2.5 A to milliamperes: 2.5 × 10³ = 2500 mA. Module 3 stems often bury milliamp or microamp values inside otherwise simple definitions.

Conventional Current Flow vs Electron Flow (Exam Trap)

This is a classic CAAS Module 3 trap.

Conventional current is defined as the direction that positive charge would move: from the positive terminal of the source, through the external circuit, to the negative terminal. Circuit diagrams, Kirchhoff’s laws as taught on the module, and most meter polarity conventions use conventional current.

Electron flow is the actual motion of free electrons in metallic conductors: from the negative terminal, through the external circuit, toward the positive terminal — opposite to conventional current.

If a question asks “direction of conventional current through the lamp,” answer from positive toward negative through the lamp. If it asks “direction of electron flow,” reverse it. Do not invent a third “both are the same” compromise — they are opposite by definition.

Resistance and Conductance

Resistance is opposition to current in a material or component. The SI unit is the ohm (Ω). One ohm is the resistance that allows one ampere when one volt is applied (from the Ohm’s-law definition used later in the module).

Conductance is the ease with which a material allows current — the reciprocal of resistance:

G = 1/R

The SI unit of conductance is the siemens (S). Older texts may say “mho,” but Module 3 expects siemens.

Worked conductance conversions

1. A heater has R = 10 Ω. Conductance G = 1/10 = 0.1 S.
2. A sensor has G = 5 mS = 0.005 S. Resistance R = 1/0.005 = 200 Ω.
3. Two equal 4 Ω resistors in parallel (preview of later topic): equivalent R = 2 Ω, so G_eq = 0.5 S — each branch contributes G = 0.25 S and conductances add in parallel.

Factors affecting resistance

Aircraft maintenance links: undersized jumpers, corroded bonding straps, and high-resistance earth returns show up as heat, voltage drop, and nuisance trips — all resistance problems dressed as “power” complaints.

Charge

Electric charge is a property of matter; the SI unit is the coulomb (C). The elementary charge on one electron is about 1.6 × 10⁻¹⁹ C, so one coulomb is an enormous number of electrons — useful as a bulk unit for circuit equations.

The fundamental relationship linking charge, current, and time is:

Q = It

where Q is charge in coulombs, I is current in amperes, and t is time in seconds.

Worked Q = It examples

1. A 2 A lamp runs for 30 minutes. t = 30 × 60 = 1800 s. Q = 2 × 1800 = 3600 C.
2. A capacitor (preview) stores Q = 0.02 C after charging at a constant 50 mA. Time t = Q/I = 0.02 / 0.05 = 0.4 s.
3. An avionics bus draws 800 mA for 45 s. Q = 0.8 × 45 = 36 C.

Rearrangements you must recognise instantly: I = Q/t and t = Q/I.

SI Unit Summary Table

Exam Scenario Mindset

On a CAAS Module 3 paper you might see: “A component has a conductance of 2.5 S. What is its resistance?” Answer 0.4 Ω (R = 1/2.5). Or: “Conventional current in a metal wire flows…” → from positive to negative, even though electrons move the other way. Or: “Charge transferred by 5 A in 2 minutes” → t = 120 s, Q = 600 C.

Keep EMF (source) distinct from voltage drop (load or wiring), keep G as the reciprocal of R, and always convert time to seconds before using Q = It. Those three habits eliminate most terminology errors before you reach Ohm’s law, Kirchhoff, and power topics.