Static electricity

For the science of static charges see Electrostatics

Static electricity refers to the buildup of electric charge on the surface of objects. The static charges remain on an object until they either bleed off to ground or are quickly neutralized by a discharge. Although charge exchange can happen whenever any two surfaces come into contact and separate, a static charge will only remain when at least one of the surfaces has a high resistance to electrical flow (an electrical insulator). The effects of static electricity are familiar to most people because we can see, feel and even hear the spark as the excess charge is neutralized when brought close to a large 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 'shock' is caused by the neutralization of charge. adams mum

History

The natural phenomenon of static electricity was known at least as early as the 6th century BC, as attested by Thales of Miletus. Scientific research into the subject began when machines were built to create it artificially, such as the friction generator developed by Otto von Guericke in the 17th century. The connection between static electricity and storm clouds was famously demonstrated by Benjamin Franklin in 1750 ^[1] ^[2]. In 1832, Michael Faraday published the results of his experiment on the identity of electricities, which proved that the electricity induced using a magnet, voltaic electricity produced by a battery, and static electricity were all the same. Since Faraday's result, the history of static electricity merged with the study of electricity in general.

Causes of static electricity

The materials we observe and interact with from day-to-day are formed from atoms and molecules that are electrically neutral, having an equal number of positive charges (protons, in the nucleus) and negative charges (electrons, in shells surrounding the nucleus). The phenomenon of static electricity requires a separation of positive and negative charges.

Contact-induced charge separation

Electrons can be exchanged between materials on contact; materials with weakly bound electrons tend to lose them, while materials with sparsely filled outer shells tend to gain them. This is known as the triboelectric effect and results in one material becoming positively charged and the other negatively charged. The polarity and strength of the charge on a material once they are separated depends on their relative positions in the triboelectric series. The triboelectric effect is the main cause of static electricity as observed in everyday life, and in common high-school science demonstrations involving rubbing different materials together (e.g. fur and an acrylic rod). Contact-induced charge separation causes your hair to stand up and causes static cling (a balloon rubbing on your hair becomes statically charged and becomes negative, and when it is near a wall it attracts to the positively charged particles in the wall).

Pressure-induced charge separation

Applied mechanical stress generates a separation of charge in certain types of crystals and ceramics molecules.

Heat-induced charge separation

Heating generates a separation of charge in the atoms or molecules of certain materials. All pyroelectric materials are also piezoelectric. The atomic or molecular properties of heat and pressure response are closely related.

Charge-induced charge separation

A charged object brought into the vicinity of an electrically neutral object will cause a separation of charge within the conductor. Charges of the same polarity are repelled and charges of the opposite polarity are attracted. As the force due to the interaction of electric charges falls off rapidly with increasing distance, the effect of the closer (opposite polarity) charges is greater and the two objects feel a force of attraction. The effect is most pronounced when the neutral object is an electrical conductor as the charges are more free to move around.

Careful grounding of part of an object with a charge-induced charge separation can permanently add or remove electrons,leaving the object with a global,permanent charge. This process is integral to the workings of the Van de Graaf Generator, a device commonly used to demonstrate the effects of static electricity.

Removal and prevention of static electricity

Removing or preventing a buildup of static charge can be as simple as opening a window or using a humidifier to increase the moisture content of the air, making the atmosphere more conductive. Air ionizers can perform the same task.^[3]

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 softeners and dryer sheets used in washing machines and clothes dryers are an example of an antistatic agent used to prevent and remove static cling.^[4]

Many semiconductor devices used in electronics are particularly sensitive to static discharge. Conductive antistatic bags are commonly used to protect such components, while people working on circuits containing them will often ground themselves using a conductive antistatic strap.^[5]^[6]

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 only limited conductivity to provide some protection against serious electric shocks from the mains voltage.^[7]

Static discharge

The spark associated with static electricity is caused by electrostatic discharge, or simply static discharge, as excess charge is neutralized by a flow of charges from or to the surroundings.

The feeling of a static electric shock is caused by the stimulation of nerves as the neutralizing current flows through the human body. Due to the ubiquitous presence of water in places inhabited by people, the accumulated charge is generally not enough to cause high current.

Despite the apparently innocuous nature of static electricity as we generally experience it, there can be significant risks associated with it in circumstances where large charges may accumulate in the presence of sensitive materials or devices.

Lightning

Lightning is a dramatic natural example of static discharge. While the details are unclear and remain a subject of debate, the initial charge separation is thought to be associated with contact between ice particles within storm clouds. In general, significant charge accumulations can only persist in regions of low electrical conductivity (very few charges free to move in the surroundings), hence the flow of neutralizing charges often results from neutral atoms and molecules in the air being torn apart to form separate positive and negative charges which then travel in opposite directions as an electric current, neutralizing the original accumulation of charge. The static charge in air typically breaks down in this way at around 30,000 volts-per-centimetre (30 kV/cm) depending on humidity.^[8] The discharge superheats the surrounding air causing the bright flash, and produces a shockwave causing the clicking sound. The lightning bolt is simply a scaled up version of the sparks seen in more domestic occurrences of static discharge. The flash occurs because the air in the discharge channel is heated to such a high temperature that it emits light by incandescence. The clap of thunder is the result of the shock wave created as the superheated air expands explosively.

Electronic components

Many semiconductor devices used in electronics are extremely sensitive to the presence of static electricity and can be damaged by a static discharge.

Static build-up in flowing flammable and ignitable materials

Discharge of static electricity can create severe hazards in those industries dealing with flammable substances, where a small electrical spark may ignite explosive mixtures. ^[9]

The flowing movement of finely powdered substances or low conductivity fluids in pipes or through mechanical agitation can build up static electricity. ^[10] 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 are: a grain silo in southwest France, a paint plant in Thailand, a factory making fiberglass mouldings 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. ^[11] ^[12] ^[13]

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. ^[14] Fluids that have low electrical conductivity (below 50 pico siemens/m), are called accumulators. Fluids having conductivities above 50 pico siemens/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 pico siemens/m is the recommended minimum value of electrical conductivity for adequate removal of charge from a fluid.

Kerosines may have conductivity ranging from <1 pico siemens/m to 20 pico siemens/m. For comparison, deionized water has a conductivity of about 10,000,000 pico siemens/m. ^[15]

An important concept for insulating fluids is the static relaxation time. This is similar to the time constant (tau) within 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 pico siemens /m will have an estimated relaxation time of about 18 seconds. The excess charge within a fluid will be almost completely dissipated after 4 to 5 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/second.

Bonding and earthing are the usual ways by which charge buildup can be prevented. For fluids with electrical conductivity below 10 pico siemens/m, bonding and earthing are not adequate for charge dissipation, and anti-static additives may be required. ^{[citation needed]}

Fueling operations

The flowing movement of flammable liquids like gasoline inside a pipe can build up static electricity. Non-polar liquids such as paraffin, gasoline, toluene, xylene, diesel, kerosene and light crude oils exhibit significant ability for charge accumulation and charge retention during high velocity flow. Static electricity can discharge into a fuel vapor. ^[16] When the electrostatic discharge energy is high enough, it can ignite a fuel vapor and air mixture. Different fuels have different flammable limits and require different levels of electrostatic discharge energy to ignite.
Electrostatic discharge while fueling with gasoline is a present danger at gas stations. 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 build up of static electricity.

The flowing movement of gases in pipes alone creates little, if any, static electricity. ^[17] It is envisaged that a charge generation mechanism will only occur when solid particles or liquid droplets are carried in the gas stream.

Mobile phones and gasoline pumps

Although there have been numerous media reports and posted warnings at gasoline pumps about the risk of fire caused by mobile phones, there has not been a confirmed case of an electrical discharge from a mobile phone ever causing a fire or explosion among gasoline fumes. To date, it is simply an urban legend. ^[18] This legend was further investigated on an episode of Mythbusters (and also on Brainiac), where the protagonists tried to ignite gasoline using a cell phone. The show showed educational and very shocking footage of how most gas pump fires start. In almost all cases, the fire is caused by the person pumping the gas re-entering the car after the fuel has begun to fill the tank, and then step out to take the pump nozzle out. When they grab the pump nozzle, the static discharge occurs from the built up of static electricity on the person, usually from friction that occurred inside the car between the carpet or seat and said person. This discharge can cause the ignition of the highly explosive gasoline vapor by the gas tank opening. This possible fire scenario has led many gas stations to remove the automatic locking mechanism on the gas pump nozzles that were designed to make it easier to fill up an empty tank, as this mechanism also allows a person to step away from the automobile during filling.

Static discharge in space exploration

Due to the extremely low humidity in extraterrestrial environments, very large static charges can accumulate, causing a major hazard for the complex electronics used in space exploration vehicles. Static electricity is thought to be a particular hazard for astronauts on planned missions to the Moon and Mars. Walking over the extremely dry terrain could cause them to accumulate a significant amount of charge; reaching out to open the airlock on their return could cause a large static discharge, potentially damaging sensitive electronics.^[19]

Ozone cracking

A static discharge in the presence of air or oxygen can create ozone. Ozone can attack rubber parts. Many elastomers are sensitive to ozone cracking. Exposure to ozone creates deep penetrative cracks in crisegtical components like gaskets and O-rings. Fuel lines are also susceptible to the problem unless preventative action is taken. Preventative measures include adding anti-ozonants to the rubber mix, or using an ozone-resistant elastomer. Fires from cracked fuel lines have been a problem on vehicles, especially in the engine compartments where ozone can be produced by electrical equipment.

Applications of static electricity

Static electricity is commonly used in xerography, air filters (particularly electrostatic precipitators), automotive paints, photocopiers, paint sprayers, theaters, flooring in operating theaters, powder testing, printers, and aircraft refueling.

Simple static electricity experiments

Note: a humid atmosphere provides a conducting path for the rapid neutralization of static charge; hence the following examples work best in dry, winter conditions.

Static electricity is notable as a physical phenomenon that can be demonstrated using simple experiments that can convey genuine understanding of the physics involved. ^[20]

Charged adhesive tape

A simple and illuminating example of the effects of static electricity can be observed using adhesive tape (such as Scotch tape, on the negative side of the triboelectric series, hence tends to gain electrons and acquire negative charge) charged by peeling.^[21]

If a length of tape adhered to a smooth surface is rapidly peeled off, the tape will acquire an excess negative charge (generally polypropylene with an acrylic adhesive^[22]). Do this with two lengths of tape and they will repel each other, demonstrating the fact that like charges repel. Each individual length of tape will experience a small attraction to almost any object as the presence of the excess negative charge induces a charge separation in nearby objects. Negative charges are pushed farther away, while positive charges are attracted, and the strength of the attractive and repulsive forces falls off quite rapidly with distance. This effect is most pronounced in materials such as metals, that conduct electricity, as the negative charges are free to move within the material.

Finally, try attaching two lengths of tape together, exhaling on them along the entire length to neutralize the charge, then rapidly pulling them apart. There will be some imbalance in the distribution of negative charge between the two pieces such that one is more positive and the other more negative; you should now find that the two lengths of tape attract each other, demonstrating the fact that opposite charges attract. Attaching the adhesive side of one length of tape to the non-adhesive side of the other reduces the chance of tearing and increases the charge imbalance, and hence the strength of the attractive force.