3.1 Electricity and Electronics Foundations

Electron Theory and the Four Circuit Quantities

Electricity is the flow of electrons through a conductor. The FAA General (AMG) knowledge test treats four quantities as the backbone of every electrical question:

Conductors (copper, aluminum) have many free electrons; insulators (rubber, glass, most plastics) have very few. A semiconductor such as silicon sits between the two and is the basis of diodes, transistors, and the solid-state devices in modern avionics. A diode allows current in one direction only (used to rectify AC to DC and to block reverse current); a transistor acts as an electronic switch or amplifier. Conventional current is drawn from positive to negative, while electron flow is negative to positive; the FAA uses conventional current in its schematics, so study to that convention.

The General curriculum also touches digital fundamentals: the binary number system (only 0 and 1) underlies digital logic and computers, and basic logic gates (AND, OR, NOT) combine binary inputs. You will not design circuits, but you must recognize that modern aircraft systems are largely digital and that a single bit represents on/off, true/false, or high/low.

Ohm's Law and Watt's Law (Worked Examples)

Ohm's Law states E = I x R (voltage equals current times resistance). Rearranged: I = E / R and R = E / I. Watt's Law states P = I x E (power equals current times voltage); combined with Ohm's Law it also gives P = I^2 x R and P = E^2 / R. Memorize the wheel; most General items are one substitution.

Worked Ohm's-Law example: A 28-volt aircraft DC bus feeds a landing-light circuit with 4 ohms of total resistance. Current = E / R = 28 / 4 = 7 amperes. The power dissipated = I x E = 7 x 28 = 196 watts (check: I^2 x R = 49 x 4 = 196 W, agrees). If a technician sizes a circuit breaker, the 7-amp draw means a 10-amp breaker is reasonable, not a 5-amp one.

Second example: A bulb draws 0.5 A from a 14-volt system. Its resistance = E / I = 14 / 0.5 = 28 ohms, and it consumes P = I x E = 0.5 x 14 = 7 watts. Common trap: candidates divide volts by watts or mix a 14-volt and 28-volt system. Always write the formula, insert units, then solve.

Series, Parallel, and Combined Circuits

The rules differ by topology and are a frequent test trap:

In a series circuit the total resistance is the simple sum, and the individual voltage drops must add up to the source voltage (a form of Kirchhoff's Voltage Law). Worked series example: three resistors of 2, 3, and 5 ohms in a 20-volt circuit give R_total = 10 ohms, so I = 20 / 10 = 2 A, and the drops are 4 V, 6 V, and 10 V (which sum to 20 V).

In a parallel circuit total resistance is always less than the smallest branch. Worked parallel example: two 6-ohm resistors in parallel give R = 1 / (1/6 + 1/6) = 1 / (2/6) = 3 ohms. The branch currents add (Kirchhoff's Current Law). Most aircraft loads are wired in parallel across the bus so each operates at full bus voltage and one failure does not open the others.

Capacitance, Inductance, Batteries, and ESD

A capacitor stores energy in an electric field between two plates separated by a dielectric; capacitance is measured in farads (usually microfarads in aircraft). A capacitor opposes a change in voltage and blocks DC while passing AC. An inductor (coil) stores energy in a magnetic field, is measured in henrys, and opposes a change in current; it passes DC but chokes AC. These reactive behaviors set up the AC topics in the next section.

Aircraft DC is supplied by lead-acid or nickel-cadmium (Ni-Cad) storage batteries rated in volts and amp-hours (capacity). 25 volts. Cells in series add voltage; cells in parallel add capacity (amp-hours) at the same voltage. So six 2-volt cells in series make a 12-volt battery, while two 12-volt 25-amp-hour batteries in parallel still read 12 volts but supply 50 amp-hours. Ni-Cad batteries can suffer thermal runaway and require different servicing, charging, and capacity-check procedures than lead-acid.

Magnetism and electromagnetic induction also belong in this foundation: current flowing through a conductor creates a magnetic field, and a magnetic field moving relative to a conductor induces a voltage. This is the basis of generators, alternators, motors, relays, and solenoids found throughout the aircraft. The strength of an electromagnet grows with current and the number of turns of wire.

Circuit Protection and ESD

Every aircraft circuit includes protection sized to the wire, not the load: a fuse melts open on overcurrent, while a circuit breaker trips and can be reset. The protective device guards the wiring from overheating during a short or overload, so a tripped breaker is a symptom to investigate, not merely reset repeatedly.

Electrostatic discharge (ESD) can destroy semiconductors and avionics modules at voltages far below what a person can feel (a perceptible spark is thousands of volts; many devices fail below 100 volts). Use grounding wrist straps, conductive mats, and approved static-shielding packaging when handling solid-state line-replaceable units. ESD awareness is now squarely in the General curriculum, not just advanced avionics study.