Static Electricity

How static charge builds

Mechanism	Description
Friction (triboelectric)	Contact + separation of two materials transfers electrons
Induction	Bring charged object near; charges in nearby object redistribute (no contact needed)
Capacitor storage	Voltage source charges plates; held even after disconnection
Photoionization	UV light strips electrons; X-rays, particle radiation
Heating	Some materials charge with thermal gradients

Triboelectric series (partial)

Position	Materials	Tendency
Top (lose electrons → +)	Glass, mica, hair, nylon, wool	Become positive
Middle (neutral)	Cotton, paper, steel, wood	Variable
Bottom (gain electrons → −)	PTFE, silicone, vinyl, polyester	Become negative

When two materials contact and separate, the higher one in the series becomes positive (loses electrons to the lower one).

Lightning and air breakdown

Quantity	Typical value
Air breakdown E field	3 × 10⁶ V/m
Cloud-to-ground potential	10⁸-10⁹ V
Lightning peak current	30,000-300,000 A
Lightning duration	~30 µs to a few ms
Charge transferred per strike	1-10 C
Energy per strike	~1-10 GJ
Air channel temperature	~30,000 K (5× hotter than Sun's surface)
Strikes per day worldwide	~9 million

JavaScript — static electricity

// Voltage on a person from triboelectric charging
function bodyVoltage(charge, capacitance = 100e-12) {
  // V = Q/C; body capacitance ~100 pF
  return charge / capacitance;
}

// Walking on carpet may generate ~10 nC
console.log(`10 nC body charge: ${bodyVoltage(10e-9).toFixed(0)} V`); // 100 V (felt)
console.log(`100 nC: ${bodyVoltage(100e-9).toFixed(0)} V`); // 1000 V (felt)
console.log(`1 µC: ${bodyVoltage(1e-6).toFixed(0)} V`); // 10000 V (definite shock)

// ESD damage threshold
function esdDamage(V) {
  if (V < 100) return 'Safe';
  if (V < 1000) return 'May damage sensitive ICs';
  if (V < 5000) return 'Damages most ICs (e.g., MOSFETs)';
  return 'Damages even less sensitive parts';
}

console.log(esdDamage(2000));  // Damages most ICs

// Force between charges
function staticForce(q1, q2, r) {
  return 8.99e9 * q1 * q2 / (r * r);
}

// 1 µC at 10 cm
console.log(`Two 1 µC at 10 cm: ${staticForce(1e-6, 1e-6, 0.1).toFixed(2)} N`);  // 0.9 N

// Coulomb breakdown distance
function breakdownDistance(V) {
  // V/d = 3e6 V/m for air
  return V / 3e6;
}

console.log(`10 kV needs ${(breakdownDistance(1e4) * 1000).toFixed(2)} mm gap`);
console.log(`100 kV needs ${(breakdownDistance(1e5) * 100).toFixed(2)} cm`);
console.log(`1 MV needs ${breakdownDistance(1e6).toFixed(2)} m`);

// Lightning energy estimate
function lightningEnergy(charge, voltage_V) {
  return 0.5 * charge * voltage_V;  // ½QV approximate
}

console.log(`5 C lightning at 100 MV: ${(lightningEnergy(5, 1e8) / 1e6).toFixed(0)} MJ`);
// 250 MJ = ~280 lbs of TNT equivalent

// Capacitor self-discharge
function capacitorDecay(V_initial, R_leak, C, t) {
  return V_initial * Math.exp(-t / (R_leak * C));
}

// 1 µF cap with 1 GΩ leakage
console.log(`After 1 hour: ${capacitorDecay(100, 1e9, 1e-6, 3600).toFixed(2)} V`);

Where static electricity matters

• Electronics manufacturing. ESD protection essential — wrist straps, ionizers, conductive workstations.
• Photocopiers and printers. Electrostatic image formation.
• Air filtration. Electrostatic precipitators clean industrial smoke.
• Food and chemical industry. Electrostatic powder coating, particle separation.
• Atmospheric physics. Lightning, atmospheric electricity, ionosphere studies.
• Painting. Electrostatic spray painting (charged paint attracts to grounded car body).
• Demonstrations. Van de Graaff generator, Wimshurst machine, Leyden jars — historic and educational.

Common mistakes

• Confusing static and current electricity. Static — accumulated, stationary. Current — flowing. Same charges, different states.
• Treating insulators as charge-free. Insulators ARE where static charge accumulates (since they don't drain). Conductors lose static unless isolated.
• Underestimating ESD damage. 50 V can damage CMOS chips. Below human-feel threshold (~3 kV).
• Ignoring humidity. Higher humidity → moisture provides leakage paths → less static buildup. Dry winter air → bigger shocks.
• Forgetting that air breakdown is local. 3 MV/m is bulk threshold. Sharp points (lightning rods) concentrate field, lower breakdown locally → preferential discharge.
• Treating Coulomb's law as instantaneous. Information about charge changes propagates at light speed. For most static problems, "instantaneous" is fine; for fast transients, retardation matters.