Key Concepts
Electric charge at rest, resulting from an imbalance of positive and negative electric charges on a surface or within a material. The imbalance of electric charges is generally produced by friction or electrostatic induction. The electric charges build up on or within a material until there is a release or discharge to another material. In this regard, static electricity is a familiarly encountered phenomenon, for instance when walking across carpet and then touching a metal doorknob. Extra electrons that have accumulated on and within the body transfer to the doorknob upon contact, resulting in an electric shock to the fingertip. Another common example of static electricity is when hair stands on end after the friction caused by taking off a knit hat (see illustration). In this instance, the negatively charged chair is drawn to the positively charged hat, while the accumulated electrons in the hair repel each other, causing the hair strands to separate. See also: Electric charge; Electric current; Electricity; Electromagnetic induction; Electron; Friction
The discovery of static electricity is usually credited to the Greek philosopher Thales. In the sixth century BCE, Thales described experiments in which rubbed amber induced movement in nearby particles. It was not until the eighteenth century, when U.S. scientist and statesman Benjamin Franklin carried out his kite-flying experiments in which he “captured” some thunderstorm electricity in a Leyden jar, that it was verified that thunderstorm electricity and static electricity were one and the same.
Triboelectrification is the process whereby charge transfer between dissimilar materials, at least one of which must have a high electrical resistivity, occurs due to rubbing or mere contact, as in the prior example of the carpet-walking and doorknob-touching. In industrial contexts, contact and separation of materials occur during powder processing and the manufacture of plastic and other materials in sheet form. See also: Electrical resistivity
Most organic and polymeric materials have volume resistivities greater than 108 ohms per meter and retain charge for periods of many hours. The magnitude of triboelectric charging may be strongly influenced by surface contamination of the contacting materials, relative humidity of the atmosphere, and the energy of rubbing. For example, during the grinding of material during powder manufacture, charge levels of about a microcoulomb per kilogram are typical. This is a relatively low level of charge. The more energetic process of pneumatically conveying a powder along a pipe may give rise to charge levels a thousand times greater and an electrostatic hazard may arise. Hazards are due to sparks to ground after a relatively large amount of static charge has accumulated on a body that is improperly grounded. See also: Electric insulator; Electric spark
In modern industry, highly insulating synthetic materials, such as plastic powders and insulating liquids, are used in large quantities in many applications. Such materials charge up readily, and large quantities of electrical energy may develop with an attendant risk of incendiary discharges. When, for example, powder is pneumatically transported along pipes, charge levels of up to about 100 microcoulombs per kilogram can develop and potentials of thousands of volts are generated within powder layers and the powder cloud. Energetic sparking from charged powder may initiate an explosion of the powder cloud. Similar problems occur when insulating liquids, such as certain fuels, are pumped along pipes, and it is essential that strict grounding procedures are followed during the refueling of aircraft, ships, and other large vehicles.
The capacity of a person for retaining charge depends upon stature, but is typically about 150 picofarads. Even the simple operations of removing items of clothing or sliding off a chair can lead to body discharges to ground of about 0.1 μC, which are energetic enough to ignite a mixture of natural gas and air. Human body capacitance is sufficiently high that, if poorly conducting shoes are worn, body potential may rise to 15,000 V or so above ground during industrial operations such as emptying bags of powder. Sparking may then occur with energy exceeding the minimum ignition energy of powder or fumes, so initiating a fire or explosion. Conducting footware should be used to prevent charge accumulation on personnel in industrial situations where triboelectrification may occur. See also: Capacitance
In the microelectronics industry, extremely low-energy discharges, arising from body potentials of only a few tens of volts, can damage microelectronics systems or corrupt computer data. During the handling of some sensitive semiconductor devices, it is imperative that operators work on metallic grounded surfaces and are themselves permanently attached to ground by conducting wrist straps. See also: Electrostatics
