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What Creates Static Electricity?

Traditionally considered a physics problem, the answer is beginning to emerge from chemistry and other sciences

Meurig W. Williams

When two objects are brought into contact and then separated, electrical charges are generated at the surfaces. Such events are called triboelectric charges, also known as contact or static charges. Triboelectricity is one of the oldest areas of scientific study, dating back to experiments by the ancient Greek philosopher Thales of Miletus, who discovered that rubbing amber against wool led to electrostatic charging. Indeed, triboelectric in Greek means “rubbing amber”; however, rubbing is not necessary because such charging also results from simple nonfrictional contacts.

2012-07WilliamsF1.jpgClick to Enlarge ImageThe buildup of this electrical potential can lead to electrostatic discharge, with consequences that can range from discomfort to disaster. Results can be as mild as a jolt we experience by touching a doorknob after walking across a rug in dry weather, or as dire as the crash of the Hindenburg, where one theory for the cause of the airship fire is that a static spark ignited a hydrogen leak. Such discharges are a major concern for NASA because the dry conditions on the Moon and Mars are ideal for triboelectric charging: An astronaut, reaching out to open an airlock after a walk on the dry surface, may cause a discharge that could zap critical electronic equipment. But not all static is a nuisance: Triboelectric charging, when controlled, is at work in products such as copiers and laser printers.

Although static electricity is a familiar subject, much still remains unknown about how and why such charges form. Research across many disciplines of science and engineering, from physics and chemistry to medicine and meteorology, is currently being conducted on triboelectricity’s various aspects. However, relatively few scientists are engaged in understanding it at a fundamental level.

Contact charge exchange between two metals is known to result from the transfer of electrons. But when at least one of the materials is an electrical insulator, there is no general understanding of what carries charges from one surface to the other. Different theories have proposed either electrons or ions. An electron is a subatomic particle carrying a negative electrical charge; an electrical current involves movement of electrons in a metal conductor. An ion, on the other hand, can carry either a positive or a negative electric charge; they are known as cations and anions, respectively. A cation has fewer electrons than protons, giving it a positive charge. An anion possesses more electrons than protons, so it has a net negative charge. Cations and anions can be atoms, molecules or polymer fragments. Evidence has been discovered for both electron and ion transfer under specific experimental conditions, but these data are limited and frequently contradictory. Recently, new research has demonstrated that charge exchange can also result from the physical transfer of tiny amounts of surface material from one substance to another. An understanding of how this occurs on a molecular level is now just beginning to emerge. It is becoming increasingly clear that more than one mechanism can occur simultaneously, and what happens may depend on the material compositions and conditions of the experiments in ways not yet known.

Remarkably, why charge exchange happens at all when insulators are involved is even less well understood than how it occurs, although the inherent complexity of the problem has long been appreciated. How does a material that by definition does not conduct electricity nonetheless gain an electrical charge? Three questions must be answered: Are the charge exchange species electrons or ions, what is the driving force for charge exchange and what limits the charge exchange? Traditionally considered to be a problem in physics, progress on finding the specific mechanisms of charge exchange did not really begin until the application of several areas of chemistry.

One reason that answers have been slow in coming is lack of incentive: Most research involving triboelectricity is applied to the development of new technologies and to solving problems, and understanding the mechanisms of charge exchange is not required for these purposes—a charge is just a charge, regardless of how and why it occurs. However, a clear picture of charging mechanisms could contribute to useful purposes when it becomes available.

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