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Static electricity is an imbalance of electric charges within or on the surface of a material. The charge remains until it can move away as an electric current or by electrical discharge. The word "static" is used to differentiate it from electric current, where an electric charge flows through an electrical conductor.
A static electric charge can be created whenever two surfaces contact and/or slide against each other and then separate. The effects of static electricity are familiar to most people because they can feel, hear, and even see sparks if the excess charge is neutralized when brought close to an electrical conductor (for example, a path to ground), or a region with an excess charge of the opposite polarity (positive or negative). The familiar phenomenon of a static shockmore specifically, an electrostatic dischargeis caused by the neutralization of a charge.
Items that are particularly sensitive to static discharge may be treated with the application of an antistatic agent, which adds a conducting surface layer that ensures any excess charge is evenly distributed. Fabric softener used in and are an example of an antistatic agent used to prevent and remove static cling.
Many semiconductor devices used in electronics are particularly sensitive to static discharge. Conductive are commonly used to protect such components. People who work on circuits that contain these devices often ground themselves with a conductive antistatic strap. (2026). 9780782143607, John Wiley and Sons. ISBN 9780782143607 (2026). 9780782143607, John Wiley and Sons. ISBN 9780782143607
In the industrial settings such as paint or flour plants as well as in hospitals, antistatic safety boots are sometimes used to prevent a buildup of static charge due to contact with the floor. These shoes have soles with good conductivity. Anti-static shoes should not be confused with insulating shoes, which provide exactly the opposite benefit some protection against serious from the mains voltage.
Within medical cable assemblies and lead wires, random triboelectric noise is generated when the various conductors, insulation, and fillers rub against each other as the cable is flexed during movement. Noise generated within a cable is often called handling noise or cable noise, but this type of unwanted signal is more accurately described as triboelectric noise. When measuring low-level signals, noise in cable or wire may present a problem. For example, the noise in an ECG or another medical signal may make accurate diagnosis difficult or even impossible. Keeping triboelectric noise at acceptable levels requires careful material selection, design, and processing as cable material is manufactured.
The feeling of an electric shock is caused by the stimulation of nerves as the current flows through the human body. The energy stored as static electricity on an object varies depending on the size of the object and its capacitance, the voltage to which it is charged, and the dielectric constant of the surrounding medium. For modelling the effect of static discharge on sensitive electronic devices, a human being is represented as a capacitor of 100 farad, charged to a voltage of 4,000 to 35,000 volts. When touching an object this energy is discharged in less than a microsecond.Carlos Hernando Díaz, Sung-Mo Kang, Charvaka Duvvury, Modeling of electrical overstress in integrated circuits Springer, 1995 page 5
The flowing movement of finely powdered substances or low conductivity fluids in pipes or through mechanical agitation can build up static electricity.Wagner, John P.; Clavijo, Fernando Rangel Electrostatic charge generation during impeller mixing of used transformer oil Department of Nuclear Engineering, Safety Engineering and Industrial Hygiene Program, Texas A&M University, College Station, online 21 August 2000; accessed Jan 2009 The flow of granules of material such as sand down a plastic chute can transfer charge, which can be measured using a multimeter connected to metal foil lining the chute at intervals, and can be roughly proportional to particulate flow.Downie, Neil A., Exploding Disk Cannons, Slimemobiles and 32 Other Projects for Saturday Science (Johns Hopkins University Press (2006), , chapter 33, pages 259-266 "Electric Sand" Dust clouds of finely powdered substances can become combustible or explosive. When there is a static discharge in a dust or vapor cloud, explosions have occurred. Among the major industrial incidents that have occurred due to static discharge are the explosion of a grain silo in southwest France, a paint plant in Thailand, a factory making fiberglass moldings in Canada, a storage tank explosion in Glenpool, Oklahoma in 2003, and a portable tank filling operation and a tank farm in Des Moines, Iowa and Valley Center, Kansas in 2007. Storage Tank Explosion and Fire in Glenpool, Oklahoma April 7, 2003 National Transportation Safety Board Static Spark Ignites Flammable Liquid during Portable Tank Filling Operation Chemical Safety Board October 29, 2007
The ability of a fluid to retain an electrostatic charge depends on its electrical conductivity. When low conductivity fluids flow through pipelines or are mechanically agitated, contact-induced charge separation called flow electrification occurs. Fluids that have low electrical conductivity (below 50 picosiemens per meter), are called accumulators. Fluids having conductivity above 50 pS/m are called non-accumulators. In non-accumulators, charges recombine as fast as they are separated and hence electrostatic charge accumulation is not significant. In the petrochemical industry, 50 pS/m is the recommended minimum value of electrical conductivity for adequate removal of charge from a fluid.
Kerosines may have conductivity ranging from less than 1 picosiemens per meter to 20 pS/m. For comparison, deionized water has a conductivity of about 10,000,000 pS/m or 10 μS/m.Chevron Corporation Aviation Fuels Technical Review 2006, accessed Dec 2008
Transformer oil is part of the electrical insulation system of large power and other electrical apparatus. Re-filling of large apparatus requires precautions against electrostatic charging of the fluid, which may damage sensitive transformer insulation.
An important concept for insulating fluids is the static relaxation time. This is similar to the time constant τ (tau) of an RC circuit. For insulating materials, it is the ratio of the static dielectric constant divided by the electrical conductivity of the material. For hydrocarbon fluids, this is sometimes approximated by dividing the number 18 by the electrical conductivity of the fluid. Thus a fluid that has an electrical conductivity of 1 pS/m has an estimated relaxation time of about 18 seconds. The excess charge in a fluid dissipates almost completely after four to five times the relaxation time, or 90 seconds for the fluid in the above example.
Charge generation increases at higher fluid velocities and larger pipe diameters, becoming quite significant in pipes 8 inches (200 mm) or larger. Static charge generation in these systems is best controlled by limiting fluid velocity. The British standard BS PD CLC/TR 50404:2003 (formerly BS-5958-Part 2) Code of Practice for Control of Undesirable Static Electricity prescribes pipe flow velocity limits. Because water content has a large impact on the fluids dielectric constant, the recommended velocity for hydrocarbon fluids containing water should be limited to 1 meter per second.
Bonding and earthing are the usual ways charge buildup can be prevented. For fluids with electrical conductivity below 10 pS/m, bonding and earthing are not adequate for charge dissipation, and anti-static additives may be required.
Electrostatic discharge while fueling with gasoline is a present danger at gas stations. "CarCare – Auto Clinic" Popular Mechanics, April 2003, p. 163. Fires have also been started at airports while refueling aircraft with kerosene. New grounding technologies, the use of conducting materials, and the addition of anti-static additives help to prevent or safely dissipate the buildup of static electricity. Customers who need to fill containers at gas stations are advised to set them on the ground first so that any static buildup will dissipate without risk of fire or explosion.
The flowing movement of gases in pipes alone creates little, if any, static electricity.Kinzing, G.E., 'Electrostatic Effects in Pneumatic Transport: Assessment, Magnitudes and Future Direction', Journal Pipelines, 4, 95–102, 1984 It is envisaged that a charge generation mechanism only occurs when solid particles or liquid droplets are carried in the gas stream.
The maximal potential is limited to about 35–40 kV, due to corona discharge dissipating the charge at higher potentials. Potentials below 3000 volts are not typically detectable by humans. Maximal potential commonly achieved on human body range between 1 and 10 kV, though in optimal conditions as high as 20–25 kV can be reached. Low relative humidity increases the charge buildup; walking 20 feet (6 m) on vinyl floor at 15% relative humidity causes buildup of voltage up to 12 kV, while at 80% humidity the voltage is only 1.5 kV.M. A. Kelly, G. E. Servais, T. V. Pfaffenbach An Investigation of Human Body Electrostatic Discharge, ISTFA ’93: The 19th International Symposium for Testing & Failure Analysis, Los Angeles, California, USA/15–19 November 1993.
As little as 0.2 millijoules may present an ignition hazard; such low spark energy is often below the threshold of human visual and auditory perception.
Typical ignition energies are:
The energy needed to damage most electronic devices is between 2 and 1000 nanojoules.
A relatively small energy, often as little as 0.2–2 millijoules, is needed to ignite a flammable mixture of a fuel and air. For the common industrial hydrocarbon gases and solvents, the minimum ignition energy required for ignition of vapor–air mixture is lowest for the vapor concentration roughly in the middle between the lower explosive limit and the upper explosive limit, and rapidly increases as the concentration deviates from this optimum to either side. Aerosols of flammable liquids may be ignited well below their flash point. Generally, liquid aerosols with particle sizes below 10 micrometers behave like vapors, particle sizes above 40 micrometers behave more like flammable dusts. Typical minimal flammable concentrations of aerosols lay between 15 and 50 g/m3. Similarly, presence of foam on the surface of a flammable liquid significantly increases ignitability. Aerosol of flammable dust can be ignited as well, resulting in a dust explosion; the lower explosive limit usually lies between 50 and 1000 g/m3; finer dusts tend to be more explosive and requiring less spark energy to set off. Simultaneous presence of flammable vapors and flammable dust can significantly decrease the ignition energy; a mere 1 vol.% of propane in air can reduce the required ignition energy of dust by 100 times. Higher than normal oxygen content in atmosphere also significantly lowers the ignition energy. Static Electricity Guidance for Plant Engineers. Graham Hearn – Wolfson Electrostatics, University of Southampton.
There are five types of electrical discharges:
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