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Electrostatics : J.P.O'rourke Electrostatics : J.P.O’Rourke How Electrostatic Voltages can vary with Humidity : Below is a listing of various ways electrostatic potentials can be generated in our everyday environment that can damage electronic equipment. Electrostatic voltages generated 10-20% relative 65-90% Relative By everyday materials Humidity Humidity 1. Walking across a carpet* 35,000 1,500 2. Walking over a vinyl floor. 12,000 250 3. Worker at a desk/bench. 6,000 100 4. Vinyl paper holders. 7,000 600 5. Plastic Bags* 20,000 1,200 6. Chairs with plastic based covering* 18,000 1,500 * Assumes no electrostatic reduction type material. Further investigation of this phenomena shows that various materials can have either a positive, negative or neutral charge. The table below shows this relative variation assuming each item is clean and dry with no external contamination present. The Greek philosopher Thales was considered the first to discover electrostatic effects when Amber attracted very small objects around 600BC. This table is very useful since it tells us that our hands are somewhat depleted of electrons (positive) and on the opposite end of the scale (bottom of table) where Teflon is located, can have an affinity for an abundance of electrons (negative). The charge on the material in the middle of the table can be considered to be relatively neutral. When movement occurs between two different types of materials especially if they are dissimilar, it will be found that one of the bodies will give up electrons more readily than the other. This results in electrons being displaced from one material and transferred to the other. The material that gave up its electrons will have a net positive charge and the material that received the electrons will have a net negative charge. This transfer of electrons happens very rapidly and then diminishes as the surface energies reach equilibrium. When charges are generated in this manner it is called the Triboelectric Effect . This effect can generate voltages anywhere from 100 to 35,000 volts as shown in an earlier table. The magnitude of this voltage depends on how quickly the materials are separated, the surface characteristics, type of material, humidity and the geometry of the surface. The kind of movement that generally is considered to cause triboelectric charging is the rubbing together of two types of materials. The process of rubbing two bodies repeatedly together, assuring very close contact, will create charge transfer when separated. This will result in one material with a net positive charge and the other with a net negative charge. However, this rubbing is not actually necessary for triboelectric charging to occur between materials. Just the process of separating two materials that have been in contact with each other for a period of time can generate a considerable electrostatic potential. This can be demonstrated by unrolling a piece of plastic tape see photos below. In the above photos the pith ball has negative charge on it which it received from a Teflon rod. In both cases a strip of tape was unrolled from the tape roll. In the left photo the pith ball is attracted to the roll part of the tape implying the roll has an abundance of positive charge on it. The right photo shows the pith ball being repelled away from the tape part that was unrolled off the roll implying that it is negative. Here, remember that lie charges repel and unlike charges attract. So the process of just pulling a piece of tape off a roll causes charge separation and electrostatic potentials to develop. Also since the unrolled tape piece is negative and the roll it came off of is positive, when the tape part is let go it will be attracted to the roll it came off of (unlike charges attract) and stick back on the roll. Unfortunately this process happens a lot with everyday materials and can cause unwanted electrostatic potentials to develop where they are not wanted causing damage to sensitive electronic devices. In an effort to better understand these electrostatic charges and their interactions with each other, let’s first take an isolated negative charge as shown Fig-1. The diagram here is shown with electric field lines all pointing in towards the negative charge. In other words the electric field lines terminate perpendicular to the surface of the negative charge in three dimensions. The electric field line concept is used to attempt to explain the action at a distance force. In other words all charges and charged objects create an electric field that extends out into the space that surrounds the charge. The presents of this charge actually alters this space, causing other charged objects that enter this space to be affected by this electric field. The strength of the field is dependent on how much charge is on the object that is creating the electric field and the distance of separation from the charged object. Negative Charge Fig-1 The electric field lines can be represented by a vector E and the force on a test charge q0 as F, so the relationship between the two can be represented by the following equation as the force exerted on the test charge, F = (q0)(E) where bold letters represent vectors ( 1 ) The electric field would then be defined as, E = ( 2 ) What this says is that the force is along the field lines in a direction that a positive test charge would go. In a diagram generally the density of the filed lines is an indicator of the intensity of the electric field, more field lines the higher the intensity. In principle, the electric field E can be defined by placing a positive test charge q0 at some point near a charged object. Then measure the force F that acts on this test charge q0 . The resultant electric field at the test may then be calculated by using equation-2. The units are Newton’s/Coulomb or more commonly called Volts/Meter. Where voltage or difference of potential is defined by, dV = Vf - Vi = - ( 3 ) This is the potential difference in volts between the two points f and i in an electric field. Interaction between charges : In summary, it should be noted that two charges are always involved in either measuring the electric field or the electric force. I other words you cannot have a force on a single isolated(means no other charge anywhere in the universe). A force(s) can only exist between two or more charges. The force between two charges q and q0 is defined by Coulombs Law, expressed in equation form as, F = K( )q = ( )( )q Coulombs Law ( 4 ) 0 0 -12 Where r is the distance between the charges, K the proportionality constant and e0 = 8.85 x 10 , the permittivity. Now that it has been established that at least two charges need to be present to have an electrostatic force, let’s look at the interaction that exists between two charges. Lets first look at the electric field distribution that exists between to negative charges shown in Fig-2. Note how the electric field lines in Fig-2 the central area between the two negative charges are compressed, bent away from each other and are actually running parallel to each other along an imaginary center line half way between the charges. This compression of field lines indicates the two charges are opposing each other, being forced apart or repelled. A diagram of two positive charges would look essentially the same the only exception would be that the electric field lines would be pointing away, emanating from each charge. Here again the force of repulsion would exist between the two like charges. Finally, let’s look at the only other interaction not consider so far and that is between a positive and negative charge. Here a much different electric field pattern results. In Fig-3 the electric field lines that emanate from the positive charge, terminate on the negative charge. The lines show a continuous flow from the positive to negative charge creating an attractive force between the two charges. Unlike Charges Attract Fig-3 So in summary it may be simply stated that like charges repel and unlike charges attract ! The generation of a specific charge by charge separation : This section will address the question on how one determines what the polarity or sign of the charge is on an unknown object. This is a good question and fortunately can be answered by referencing the triboelectric table mentioned earlier. In this table it is noted that the materials near the top of the table tend to have an affinity to give up negative charges (electrons) and the material at the bottom of the table have an affinity to accept negative charges (electrons). This relationship can be used to our advantage since the materials at the top of the table would tend to have a net positive charge on them and those at the bottom a net negative charge. So with this in mind, if a piece of rabbit’s fur from the top of the table is rubbed against a piece of Teflon rod at the bottom, a decent charge separation should result as shown in Fig-4. Fig-4 Here the contact or rubbing action is necessary to assure better contact between the fur and the Teflon resulting in good charge transfer between the two materials. The fur easily gives up some of its electrons and the Teflon will easily accept them. Separating the two materials, results in the Teflon having an excess of electrons (negative charge) and the fur a depletion of electrons resulting in the fur being positively charged.
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