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From Wikipedia, the free encyclopedia
- For the science of static charges see Electrostatics
Static electricity refers to the accumulation of excess electric charge in a region with poor electrical conductivity, (i.e. an instulator) such that the charge accumulation persists. 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 an electrical conductor forming a path to ground, or a region with an excess charge of the opposite polarity (positive or negative).
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[edit] History
Before the year 1832, when Michael Faraday published the results of his experiment on the identity of electricities, physicists thought static electricity was somehow different from other electrical charges. Faraday proved that the electricity induced using a magnet, voltaic electricity produced by a battery, and static electricity are all the same.
[edit] 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 sustained separation of positive and negative charges.
[edit] Triboelectric effect
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 materials 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)
[edit] Electrostatic induction
A charged object brought into the vicinity of an electrical conductor will cause a separation of charge within the conductor as charges of the same polarity are repelled and charges with the opposite polarity are attracted. Although the conductor as a whole remains neutral, it will contain regions of excess positive and negative charge which themselves act as charged objects.
Careful grounding of part of an object with such a charge imbalance 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 static electricity.
[edit] 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. 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 discharge superheats the surrounding air causing the bright flash, and produces a shockwave causing the clicking sound.
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 a dangerously high current.
[edit] Lightning
Lightning is a dramatic natural example of static discharge. While the details are unclear and remain the subject of debate, the initial charge separation is thought to be associated with contact between ice particles within storm clouds. Whatever the cause may be, the resulting 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 shockwave created as the superheated air rapidly expands.
[edit] Simple 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.[1]
[edit] 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.[2]
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[3]). Do this with two lengths of tape and they will repel each other, demonstrating the fact that like charges repel.
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.
[edit] Applications
Static electricity is commonly used in xerography, air filters (particularly electrostatic precipitators), and some automotive paints.
[edit] Risks
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.
[edit] The chemical industry
Discharge of static electricity can create severe hazards in those industries dealing with flammable substances, where a small electrical spark may ignite explosive mixtures with devastating consequences.
A similar charging mechanism can occur within low conductivity fluids flowing through pipelines - a process called flow electrification. Fluids which have low electrical conductivity (below 50 pico-siemens/cm, where pico-siemens/cm is a measure of electrical conductivity), are called accumulators. Fluids having conductivities above 50 pico siemens/cm are called non-accumulators. In non-accumulators, charges recombine as fast as they are separated and hence electrostatic charge generation is not significant. In the petrochemical industry, 50 pico siemens/cm is the recommended minimum value of electrical conductivity for adequate removal of charge from a fluid.
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/cm 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 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 velocity limits. Because of its large impact on dielectric constant, the recommended velocity for hydrocarbon fluids containing water should be limited to 1 m/sec.
Bonding and earthing are the usual ways by which charge buildup can be prevented. For fluids with electrical conductivity below 10 pico-siemens/cm, bonding and earthing are not adequate for charge dissipation, and anti-static additives may be required.
Applicable standards:
- BS PD CLC/TR 50404:2003 Code of Practice for Control of Undesirable Static Electricity
- NFPA 77 (2007) Recommended Practice on Static Electricity
- API RP 2003 (1998) Protection Against Ignitions Arising Out of Static, Lightning, and Stray Currents
[edit] 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.
[edit] Space exploration
Due to the extremely low humidity in extra-terrestrial 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.[4]
[edit] References
- ^ Kids science projects. Retrieved on 2008-01-20.
- ^ H. Yasuro, H. Makoto and I. Isao (2007). "Charging of Adhesive Tapes on Peeling". Journal of the Adhesion Society of Japan 43 (3): 97-103.
- ^ 3M Material Safety Data Sheet. Retrieved on 2008-01-20.
- ^ NASA - Crackling Planets. Retrieved on 2008-01-20.
