2.3 Static Electricity, Coulomb's Law & Conduction

Static Electricity and Charge Distribution

CAAS SAR-66 Module 3 topic 3.2 Static Electricity and Conduction asks you to explain how charge can sit almost motionless on a surface, how charged objects interact, and how that charge eventually moves through different media.

Static electricity is an imbalance of electric charge on the surface of an object. Electrons are transferred by friction (triboelectric charging), contact and separation, induction, or ionization. On a conductor, excess charge redistributes quickly across the outer surface and concentrates at sharp points or edges. On an insulator, charge stays localised where it was deposited—hence a plastic cover can hold a “patch” of charge that zaps an adjacent pin.

Distribution rules technicians use:

• Excess charge on an isolated conductor resides on the outer surface.
• Charge density is highest at sharp extremities (wing tips, probe ends, fastener corners)—one reason static dischargers (wicks) are fitted on trailing edges.
• Humidity lets a thin moisture film leak charge away; very dry hangars and winter ramps increase static problems.

Scenario: Refuelling a composite aircraft on a dry day without bonding/grounding cables can leave the airframe and tanker at different potentials. A spark in the vapour zone is an unacceptable risk. Procedures exist specifically to equalise potential before fuel flows.

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Electrostatic Laws of Attraction and Repulsion

Two experimental laws underpin every static question:

1. Like charges repel — two negatively charged panels push apart; two positively charged ions likewise.
2. Unlike charges attract — a negative electron cloud is pulled toward a positive nucleus; a charged dust particle sticks to an oppositely charged surface.

Induction lets a charged object rearrange charges in a nearby conductor without touching it. Bring a negatively charged wand near an isolated metal tool: free electrons in the tool are repelled to the far side, leaving the near side positive. If you briefly ground the far side, electrons leave permanently and the tool retains a net positive charge after the wand is removed. Induction explains unexpected shocks from insulated work stands and why conductive mats are grounded.

Trap: Attraction to a charged object does not prove the object’s polarity by itself—neutral bits of paper are attracted to either polarity because of induced charge separation (polarization).

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Units of Charge and Coulomb’s Law

The SI unit of charge is the coulomb (C). One coulomb is a very large practical charge; lightning strokes involve tens of coulombs, while ESD events that kill semiconductors may involve only nanocoulombs (10⁻⁹ C) at high voltage.

Elementary charge: e ≈ 1.6 × 10⁻¹⁹ C. Any macroscopic charge is an integer multiple of e.

Coulomb’s law quantifies the electrostatic force between two point charges:

F = k q₁ q₂ / r²

Where:

• F = magnitude of the force (newtons)
• q₁, q₂ = charges (coulombs)
• r = separation distance (metres)
• k = Coulomb’s constant ≈ 9 × 10⁹ N·m²/C² (in free space / air approximation)

The force is attractive if charges have opposite signs and repulsive if signs are the same. Force direction lies along the line joining the charges.

Worked example: Two point charges of +2 μC and +3 μC are 0.5 m apart in air.

1. Convert: q₁ = 2 × 10⁻⁶ C, q₂ = 3 × 10⁻⁶ C, r = 0.5 m.
2. F = (9 × 10⁹)(2 × 10⁻⁶)(3 × 10⁻⁶) / (0.5)²
3. Numerator = (9 × 10⁹)(6 × 10⁻¹²) = 5.4 × 10⁻²
4. Denominator = 0.25
5. F = 0.216 N (repulsive, since both positive)

Scaling traps for the exam:

• Double one charge → force doubles.
• Double both charges → force quadruples.
• Double the distance → force falls to one quarter (inverse-square law).
• Halve the distance → force becomes four times larger—why approaching charged surfaces raises spark risk quickly.

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Conduction in Solids, Liquids, Gases, and Vacuum

Conduction is the organised movement of charge under an electric field. The carriers depend on the medium:

Solids

In metallic solids, free electrons drift. Ionic solids usually do not conduct until molten or dissolved. Semiconductors use electrons and holes. Insulating solids conduct only negligibly until breakdown (puncture).

Liquids

Pure water is a poor conductor; dissolved salts create mobile ions that carry current (electrolytic conduction). Battery electrolytes, acid spills, and contaminated wash water all provide ionic paths—dangerous across exposed terminals.

Gases

Gases are normally insulating. Above a critical field they ionize; electrons and ions avalanche into a spark or arc. Lightning, ignition sparks, and corona are gaseous conduction. Pressure, electrode shape, and humidity all shift the breakdown voltage (Paschen behaviour is beyond Level 2 detail, but “air can suddenly conduct” is examinable).

Vacuum

A perfect vacuum has no resident carriers. Current still flows if charged particles are injected across the gap—thermionic electrons from a hot cathode, or field-emitted electrons from a sharp point. Vacuum interrupters and old electron tubes exploit this. In Module 3 language: vacuum conduction requires a source of free charges; the vacuum itself is not an electron sea.

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ESD Awareness for Aircraft Maintenance

Electrostatic discharge (ESD) is the sudden transfer of static charge. Humans walking on certain floors can charge to several kilovolts; a discharge of only a few tens of volts can destroy unprotected MOSFET gates even when the technician feels nothing.

Aircraft-focused controls:

• Bonding and grounding during fuelling, oxygen servicing, and painting.
• Static dischargers on airframes to bleed precipitation static in flight.
• ESD workstations for avionics: wrist straps, heel straps, grounded mats, dissipative bags.
• Humidity and cleaners — follow AMM chemistry; some solvents increase static.
• Composite structures may not provide the continuous conductive skin of older aluminium aircraft; follow specific bonding jumpers and conductive paths in the maintenance data.

Exam framing: Static theory (attraction, repulsion, Coulomb’s law) explains why procedures exist; ESD awareness is how you apply that theory without damaging a S$ six-figure LRU or creating an ignition source.

If you can state the inverse-square force law, name the carriers in each medium, and describe one hangar ESD control, you have topic 3.2 at the Level 1–2 depth Module 3 expects.