1、W : . . _- . i ,. . - .- . . I Revised 1969 - is perhaps the oldest known manifestation of electricity. The ancient Greeks, such as Tliales of Miletus (640-546 B.C.) spec- ulated and wrote about the strange property which amber developed when rubbed with a dissimilar material. The force of attractio
2、n or repulsion between materials after rubbing with amber was, for almost 2,000 years, the extent of mans knowledge of electricity. The name “electricity? is derived from the Greek word for amber wliich is “elektron.” Today, static electricity is not considered a phenomenon which excites mans curios
3、ity. It is considered by most people to be a nuisance which causes shocks, unruly hair, clinging clothes, or other annoyances. To people who are concerned with industrial safety, it is more than an annoyance, it is a hazard which iii many areas can be dangerous. This bulletin will discuss static ele
4、ctrical phenomena not as an exhaustive treatise but as a short survey of possible causes and coil- trols. The explanations used have been simplified and are ncluded to develop a practical basis for understanding and discussion. * ELECTRICITY AND ATOMS Electricity Electricity behaves as though it wer
5、e a weightless, invisible fluid which freely flows through certain materials such as metals. Benja- min Franklin considered electricity to be such a fluid. He suggested that this substance covered tlie surface of all objects. Tlie electrical effects with which lie -as familiar, such as the attractio
6、n or repul- sion of charged objects, were explained ns the presence of more fluid upon one of the objects t.han upon tlie other. An over supply of this hypothetical fluid lie indicated by “plus,” and il deficiency by (minus.” Charles Du Fay suggested that electricity consisted of two fluids. Neutral
7、 objects were supposed to have equal amounts 1 - . Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-= 3535787 0279385 757 W of the different fluids. When objects exhibited electrical properties, the effects indicated that the object contained more of
8、one fluid than the other. If these speculations are considered within the context of modern atomic tlieory, some degree of correctness must be recog- nized in each hypothesis. Atoms All matter is composed of small particles called atoms. These parti- cles are the smallest entities of matter which re
9、tain the chemical cliaracteristics of the original material. Atoms consist of a relatively heavy and positively charged nucleus surrounded by a negatively charged cloud. The nucleus contains within its mass particles called neutrons which have no charge and particles called protons which carry unit
10、positive charges. The negatively charged cloud surround- ing the nucleus is composed of electrons, each of which carries a unit negative charge. All atoms in their normal state have as many posi- tive as negative charges and are therefore electrically neutral. Ions and Ionization The electrons surro
11、unding the nucleus have a large degree of mobil- ity, particularly. those electrons which occupy the outer limits of the atom. Protons lack individual mobility. and move only as the nucleus of the atom moves. The outermost electrons at times may leave the influence of the parent atom and migrate to
12、another atom. This may be caused by collision of atoms or simply by the close approach of one atom to another. If the effect. of a collision is great enough to cause a complete separation of the neggtive electron from the parent atom, the result is called ionization. The charged par- ticles resultin
13、g from the process are called ions. Static electrical effects are caused by the accumulation of t.hese charged particles mithin or on a body. Induction Like charges repel each other and unlike charges attract each other. This was one of the first properties of charged bodies observed by early invest
14、igators. Although electrons are highly mobile and can migrate individually, the proton fixed in the nucleus can only move by dragging along the entire atom, or in solids, the body which contains the atom. These charges accumulated upon a body can, by reasons of attraction or repulsion, induce a char
15、ge upon an adja- cent object. This electrostatic induced charge on an object is actu- ally the collection of similar charges on the surface of an object caused by the forces of attraction or repulsion transmitted t.hroiigh space from ni1 adjacent. charged object. Charge accumulation by 2 Provided by
16、 IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-3535789 0279386 693 m I induction is of major importance when considering the problems of static electricity. Potential When oppositely charged bodies are separated, work must be done against a force of attraction
17、. The expenditure of eiiergy repre- sented by this work appears as nii increase of the electrical tension between the charged bodies. This tension is called potential and is expressed in terms of volts. High static voltage does not produce the shock hazard as current. voltage does, because of tlie e
18、xtremely low nmperage. It is more practical when dealing with electricity to speak of the potential difference between bodies rather than the potential of the bodies with respect to zero potential. If, for exam- ple, tlie electrical effects of importance are between two bodies, A and 13, it is more
19、important to know the potential difference between A and R, rather tlim the potential of A 1vit.h respect to zero and tlie potential of R with respect, to zero. Potential difference is also expressed in volts. Conduction and Resistance Electricity, whether it is current flowing in a wire or an accum
20、u- lation of static chtwges, acts as though it were a fluid. If a charge of electricity is impressed on a body, it. tends to distribute itself or flow over the entire surface. The process of distributing or trans- mitting electricity over n surface is called conduction. Materials which readily distr
21、ibute elect.rica1 cliarges are called good conduc- tors. Metals in general are classified as good conductors. Non- metallic substances are classified as semiconductors or as noiicoiiduc- tors. Although such classifications have been made, there is no sharp dividing line separating conductors and non
22、conductors. The property of a substance which opposes tlie flow of electric cur- rent is called its resistance. The resistance of a substa.nce, meas- ured in ohms, is important when methods of static control are applied, particularly when grounding and bonding are considered. Static electricity accu
23、mulates on bodies which are electrically iso- lated by materials which are essentially nonconductors. The surround- ing nonconductive material of a charged body is called an insulator. This material can be a gas, liquid, or solid. The amount of charge which will accumulate upon a body depends upon t
24、he capacity of the body and tlie insulating properties of the surrounding medium. For exainple, tlyo charged bodies separated by air could build up a poteii- tia1 difference until this reaches the point of the “breakdown” volt- age of the air. This is the point at which the voltage becomes great eno
25、ugh to ionize the air so that it will form a conducting path between the two bodies and dissipate tlie charge. The major hazard of static ._ 3 -_ Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-3535789 0279387 52T electricity is t.he production of sp
26、ark discharge due to the brea.kdomn of the insulating.materia1 sepa-rating the two charges. Causes of Static Electricity Static electricity is produced by the contact and separation of ma- terials. When materials are in contact, electrons can move across the interface from one substance to another.
27、Separation of the materials prevents the immediate reestablishment of electrical neutrality and re- sults in the accumulation of a negative charge upon one substance, and on the other, an equal and opposite positive charge. The potential developed is the amount of work which must be done against the
28、 force of attraction opposing t.he charge separation. A surface which has a deficiency or excess of only one electron in 100,000 atoms is very strongly cha.rged. DETECTION OF STATIC ELECTRICITY The presence of static electricity is usually demonstrated by shock, sparks, or the attraction or repulsio
29、n of light particles to surfaces. Alt.hough the presence of static electricity can be detected by sensory perception or by the attraction or repulsion of light objects, its extent should generally be estimated by instruments. Such instruments as the electroscope, neon glow tubes, electrostatic volt
30、meters, and vacuum tube electrometers are available. Gold leaf Electroscope The simplest device for purposes of detecting and measuring static electricity is the gold leaf electroscope. This instrument, illustrated in fipire 1, has a thin gold leaf (or other metal) fastened to a metal rod. This rod
31、is contained in a box from which it is insulated by an amber plug. An observation window permits a view of the gold leaf imposed on a calibrated scale. The rod terminates outside the case in a nietal knob. If the hob is touched to a charged surface, the gold leaf is repelled from the post and moves
32、across the scale. This repulsion is caused by t.he presence of like charges on the leaf and on the post. The angle of separation of the gold leaf from the post. is roughly proportional to the relative potential, but the polarity of the charge must be determined by comparison mit.h a charge of known
33、polarity. This is a qualitative instrument at best, but is use- ful for locating static accumulations. 4 . PRBDLEM HARD COPY- I Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-3535789 0279188 466 -Metal Knob Charged body -A -Am ber Insulator Glass an
34、d metal case Calibrated Scale Gold leaf FIGURE 1.4old leaf electroscope. Neon Glow Tubes Glow tubes similar to those used to check automobile spark plugs may also be used to detect static electricity. These tubes are available in low enough wattages to actually detect static accumulations at po- ten
35、tials as low as 100 volts. This detector is useful to determine the presence of static electricity but does not indicate the amount of the accumulation. Electrostatic Voltmeters or Electrometers The so-called electrostatic voltmeters or electrometers are used gen- erally to measure very high voltage
36、s such as those produced in a Van de Graaff generator. Small, portable, accurately calibrated instru- ments are available to measure voltages in the range of 100 to 5,000 volts. These are not instruments which depend upon the passage of a current. They operate by measuring the movement between fixed
37、 and mobile vanes on which a voltage is impressed. The stationary set of vanes is highly insulated. When some part of the instrument or probe is held a short distance away from the surface to be tested, the needle of the dial is deflected. This needle is attached to the movable vanes which are sprin
38、g loaded. The deflection, due to repul- sion between the vanes, is proportional to the amount of charge on the surface touched by the probe. I I The Vacuum Tube Electrometer The vacuum tube electrometer lias within it a siinple radio tube which contains a heated filament as cz source of electrons ai
39、id a plate upon which cz strong positive charge is impressed. The circuit. is coin- pleted by tlie flow of electrons from the filainent to tlie plate. Be- tween the filament and the plate, a grid is imposed. The amount of current flowing from the filament to the plate will be modified by any charge
40、impressed upon the grid. If a positive charge is placed upoii the grid, it accelerates tlie electrons flowing from tlie filament to the 5 Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-W 3515789 0279189 3T2 W plate and thus increases the current ; i
41、f a negative charge is impressed upon the grid, it reduces the current by impeding the flow of electrons from the filament to the plate. The grid is connected to a probe which may be a tinsel bar or simply a bare wire. If the probe is held near a positive static charge, a posi- tive charge is induce
42、d on the grid. In the same manner, a negative charge may be induced. An indicating meter placed in the plats cir- cuit indicates changes in the current flowing in the circuit. This in- strument can be made to measure the amount of static charge and its polarity. Figure 2 illustrates the principle of
43、 the vacuum tube static detedor. I F Ground AW- A T-Vacuum tube M - Meter C-C Battery to adjust P- Plate Pr - Probe grid-plate circuit current F - Fi lameni R- Resistance to B - B Battery to impress a G - Grid control grid leakage charge on plate filament A-A Battery to heat FIGURE 2.-Vacuum tube de
44、tector. This instrument may be used either as a portable field instrument or as zl recording meter. When used as a recording meter, it will indicate the relative intensities of static accumulation over a period of time. A modification of this instrument is called a “feed mill.” These instruments are
45、 used to monitor charge production in an area and at a distance. They are used generally in research but rarely used as field instruments. Static alarms are available commercially for plant use in areas in which the accumulation of static charges may be very hazardous. These alarms consist of electr
46、ostatic voltmeters together with their detecting antennae connected electronically to visual or audible alarm systems. These instruments must bs used with care, and operators should be tmined in their use. Erroneous results may be obtained with a hand held instrument if the operator becomes charged.
47、 Readings with the probe should always be checked against readings made on adjacent grounded equipment. All readings should be taken under the same 6 Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-= 3515789 0279390 014 conditions with respect to the
48、 position of surrounding conductors. The measurements are usually given as kilovolts per unit distance. CONTROL OF STATIC ELECTRICITY The objective of most static control measures is to provide a path by which separated charges may recombine before sparking potentials are reached. Three common ways
49、are generally employed to ac- complish this purpose. They are : 1. Bonding and grounding. 2. Ionization of the surrounding media. 3. Humidification. Bonding and Grounding Static sparking does not occur between objects at the same potential. Bonding is the term used to indicate the equalization of potential be- tween two conductive bodies by connecting them together by means of a conducting wire. Objects bonded together have a zero potential difference wi
copyright@ 2008-2019 麦多课文库（www.mydoc123.com）网站版权所有
备案/许可证编号：苏ICP备17064731号-1