Module 3.1-3.4 Electron theory, static electricity, terminology and generation
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Module 3 is 20 questions (A), 52 (B1/B2/B2L) and 24 (B3) under EASA. These four opening sub-modules are mostly definitions - learn them precisely.
Knowledge levels
| Sub-module | A | B1 / B2 / B2L | B3 |
|---|---|---|---|
| 3.1 Electron theory | 1 | 1 | 1 |
| 3.2 Static electricity and conduction | 1 | 2 | 1 |
| 3.3 Electrical terminology | 1 | 2 | 1 |
| 3.4 Generation of electricity | 1 | 1 | 1 |
3.1 Electron theory
- Atoms contain positive protons and neutral neutrons in the nucleus, with negative electrons in shells around it (see Module 2.1).
- Valence electrons in the outer shell can be freed. Free electrons moving through a material form an electric current.
- Conductors (copper, aluminium, silver, gold) have 1-3 loosely held valence electrons and many free electrons.
- Insulators (rubber, glass, PTFE, mica) have full or nearly full valence shells and almost no free electrons.
- Semiconductors (silicon, germanium) have 4 valence electrons; their conductivity lies between and can be controlled by doping (Module 4).
- Ions: an atom that loses electrons becomes a positive ion; one that gains electrons becomes a negative ion. Ions carry current in liquids (electrolytes) and gases.
Key fact: Silver is the best conductor, but copper is used for most aircraft wiring because of cost; aluminium is used for large cables where weight matters.
3.2 Static electricity and conduction
- Static electricity is a build-up of charge on a surface, usually by friction (triboelectric effect) - e.g. an aircraft flying through dry air, snow or dust, or fuel flowing through a hose.
- Laws of electrostatics: like charges repel, unlike charges attract.
- Coulomb's law: F = k Q₁Q₂ / d², where k ≈ 9 × 10⁹ N·m²/C². The force is proportional to the product of the charges and inversely proportional to the square of the distance.
- Unit of charge: the coulomb (C). One electron carries 1.6 × 10⁻¹⁹ C; 1 C ≈ 6.24 × 10¹⁸ electrons.
- Charge resides on the outer surface of a conductor and concentrates at sharp points - the reason static dischargers (wicks) on trailing edges bleed charge to the air at sharp tips and so reduce radio interference.
- Bonding straps and grounding during refuelling equalise potential to prevent sparks.
Worked example - Coulomb's law
Two charges are 0.2 m apart and repel with 9 N. What is the force at 0.4 m?
- Distance doubles → force divides by 2² = 4
- F = 9 / 4 = 2.25 N
Conduction in different media
| Medium | Charge carriers |
|---|---|
| Solids (metals) | free electrons |
| Liquids (electrolytes) | positive and negative ions |
| Gases | ions and electrons after ionisation (arcs, lightning, neon lamps) |
| Vacuum | electrons emitted from a heated cathode (thermionic emission, e.g. CRT) |
3.3 Electrical terminology
| Term | Symbol | Unit | Meaning |
|---|---|---|---|
| Charge | Q | coulomb (C) | quantity of electricity |
| Current | I | ampere (A) | rate of flow of charge: I = Q / t (1 A = 1 C/s) |
| Electromotive force (EMF) | E | volt (V) | the energy per unit charge produced by a source (open-circuit voltage) |
| Potential difference (PD) | V or U | volt (V) | energy per unit charge between two points of a circuit |
| Resistance | R | ohm (Ω) | opposition to current: R = V / I |
| Conductance | G | siemens (S) | 1 / R |
- Conventional current flow is from positive to negative (used on most diagrams and for hand rules).
- Electron flow is from negative to positive (the actual movement of electrons).
- Factors affecting resistance of a conductor: length (proportional), cross-sectional area (inversely proportional), material (resistivity) and temperature.
Worked example - charge and current
A battery delivers 5 A for 2 minutes. How much charge flows?
- Q = I × t = 5 × 120 = 600 C
Key fact: EMF is the voltage of the source with no load; the terminal PD under load is lower because of the source's internal resistance (Module 3.6).
3.4 Generation of electricity
| Method | Principle | Aircraft examples |
|---|---|---|
| Light | photovoltaic effect - light frees electrons in a semiconductor junction | solar cells, photocells, light sensors |
| Heat | thermoelectric (Seebeck) effect - two dissimilar metals joined, a temperature difference produces EMF | thermocouples for EGT, CHT |
| Friction | triboelectric effect - charge transfer between rubbing materials | static build-up on the airframe |
| Pressure | piezoelectric effect - crystals (quartz) under mechanical stress produce a voltage | pressure transducers, vibration pick-ups, crystal oscillators |
| Chemical reaction | electrochemical cells | lead-acid, NiCd, lithium batteries |
| Magnetism and motion | electromagnetic induction - a conductor cutting a magnetic field | generators, alternators, magnetos, tachometer generators |
Key fact: Almost all aircraft electrical power comes from magnetism + motion (generators); batteries provide emergency and starting power.
Exam tips
- Distinguish EMF (source, open circuit) from PD (across a component).
- Coulomb's law follows an inverse-square law - doubling distance quarters the force.
- Thermocouple = heat; piezo = pressure; photovoltaic = light.