“Electric charge is a fundamental property of matter,” according to Michael Richmond, a physics professor at the Rochester Institute of Technology. Nearly all electric charge in the universe is carried by protons and electrons. Protons are said to have a charge of +1 electron unit, while electrons have a charge of −1, although these signs are completely arbitrary. Because protons are generally confined to atomic nuclei, which are in turn imbedded inside atoms, they are not nearly as free to move as are electrons. Therefore, when we talk about electric current, we nearly always mean the flow of electrons, and when we talk about static electricity, we generally mean an imbalance between negative and positive charges in objects.
Causes of static charge buildup
One common cause of static charge buildup is contact between solid materials. According to the University of Hawaii, “When two objects are rubbed together to create static electricity, one object gives up electrons and becomes more positively charged while the other material collects electrons and becomes more negatively charged.” This is because one material has weakly bound electrons, and the other has many vacancies in its outer electron shells, so electrons can move from the former to the latter creating a charge imbalance after the materials are separated. Materials that can lose or gain electrons in this way are called triboelectric, according to Northwestern University. One common example of this would be shuffling your feet across carpet, particularly in low humidity which makes the air less conductive and increases the effect.
Because like charges repel each other, they tend to migrate to the extremities of the charged object in order to get away from each other. This is what causes your hair to stand on end when your body takes on a static charge, according to the Library of Congress. When you then touch a grounded piece of metal such as a screw on a light switch plate, this provides a path to ground for the charge that has built up in your body. This sudden discharge creates a visible and audible spark through the air between your finger and the screw. This is due to the high potential difference between your body and the ground which can be as much as 25,000 volts.
Dangers of static charge buildup
In addition to causing in a painful shock, these sudden high-voltage discharges can provide a source of ignition for flammable substances, according to the Occupational Safety and Health Administration (OSHA). Static shock can also damage delicate electronics. According to NASA, a simple spark from a finger can damage sensitive components and render them unusable, so precautions must be taken such as keeping circuit boards in conductive plastic bags and wearing grounding straps to dissipate static charge continuously from your body.
Another source of static charge is the motion of fluids through a pipe or hose. If that fluid is flammable — such as gasoline — a spark from a sudden discharge could result in a fire or explosion. People who handle liquid fuels should take great care to avoid charge buildup and sudden discharge. In an interview, Daniel Marsh, professor of physics at Missouri Southern State University, warned that when putting gasoline in your car, you should always touch a metal part of the car after getting out to dissipate any charge that might have developed by sliding across the seat. Also, when buying gas for your lawn mower, you should always take the can out of your car and place it on the ground while filling it. This dissipates the static charge continuously and keeps it from building up enough to create a spark.
Large tank farms present an even greater danger of fire and explosions, so the National Transportation and Safety Board (NTSB) has issued guidelines that include minimizing static generation, preventing charge accumulation, avoiding spark discharge, and controlling the environment inside the tank.
Moving gas and vapor can also generate static charge. The most familiar case of this is lightning. According to Martin A. Uman, author of “All About Lightning” (Dover, 1987), Benjamin Franklin proved that lightning was a form of static electricity when he and his son flew a kite during a thunderstorm. They attached a key to the kite string, and the wet string conducted charge from the cloud to the key which gave off sparks when he touched it. (Contrary to some versions of the legend, the kite was not struck by lightning. If it had been, the results could have been disastrous.)
Franklin in fact shaped the way we think about electricity. He became interested in studying electricity in 1742. Until then, most people thought that electrical effects were the result of mixing of two different electrical fluids. However, Franklin became convinced that there was only one single electric fluid and that objects could have an excess or deficiency of this fluid. He invented the terms “positive” and “negative,” referring to an excess or deficiency, according to the University of Arizona. Today, we know that the “fluid” was actually electrons, but those weren’t discovered for about 150 years.
According to the Jet Propulsion Laboratory, clouds develop zones of static charge due to warm water droplets in updrafts exchanging electrons cold ice crystals in downdrafts. According to NASA, the potential between these atmospheric charges and the ground can exceed 300,000 volts, so the consequences of being struck by lightning can be deadly. In a lightning strike, the current tends to move over the surface of the body in a process called “external flashover,” which can cause severe burns, particularly at the initial point of contact. Some of the current, though, can travel through the body and damage the nervous system, according to the National Weather Service. Additionally, the concussion from the blast can cause traumatic internal injuries and permanent hearing loss, and the bright flash can cause temporary or permanent vision damage. As an example of the tremendous energy released in a lightning strike, Marsh told Live Science about his personal observation of a large oak tree that was literally split in half by high-pressure steam created by a lightning strike.
If you can hear thunder, generally, you are already within striking range, according to the University of Florida. If you are outdoors when a storm approaches, you should immediately seek shelter in a building or vehicle and avoid touching any metal. If you cannot get inside, move away from tall objects such as trees, towers or hilltops, squat down, and if possible, balance on the balls of your feet making as little contact with the ground as possible, according to Brigham Young University.
Applications of static electricity
While static electricity can be a nuisance or even a danger, as in the case of static cling or static shock, in other cases it can be quite useful. For instance, static charges can be induced by electrical current. One example of this is a capacitor, so named because it has the capacity to store electric charge, analogous to how a spring stores mechanical energy. A voltage applied to capacitor creates a charge difference between the plates. If the capacitor is charged and the voltage is switched off, it can retain the charge for some time. This can be useful, as in the case of supercapacitors, which can replace rechargeable batteries in some applications, but it can also be dangerous. Electronic equipment such as older CRT computer monitors and television sets contain large capacitors that can retain a charge with up to 25,000 volts, which can cause injury or death even after the device has been turned off for several days.
Another way to create a useful static charge is with mechanical strain. In piezoelectric materials, electrons can literally be squeezed out of place and forced to move from the region that is under strain. The voltage due to the resulting charge imbalance can then be harnessed to do work. One application is energy harvesting, whereby low-power devices can operate on energy produced by environmental vibrations.
Another application is for crystal microphones. Sound waves in the air can deflect a diaphragm connected to a piezoelectric member that converts the sound waves to an electrical signal. In the inverse operation, the electric signal can cause a piezoelectric transducer in a loudspeaker to move, thus reproducing the sound.
Localized static charges can also be affected by an intense light. This is the principle behind photocopiers and laser printers. In photocopiers, the light may come from a projected image of a sheet of paper; in laser printers, the image is traced onto the drum by a scanning laser beam. The entire drum is initially charged by a coronal discharge wire that gives off free electrons through the air, exploiting the same principle that causes St. Elmo’s fire. The electrons from the wire are then attracted to a positively charged drum. An image is then projected onto the photoconductive drum, and the charge is dissipated from the illuminated areas, while the dark areas of the image remain charged. The charged areas on the drum can then attract oppositely charged toner particles which are then rolled onto the paper, which is backed by a positively charged roller, and fused in place by an electric heating element.
Marsh noted that coal-fired power plants use electrostatic precipitators to collect particulates from smokestacks so they can be disposed of as solid waste rather than being discharged into the air. In another application, he described how static charge is applied to herbicides that are sprayed onto weeds in a fine mist. The charged droplets are attracted to and distributed evenly over the leaves of the undesirable plants rather than falling onto the ground and being wasted. The same principle is used for electrostatic spray painting so more paint goes onto the target and less in the air and on the walls and floor of the paint room.