Atomic Structure — Where It All Starts
To understand electron flow, you first need to understand the basic structure of an atom. At the center of every atom is the nucleus, which contains protons (positive charge) and neutrons (neutral — no charge). Orbiting the nucleus in shells are electrons, which carry a negative charge.
The electrons in the outermost shell are called valence electrons. These are the electrons that can break free from their atom and jump to a neighboring atom. When valence electrons move from atom to atom through a conductor, that movement is what we call electrical current.
What Makes Electrons Flow?
Electrons don't move on their own — they need a push. That push is called voltage, which can be thought of as electrical pressure. A battery creates a difference in electrical charge between its positive and negative terminals. This difference is called a potential difference.
For example, a 12-volt aircraft battery has 12 volts of potential difference between its terminals. A 24-volt battery has greater potential difference and can push electrons with more force. The key principle: no voltage difference means no electron flow.
Electron Flow Model
The electron flow model describes what is actually happening at the atomic level. Electrons carry a negative charge — they are repelled by the negative terminal and attracted to the positive terminal. Therefore, electrons physically move from negative to positive.
The FAA officially defines current flow using electron theory — negative to positive. This is stated in FAA-H-8083-30B, page 12-14 (Aviation Maintenance Technician Handbook — General). For your FAA written exam and oral test, this is the answer: current flows from negative to positive.
Conventional Flow Model
The conventional flow model describes current as flowing from positive to negative — the exact opposite of electron flow. This model originated over 270 years ago when Benjamin Franklin guessed wrong about which direction charge carriers moved. Despite being incorrect at the atomic level, the convention stuck.
Most electrical engineering textbooks and many aviation wiring schematics still use the conventional flow model. This is why you need to understand both — the FAA will test you on electron flow theory, but the prints you read in the field may reference conventional flow.
Why You Need to Know Both Models
The FAA expects you to understand both models and explain the difference:
- Electron flow (FAA position): Current flows from negative to positive — this is what actually happens at the atomic level
- Conventional flow: Current flows from positive to negative — used in many schematics and diagrams you'll encounter in the field
A Designated Maintenance Examiner (DME) can ask you about either model during your oral and practical exam. You must be able to describe both clearly.
From a practical standpoint as an A&P mechanic working on general aviation aircraft, the direction of electron flow rarely changes how you troubleshoot a circuit. Most troubleshooting is about finding where a circuit is broken — open circuits, bad grounds, faulty components — not which direction electrons are moving. However, there are specific cases where polarity matters, which we'll cover next.
Polarity Matters — Real World Applications
Aircraft Battery Voltage Check
Using a multimeter set to DC volts, you can check the voltage across an aircraft battery by placing the red lead on positive and the black lead on negative. A healthy 12-volt battery should read approximately 12.5–12.8 volts. If you reverse the leads, you'll get a negative reading — a practical demonstration of polarity and electron flow direction.
LED Landing Lights
LED landing lights are a real-world example where polarity is critical. Unlike incandescent bulbs, LEDs are diodes — they only allow electricity to flow in one direction. If you hook up an LED landing light with reversed polarity, it simply won't work. Always check the positive and negative markings on the light and connect accordingly.
Reading Aircraft Wiring Diagrams
Understanding electron theory helps when reading wiring diagrams — something you'll do frequently as an A&P mechanic. Here's how a typical landing light circuit works on a Cessna 172:
- Bus bar — The circuit originates from a bus bar (a common junction point behind the instrument panel where multiple circuits connect)
- Circuit breaker — The circuit passes through a 10-amp circuit breaker for protection
- Wire — Wire numbers on schematics tell you the location, type, size, and conductor material
- Connection points — Disconnect junctions (like a firewall pass-through or wing disconnect) where wires join together
- Light — Power flows through the landing or taxi light
- Ground — The return path completes the circuit back to ground
In many cases, the landing light and taxi light share a common ground wire. Every circuit must be complete — power side and ground side — for current to flow.
Basic Multimeter Functions for A&P Mechanics
A basic multimeter (under $50) is an essential tool for aircraft maintenance. The key functions you'll use:
- DC Volts (V with solid line and dots) — Used to check battery voltage, verify power at components, and test circuits. This is your most-used setting in general aviation.
- AC Volts (V with squiggly line) — Less common in GA but available for AC systems
- Ohms / Resistance (Ω) — Used to test grounds, check spark plug health, and measure component resistance
- Continuity (speaker/diode symbol) — Beeps when there's a complete path. Essential for tracing wires through an aircraft or finding breaks in wiring
- DC Amps — Used occasionally for checking alternator output. Be aware of your meter's fuse rating (often 10 amps) to avoid blowing the fuse
Lead placement: The black (common/ground) lead goes in the center port. The red lead goes in the appropriate port depending on whether you're measuring volts or amps.
Frequently Asked Questions
What is the difference between conventional flow and electron flow?
Conventional flow describes current moving from positive to negative — this is the older model based on Benjamin Franklin's assumption. Electron flow describes current moving from negative to positive — this is what actually happens at the atomic level as electrons are repelled from the negative terminal and attracted to the positive terminal. The FAA officially uses the electron flow model (negative to positive) as stated in FAA-H-8083-30B.
Which model does the FAA use — conventional flow or electron flow?
The FAA officially adopts the electron flow model, which states that current flows from negative to positive. This is documented in the Aviation Maintenance Technician Handbook — General (FAA-H-8083-30B), Chapter 12, page 12-14. For your written and oral exams, always answer using electron flow theory unless specifically asked about conventional flow.
Why does polarity matter when installing LED landing lights?
LED landing lights contain diodes, which only allow electricity to flow in one direction. If you connect an LED light with reversed polarity (positive and negative swapped), it will not illuminate. Unlike traditional incandescent bulbs that work regardless of polarity, LEDs require correct orientation. Always verify the positive and negative terminals before installation.
