physics//electromagnetism//electric charge//static electricity

Static electricity is electric charge that has built up on an object and stays there because it has no conducting path to leave by, and it matters to engineers mostly as a hazard: the spark that kills a chip on a workbench, or ignites dust or fuel vapour. It is what shocks a hand reaching for a doorknob after walking across a carpet.

Static electricity is electric charge that has built up on an object and stays there because it has no conducting path to leave by, and it matters to engineers mostly as a hazard: the spark that kills a chip on a workbench, or ignites dust or fuel vapour. It is what shocks a hand reaching for a doorknob after walking across a carpet.

The charge usually comes from contact between two different materials, the triboelectric effect. When two surfaces touch, electrons cross from the one that holds them more loosely to the one that holds them more tightly, and when the surfaces separate each keeps its share: one side ends up with extra electrons, the other with a deficit. Rubbing does not create the charge; it multiplies the points of contact and separation, so a shoe sole dragged across a carpet can leave a body at several thousand volts. Why a given pair of materials exchanges charge in a given direction is only partly understood (electron transfer dominates, but ions and fragments of material can move too).

The charge waits for a path.

A body at several thousand volts carries a tiny amount of charge, harmless in itself; the event is the discharge, when the electric field near a conductor grows strong enough for the air to give way.

Dry air makes it worse. Humid air and damp surfaces let the charge leak away slowly, which is why static shocks are a winter, heated-room phenomenon.

The sparks are short because the air holds out. Air breaks down at about 3 kV per millimetre (dielectric breakdown), so a few thousand volts jump only a few millimetres, the moment the hand is close enough.

Electronics are protected by giving the charge a slow path first: wrist straps, conductive mats and grounded benches drain it before it can arc into a circuit.