Useful Static Electricity
Most people are unaware of the important applications that are based on electrostatic principles.
The term static electricity most likely brings to people's minds such phenomena as nasty shocks, clinging clothes, dirty TV screens, exploding tankers, and, especially to the readers of this magazine, ruined electronic components and circuits (also known as ESD problems). And one must admit that when the talk in the electronics industry is about static electricity, it is rarely for its virtues, but rather becausealmost without exceptionit presents itself as a nuisance or sometimes even as a risk.
This is slightly unfair to static electricity, although understandable considering that most people know only of its negative characteristics. Most people are unaware that static electricityor rather, electrostatic principlesare vital for painting the refrigerator, the quality of sandpaper, the functioning of the photocopier and, even more importantly, the cleaning of smoke from coal-fired power plants. These are only a few examples of important applications of static electricity.
It may at first sight seem funny that the cause of the apparently unpredictable and bothersome expressions of static may also be put to good use. But from a physicist's point of view, it is not surprising at all. Most applications of static electricity are related to the handling of particulates, whether airborne or suspended in liquid. The following characteristics are common to electrostatic processes:
- The force from an electric field on a small, charged particle may be much larger than gravity.
- It is very easy to monitor the electric force by monitoring the electric field.
- The electric forces acting upon airborne particles interfere insignificantly with the air and its movement.
In all electrostatics applications, some materials must be electrified, meaning they must be able to respond to an electric field. This can be done either by giving the materials a net charge or by exposing them to induction or polarization.1,2 The three most significant electrification processes are corona electrification, contact and triboelectrification, and induction and polarization in an electric field. Precipitation uses corona electrification. Often, two or more electrification processes are active. For example:
- Separation uses both corona and triboelectrification.
- Surface treatment uses contact and triboelectrification and polarization.
This article focuses on examples of all three of these applications.
The principle of electrostatic precipitation is the charging of airborne (solid or liquid) particles with the same polarity, which consequently makes them move in the same direction in an electric field.
Many industrial processes produce airborne particulates to such an extent that it is necessary to clean the affected air before releasing it into the environment. A prime example is the production of fly ash in coal-fired electric power generation. A lot of other industries also produce particulate air pollution on a large scale.
Even everyday activities, such as cooking and cleaning (not to mention smoking) produce a particle concentration in the air, often at unacceptable levels. Although particulates can be removed from the air by mechanical filters, for industrial pollution, the world would be at a loss without electrostatic precipitation, usually in the form of the electrofilter.
The electrofilter is the oldest application of static electric principles to be put to industrial use, dating back to 1906 when Frederick Cottrell built his first precipitator. Figure 1 shows a simplified drawing of an electrofilter, and Figure 2 illustrates its working principles.
Figure 1. Overview of an electrofilter.
Figure 2. Electrofilter, working principle.
A single corona wire (or series of them) is kept at a high potential, which causes positive and negative ions to be formed within a thin sheath around the wire. In the figure, the negative ions are attracted to the wire where they are neutralized. The positive ions, however, move toward the grounded electrodes, forming a highly concentrated region of positive ions. When the dirty air is drawn through this region, the ions will tend to attach to particles in the air.
The airflow carries the charged particles into the precipitation volume, where they move toward the collecting electrodes and where, eventually, they may plate out. Whether a particle will actually land on the collecting electrodes depends on many factors, including flow rate, particle size, particle charge, filter dimensions, and field strength.
Several methods can be used to remove the plated-out materials from the collecting electrodes. For industrial plants, big mechanical shakers may be needed for cleaning; for household electrofilters, it may be sufficient to place the filter unit in the dishwasher every other week to remove lingering particulates.
The principle of electrostatic separation is the charging of components in particulate mixtures with opposite polarities (or leaving conductive materials uncharged) and subsequently separating the components by mutual repulsion or by an external electric field, possibly aided by gravity.
Figure 3 depicts this process using a drum separator. A mixture of conductive and insulative particles is fed from a hopper onto a grounded drum, and all the particles are charged (positively in the case shown) by a corona discharge. When the particles leave the corona region, the conductive particles lose their charge to the drum and move away by gravity and "centrifugal" forces. The insulative particles stick to the drum until a brush or scraper, possibly assisted by an ac corona discharge, removes them.
Figure 3. Electrostatic drum separator.
The charging of the particles to be separated can be achieved using a variety of methods. Selecting the appropriate method depends on several factors, including the properties of the materials themselves, the state of their surfaces, and external parameters such as electric fields and temperature gradients across interfaces. Table I lists a few of the many material mixtures that in particle form can be separated electrostatically to produce a commercial benefit.
|Material A||Material B|
Barley, rice||Rodent excrement|
Table I. Electrostatically separable mixtures.
Nearly all products manufactured today are coated in some way. This is true for the paper we write on, the clothes we wear, the cars we drive, and the furniture we sit on. For practical, economic, and environmental reasons, it is desirable that products be coated with only the necessary amount of material, and this consideration makes electrostatic coating superior to most other methods.
In an earlier column, we discussed spray painting as a method of surface treatment using electrostatics.3 This section addresses a completely different type of coating used for manufacturing sandpaper. Figure 4 illustrates the principle used in this coating process.
Figure 4. Manufacturing of sandpaper.
Two plane electrodesone grounded while the other is connected to a high-voltage supplyare used to establish an electric field. A continuous belt of a suitable semiconductive material passes through the field in contact with the lower electrode. The paper or textile for the sandpaper backing is fed from a stock roll through the field, in contact with the upper electrode. Before the paper enters the field, a layer of adhesive is applied to the side of the paper that will eventually face downward.
An abrasive such as ground agate is fed from a hopper onto the lower belt outside the field. When the abrasive enters the field, the polarized grains will be charged directly with the same polarity as the lower electrode.
The grains will align their longest axes with the field, and when the field strength is high enough, they will move toward the upper electrode and embed themselves in the adhesive with a sharp tail protruding. If a grain hits a point already occupied by another grain, the late arriver is neutralized, charged with the opposite polarity, and repelled. It will then fall to the lower belt, repeating the process.
When the grit-covered face leaves the field, a shaker removes loose material and a drier sets the adhesive. The electrostatic deposition of the abrasive provides a more uniform
distribution and a more beneficial orientation of the single grains than
is possible by a purely mechanical process.
The three topics treated here are only a few of many useful applications of electrostatic principles. Equally valuable applications include electret microphones and filters, electrostatic motors, and the vital field of electrostatic image forming known as xerography, without which we wouldn't have photocopies. The purpose of this short survey is to provide an overview of the possibilities of electrostatics. Perhaps the next time you are frustrated by an ESD problem, you might even remember that static electricity can also be looked at as a good thing.
1. Niels Jonassen, "Induction: What it Means to ESD," in Mr. Static, Compliance Engineering 17, no. 4 (2000): 2834.
2. Niels Jonassen, "Polarization, for Better or Worse," in Mr. Static, Compliance Engineering 17, no. 5 (2000): 3440.
3. Niels Jonassen, "Is Static Electricity Static?," in Mr. Static, Compliance Engineering 17, no. 1 (2000): 3036.
Niels Jonassen, MSc, DSc, worked for 40 years at the Technical University of Denmark, where he conducted classes in electromagnetism, static and atmospheric electricity, airborne radioactivity, and indoor climate. After retiring, he divided his time among the
laboratory, his home, and Thailand, writing on static
electricity topics and pursuing cooking classes. He passed away in 2006.
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