Chapter 15 - Static Electricity

Abstract

A static charge is formed whenever two surfaces are in relative motion, for example, when a liquid flows past the walls of a pipeline, when liquid droplets or solid particles move through the air, or when someone walks, gets up from a seat, or removes an article of clothing. Static electricity (static for short) has been blamed for many fires and explosions, sometimes correctly. Sometimes, however, investigators have failed to find any other source of ignition. So they assume that it must have been static even though they are unable to show precisely how a static charge could have been formed and discharged. Many static charges flow rapidly to earth as soon as they are formed. But if a charge is formed on a nonconductor or on a conductor that is not grounded, it can remain for some time. If the level of the charge, the voltage, is high enough, the static will discharge by means of a spark, which can ignite any flammable vapors that may be present. On a number of occasions, people receive a mild electric shock while using a carbon dioxide fire extinguisher. The gas jets from the extinguishers contain small particles of solid carbon dioxide, so a charge will collect on the horn of the extinguisher and may pass to earth via the hand of the person who is holding the horn. The water droplets from steam jets are normally charged, and discharges sometimes occur from the jets to neighboring grounded pipes. These discharges are of the corona type rather than true sparks and may be visible at night; they look like small flames.