By far most commonly during winter when the atmosphere is dry, we are likely to experience triboelectric charging. Getting out of a car or removing a jacket made of synthetic fabric can lead to a startling electric shock. Nuisance electrostatic charging has become more and more common in the modern world, due primarily to widespread use of synthetic fabrics and plastics. These materials are good insulators and do not dissipate charge once it has been separated. Examples of such charging, such as those below, can be used effectively in electrostatic demonstrations.
CLICK HERE for a triboelectric series & brief discussion of the physics of triboelectification.
The commonest demonstration of triboelectrification in the lecture hall involves rubbing a rod, a sheet, or other solid object with a cloth, and then showing that the rod has become electrified. There are many choices for materials these days, and so only a few of the more reliable and readily available materials are listed below.
Because of their conductive properties, rubbing cloths made of wool, silk, cotton, and most other natural fibers usually retain no charge because of rapid leakage to ground through the body. Exceptions include certain synthetic fabrics, such a polyethylene filter media, etc.
In all but the purest of materials, the nature of triboelectrification is very
complex. Part I of Harper's classic book [Harper, 1967] provides an excellent
discussion of the phenomenology.
When a CO2 fire extinguisher is operated, rapid expansion of the gas causes cooling which manifests itself as a "fog" consisting of dry ice (i.e., CO2) particles and, if the relative humidity is high, water ice formed from moisture in the air. These entrained ice particles move very rapidly and any that collide with the plastic horn of the fire extinguisher are likely to become triboelectrically charged. The charging of these ice particles may seem surprising, but in fact triboelectrification is commonly observed with wind-blown snow during very cold weather.
A simple demonstration of this effect can be achieved with a CO2 bicycle tire inflater with a plastic horn attached to it. The horn can be made out of the plastic cover from an inexpensive report binder, rolled up into a cone. See the figure below. The TESV or the moving charge sensor may be used to detect the charging.
This demonstration reveals the somewhat ironic result that even such a
conventional safety device as a fire extinguisher can create a significant
electrostatic hazard in certain applications. The hazard arises when an extinguisher
is used to inert a shipboard fuel tank. This practice, called "hatch snuffing", is
done through an open hatch. If sufficient charge is separated, a brush electrostatic
discharge can occur, igniting flammable vapors or mist present just outside the hatch
opening. The risk associated with this phenomenon can be avoided simply by removing
the plastic horn from the CO2 fire extinguisher [Leonard and Clark, 1975].
Without the horn, entrained ice particles have no surface to interact with and against
which to become charged.
Desoldering tools apply strong suction to remove excess molten solder from a printed circuit board or connection terminal. These tools come in a variety of sizes and types and are likely to be constructed of plastic. The device is essentially a spring-loaded piston which is first cocked and then released to create a temporary partial vacuum in the chamber which sucks up the excess molten solder. Refer to the figure below. The nozzle area of these tools can become triboelectrically charged by contact with bits of solder and other debris being drawn at very high speed through the aperture into the piston. If the charging is sufficient and the nozzle makes contact with a wire or circuit element, a small electrostatic discharge can occur. Sensitive electronic components can be damaged by these small electrostatic discharges.
Desoldering tools made
with antistatic plastic are available. The slightly conductive plastic probably does
not significantly decrease triboelectric charging, but does more rapidly dissipate this
charge from the nozzle, reducing the chance of damage to electronic components.
When clear plastic tape is pulled from a roll, the end of the tape almost always exhibits an annoying "static cling" effect. This phenomenon shows that the tape has become tribocharged during separation from the roll. If the dispensed tape is charged negatively, then there must be positive charge remaining on the roll. This fact is readily demonstrated by cutting away part of the top of a plastic tape dispenser to reveal more of the roll. The existence of both signs of charge can be shown readily using the TESV instrument described elsewhere at this site.
In the packaging of sensitive VLSI chips and circuit boards for shipment, it is
quite important to avoid this charge. Furthermore, in cleanroom fabrication
facilities, serious contamination problems can be caused by static adhesion of dust
particles to electrically charged tape. Antistatic tapes and tape dispensers are now
widely available on the market to deal with these problems. The charging phenomena
that occur when tape or any insulating sheet material is peeled from a surface or
backing are complex, and have been the subject of much study [Walker, 1987 & 1988;
Scudiero et al., 1998].
When stackable, plastic objects such as chairs, buckets, or funnels are pulled apart, a separation of electrostatic charge often occurs so that opposite sign charge accumulates on the surfaces being separated. This phenomenon, depicted below, has been implicated in a number of ESD ignitions involving flammable liquids [Lüttgens, 1985]. Such incidents are likely to be serious because of the high probability of direct worker exposure in the event of an ignition of flammable material. In the incident mentioned by Lüttgens, a worker was burned when the flammable vapors of the solvent caught fire.
A good question to ask is why buckets made of the same material could ever become charged when separated. The explanation is that plastic is neither a pure nor a homogeneous material, so the two surfaces being separated are NOT identical and thus can become charged. Even if the two buckets were identical, their surfaces become contaminated with dirt, so triboelectrification is still likely to occur.
A plastic bucket with an insulating grip on the metal handle is more likely to stay
charged when held by its handle. Note that, if the bucket contains water or has
moisture on its inside, then it may behave like a Leyden jar, storing the charge at
lower potential and possibly increasing the likelihood that any capacitive spark might
Many people experience static electricity on a daily basis in an automobile or truck, especially in the dry winter season. Sliding across a seat to get out of a vehicle, it is annoyingly common to experience a shock when you touch the door handle or metal frame of the car. The charging occurs as the fabric of your pants, skirt, or coat makes sliding contact with the car seat material. CLICK HERE to read some discussion on the Car Talk website about static charging in winter. John Chubb has investigated this phenomena extensively. Auto manufacturers could solve the problem permanently, were they to install antistatic fabric in car seats. I do not know why they don't do this! To avoid this unpleasant phenomenon, you must slowly dissipate the charge before touching any metal components of the car. Though sometimes quite startling, these shocks are not generally thought to be dangerous to a driver or passenger.
On the other hand, the situation is very different for a car or truck in a gasoline
filling station. During fueling, a spark can ignite gasoline fumes and cause serious
fires, as reported by the Petroleum Equipment
Institute. Furthermore, as reported by Chevron,
there is a serious risk when a fuel container sitting in the bed of pickup trucks
with a plastic liner is filled.
To demonstrate triboelectrification of different materials in a lecture, it is very important to start each test with materials that are uncharged. Starting with zero net charge is necessary to convince student that it is the rubbing that causes the separation of the charge. Many easily charged materials, such as PVC or Teflon, are very highly insulating and retain their charge long after being rubbed. Therefore, using them more than one time requires that the lecturer be able to discharge them quickly. One fairly effective way to discharge objects is with the small open flame of a Bunsen burner or alcohol lamp. But this solution is often inconvenient and may present a safety risk. A better solution is to use a small ionizing air blower. These simple devices are essentially fans with a high voltage AC voltage source and a set of corona points to produce equal amounts of positive and negative ions. Even the smallest model blower will effectively neutralize almost any charged insulating object within a few seconds.
Use of an ionizing air blower greatly increases the predictability of many
electrostatics experiments and, in particular, facilitates the triboelectric charging
demonstrations described on this page. CLICK HERE for a
technical summary of their operation.
W.R. Harper, Contact and Frictional Electrification, Oxford Univ. Press, United Kingdom, 1967.
J.T. Leonard and R.C. Clark, "Generation of static electricity by carbon dioxide in inerting and fire extinguishing systems," Inst. Phys. Conf., ser. 27, 1975, pp. 301-310.
G. Lüttgens, "Collection of accidents caused by static electricity," J. Electrostatics, Vol. 16, 1985, pp. 247-255.
J. Walker, "The Amateur Scientist," Scientific American, Vol. 257, 1987, pp. 138-141; Vol. 258, 1988, pp. 114-117.
L. Scudiero, S. C. Langford, and J. T. Dickinson, "Electrical currents
produced by peeling a pressure sensitive adhesive: Role of electrostatic forces in
adhesion", Mittal Festschrift, W. J. Van Ooij and H. R. Anderson, eds., (VSP:
Utrecht, NL) 1998, pp. 597-613.