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(

)q0 = (

)(

)q0 Coulombs Law ( 4 )

Where r is the distance between the charges, K the proportionality constant and e0 = 8.85 x 10-12, 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. So this has accomplished what was intended, that was to separate and then

isolate some positive and negative charge. The final result is a Teflon rod that has a lot of excess

negative charge that can be used as a negative charge source making it possible to identify the polarity

of unknown charged sources.

Now let’s make a simple device that can be used to identify the polarity of unknown objects.

The simplest solution is to make a some kind of vertical stand with an extended arm that can hold a small piece of chargeable light material on the end of a reasonable length of insul ating tread. The stand shown in Fig-5 is one possible solution. Here the stand and extended arm are made of metal for sturdiness and weight and the chargeable ball made of Pith (a dried out spongy plant stem material) suspended by a silk thread.

Fig-5

Initially, the pith ball is uncharged and hanging vertically straight down from the extended arm. The

negatively charged teflon rod is then moved towards and touched up against the pith ball. The pith ball

well generally just hang there unaffected by the negatively charged rod since the pith ball has no net

charge on it and is said to be neutral. When the two touch, some negative charge is then transferred to

the pith material and the pith ball is quickly repelled away from the rod. This is depicted in Fig-5, where

the position of the pith ball is shown in a before and after position. The final result is shown in Fi

Fig-6

where the pith ball now wants nothing to do with the negatively charged rod and will be repelled away

from the rod no matter what direction the rod is moved towards the pith ball. The result now is the pith

ball becomes a polarity and general voltage intensity detector.

The polarity and general voltage intensity of any object brought near the pith ball can be determined by

watching the action of the pith ball. An example of what this might look like is shown in Fig-7 where a

positively charged sphere is brought near the negatively charged pith ball. Here the thread and pith ball

are lined up with the center of mass of the sphere indicating a strong electric field and the sphere has

positive charge on it because unlike charges attract. If the electric field on the sphere was not so intense

the thread would not be stretched out to a straight line but curved and the pith ball lower in height.

It should be noted that the pith balls used in this article are all coated with an aluminum spray paint to

make the surface a conductor. This means the charge will evenly distribute over the entire surface. This

is important because if the surface was non-conducting (insulator) the charge would stay on the area of

the surface it was deposited and not move around. If this was the case the pith ball would rotate around

depending on its charge and the charge on the object brought near it.

Fig-7

Another example of this is shown in Fig-8 where two aluminum coated pith balls are suspended from a

single point. Here the two the pith balls are being forced apart which implies they both have the same

polarity charge on them because like charges repel. What is not known is the polarity. To determine the

polarity a charged Teflon rod could be brought near them. If the balls are attracted to the rod, the

charge on the two balls would be positive since the Teflon is always negative. If the two balls are

repelled away, they are both negative.

Fig-8

The next example is a little more challenging but still involves basic electrostatics principles. The setup in Fig-9 shows a ping pong ball mounted on a tooth pick that can easily spin around in the wood /plastic holder. The tooth pick has sharpened points at both ends so it can spin with little effort in its mounting. There are two electrodes, one mounted on each side of the ping pong ball as shown, one is positive and the other negative. The red line was put on the ball in order to detect if the ball rotates when the high voltage is applied to the two electrodes. One is labeled plus(+) and the other minus(-). What happens when the high voltage (75KV) is turned on ? The answer is, the ball rotates at a very high speed. If you were observing it the ping pong ball would seem just yellow and the red line would no longer be visible because it was spinning so fast.

Fig-9 Fig-10

Fig-11 Fig-12

To help explain this I put little plus(+) and minus(-) signs in the ping pong ball to show what happens as

seen in Fig-10, Fig-11 and Fig-12. The ball is then slowly rotated to show what is happening.

Starting with Fig-9, the ball is stationary when the high voltage is turned on. After a few seconds a hiss or crackling sound is heard and then the ball starts to spin. The direction of spin could be either direction, which is random when it starts up. Wonderful, but why does it spin ? Let’s analyze what is happening here. When the high voltage is turned on it takes a little time to build up to about 75,000 volts. When the voltage is high enough, charge will jump from the brass electrode to the surface of the ping pong ball right opposite the electrode on both sides of the ball. That means positive charge will be deposited on the left side of the ping pong ball near the electrode and negative charge deposited on the right side. As this is happening, the ball starts to spin as noted in fig-10, Fig-11 and Fig-12. In each frame the ball is seen rotating a little more in each figure. To answer why it spins is simple because like charges repel and unlike charges attract. A more detailed explanation would be as follows. The positive charge that leaps from the positive

electrode to the surface of the ping pong ball is positive. Since that positive charge now exists on a

insulating surface(plastic) of the ball it can’t move around and stays right there. However it is right near

the positive electrode and since like charges repel the surface of the ball starts to rotate away from the

positive electrode. A similar thing happens at the negative electrode. As this positive charge area moves

away, it is moving towards the negative electrode which adds more speed to its motion since unlike

charges attract. So in summary, the positive electrode pushes the positive charge area on the ball away

while the negative electrode attracts the positive area to it. A similar thing happens for the negati ve

charged area and the ball spins with is rotational speed essentially limited by friction at the pivot points.

Another example is shown in Fig-13, where a plume of plastic ribbons is attached to the dome of a small Van De Graaff generator. The head voltage on this generator is about a negative 400,000 volts. The ribbons that hit the dome head get negative charge transferred to them causing then to get repelled from the dome and each other. These are the ones standing up and away from the dome. These ribbons are moving around with the air movement in the room and bounce up and down and occasionally hit the dome head and again bounce back up. Dirt, dust and other contamination all play a part in the somewhat random behavior of the ribbons of this plume by leaking down the charge on each ribbon.

FIG-13

The table model Van De Graaff electrostatic generator is one of the more popular types of generators used and depending on the size, can generate upwards of 500,000 volts. In Figure 14 an electrostatic view of this generator is shown. This is an excellent example of the triboelectric effect in action. The generator is started by a motor driven bottom felt roller that drives a rubber belt which wraps around a top plastic roller in the head of the unit. Note the triboelectric effect happens at the bottom and top rollers because the rubber belt is being rolled on and then off of each roller. This results in the same action as the plastic tape example back in the beginning of these notes. A small sharp pointed wire comb at the bottom brings electrons to the felt roller from earth ground. The top comb will then pull those electrons off the belt and distribute them on to dome on top of the unit. This process very quickly creates enough negative charge in the dome that could typically cause a dome to base discharge if the dome is not discharged to an adjoining device. This charge and discharge continues until the motor is turned off or the dome acquires dirt, hairs or other contaminates that cause it to leak off dome electrons greatly reducing its electrostatic potential. If the unit fails to generate any appreciable electrostatic voltage with the motor running the unit will then have to be completely cleaned removing all contaminates using alcohol.

Van De Graaff Electrostatic Generator

Fig-14

In our final demonstration a lower voltage generator will need to be used to show its operation. The Van

De Graaff makes a spectacular show with its large sparks and high voltage but is not practical when it

comes to running a lot of electrostatic demos. This is because its voltage is just too high causing

discharges in places not desired making the demo inoperative.

The generator of choice for a lot of these typical demos is the Wimshurst generator since the output

voltage is lower and more available charge which is stored in two Leyden jars. The Wimshurst generator

operates by the method of induction to separate and build up and store charges. The process of

induction can be shown by the charging of an isolated sphere that is initially electrically neutral. The

Fig-15 FIG-16 Fig-17 method to do this is demonstrated in figures 15, 16 and 17.

In figure-15 a charged Teflon rod is brought very close to the neutral metal sphere. The electric field from the rod causes the free electrons to flow to the opposite end of the sphere since like charges repel. So by induction(the rod never touches the sphere), one side of the sphere is positive and the other side negative. The diagram in figure-16 shows a ground wire momentarily touching the sphere allowing the electrons to be repelled(pushed) right off the sphere. Then in figure-17 the ground wire and the Teflon rod are taken away leaving the positive charges to evenly redistribute over the surface of the sphere. The final result is a metal sphere with a net positive charge that was obtained by the method of induction. What is important to note here is that the charged Teflon rod never touched the sphere. The induction process just described is the method which the Wimshurst generator uses to generate and then separate charges. Once the charges are separated they are then stored in two Leyden jars (capacitors) which are both connected to the output terminals. Typically the basic operation of this generator starts with some charge isolated on one of two closely placed plastic discs. These two discs are mounted on a stand with a rod running through their centers and spaced about a quarter inch apart. A pulley wheel is attached to the outside of each wheel with a opposite set of pulley wheels mounted below the discs. On one side an O-ring is directly connected to the pulleys of one side and another O-ring is twisted to look like a figure eight and the connected to the two pulleys. The bottom two pulleys are mounted on a single rod and are either turned by a hand crank or an electric motor. When the bottom rod starts rotating, one of the discs starts rotating clockwise and the other counter-clockwise. Many metal foil strips are glued to the outside surface of both discs. These are used to collect the generated charges in that area of the mounted foil strips so it distributes the charges evenly. There is also one each side a neutralizing rod with brushes at each end which distributes the correct charges to the foil strips as they rotate. Finally, on the left and right sides of both discs are brushes that are connected to the Leyden jars and output terminals that collect these charges from the foil strips of the oppositely spinning discs. Diagrammatically, this process is shown in figure -18.

Fig-18

In figure-19 is a Wimshurst generator in operation with a set of discharge ball electrodes connected to its output terminals. When the electrodes are moved closer together a discharge as shown will result.

Fig-19 The final demonstration that emphasizes basic electrostatic principles is shown in figures 20 and 21.

Fig-20 Fig-21 In this setup there are six brass plates attached to the inside of a plastic cylindrical section. Note, every other plate is connected together yielding two isolated sets of three plates. One set is connected to the positive electrode of the Wimshurst and the other to the negative electrode. When the generator is turned on each adjacent plate will have an opposite charge. Next a metal conducting surface ping pong ball is tossed into the cylinder to start it rolling around the cylinder in one direction. After a few moments it is noticed that the ball continues to rotate around the cylinder at a constant speed and not slowing down as shown in Figures 20 and 21. The operation on this demonstration is dependent on the principles of unlike charges attract and lie charges repel. When the moving conducting surface ball touches one plate it immediately gains the charge that is on that plate. Remembering, like charges are repelled, the ball is pushed way from that plate. Since the ball was initially given a circular motion around the cylinder the ball will be pushed away from the initial plate in a circular motion. When it leaves that plate it will be immediately attracted to the next adjacent plate since that plate has an opposite charge on it. Knowing unlike charges attract the ball is pulled towards that plate and when it contacts that plate this opposite charge is now transferred

to the ball. This starts the whole process all over again keeping the ball moving around until the generator is turned off.