Static Electricity

WHAT IS STATIC ELECTRICITY ?

Everything we see is made up of tiny little parts called atoms. So what are atoms made of? In the middle of each atom is a "nucleus." The nucleus contains two kinds of tiny particles, called protons and neutrons. Orbiting around the nucleus are even smaller particles called electrons. The 115 kinds of atoms are different from each other because they have different numbers of protons, neutrons and electrons.

Protons, electrons and neutrons. These are very different from each other in many ways. One way they are different is their "charge." Protons have a positive (+) charge. Electrons have a negative (-) charge. Neutrons have no charge.

Usually, atoms have the same number of electrons and protons. Then the atom has no charge, it is "neutral." But if you rub things together, electrons can move from one atom to another. Some atoms get extra electrons. They have a negative charge. Other atoms lose electrons. They have a positive charge. When charges are separated like this, it is called static electricity.

The nucleus is large compared to the electrons. The atom is mostly empty space. And the electrons are very far away from the nucleus. While this model is not completely accurate, we can use it to help us understand static electricity.

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ELECTRONS CAN MOVE

The protons and neutrons in the nucleus are held together very tightly. Normally the nucleus does not change. But some of the outer electrons are held very loosely. They can move from one atom to another. An atom that looses electrons has more positive charges (protons) than negative charges (electrons). It is positively charged. An atom that gains electrons has more negative than positive particles. It has a negative charge. A charged atom is called an "ion."

Some materials hold their electrons very tightly. Electrons do not move through them very well. These things are called insulators. Plastic, cloth, glass and dry air are good insulators. Other materials have some loosely held electrons, which move through them very easily. These are called conductors. Most metals are good conductors.

Conductors and Insulators

 •Conductors are materials in which the electrons can move rather freely (i.e. they readily conduct a flow of electrons).

•Examples of conductors are metals such as copper, silver, Aluminum plus salt water solutions (the human body falls into this category).

•Non-conductors or Insulators are materials in which the electrons are more tightly bound to the atoms and generally are not free to move.

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•Examples of insulators are wood, plastic, stone; in short, any non-metal.

•The earth acts as a large conductor and has a very large capacity to absorb charge concentrations from smaller conductors.

•So any charge on a conductor will be lost if there is a path to ground.

•Substances that fall between the metals and the insulators are called semiconductors.

•Semiconductors such as Silicon and Germanium are widely used in modern electronics since their properties may be radically altered by the addition of small amounts of impurity atoms.

•Superconductors are perfect conductors in the sense that they offer no resistance to the flow of charges.

How can we move electrons from one place to another?

One very common way is to rub two objects together. If they are made of different materials, and are both insulators, electrons may be transferred (or moved) from one to the other. The more rubbing, the more electrons move, and the larger the charges built up. (Scientists believe that it is not the rubbing or friction that causes electrons to move. It is simply the contact between two different materials. Rubbing just increases the contact area between them.)

Static electricity is the imbalance of positive and negative charges.

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If two things have different charges, they attract, or pull towards each other. If two things have the same charge, they repel, or push away from each other.

Unlike charges attract

When a positive charged particle (+) like a proton is near a negative charged particle (-) like an electron, the electrical field goes from one to the other.

Like charges repel

When particles have the same charge, they repel each other.

Like charges push away from each other

A charged object will also attract something that is neutral. Think about how you can make a balloon stick to the wall. If you charge a balloon by rubbing it on your hair,

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it picks up extra electrons and has a negative charge. Holding it near a neutral object will make the charges in that object move. If it is a conductor, many electrons move easily to the other side, as far from the balloon as possible. If it is an insulator, the electrons in the atoms and molecules can only move very slightly to one side, away from the balloon. In either case, there are more positive charges closer to the negative balloon. Opposites attract. The balloon sticks. (At least until the electrons on the balloon slowly leak off.) It works the same way for neutral and positively charged objects.

So what does all this have to do with shocks? Or hair full of static? When you take off your wool hat, it rubs against your hair. Electrons move from your hair to the hat. Now each of the hairs has the same positive charge.

Remember, things with the same charge repel each other. So the hairs try to get as far from each other as possible. The farthest they can get is by standing up and away from the others. Bad hair day!

As you walk across a carpet, electrons move from the rug to you. Now you have extra electrons. Touch a door knob and ZAP! The door knob is a conductor. The electrons move from you to the knob. You get a shock.

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We usually only notice static electricity in the winter when the air is very dry. During the summer, the air is more humid. The water in the air helps electrons move off you more quickly, so you can not build up as big a charge.

TRIBOELECTRIC SERIES

Scientists have ranked materials in order of their ability to hold or give up electrons. This ranking is called the triboelectric series.

Dry human skin, rabbit fur and perspex have the greatest tendency to give up electrons when rubbed on something and become positively ( + ) charged. Teflon and polythene have the greatest tendency to become negatively charged ( - ) when rubbed. If you want to create static electricity, rubbing fur on Teflon should give the best results.Become positive in charge

The following materials will tend to give up electrons when brought in contact with other materials. They are listed from those with the greatest tendency to give electrons to those that barely give up electrons.

Dry human skin Greatest tendency to giving up electrons and becoming highly positive (+) in charge

Perspex or acetate  

Rabbit fur Fur is often used to create static electricity

Glass The glass on your TV screen gets charged and collects dust

Human hair "Flyaway hair" is a good example of having a moderate positive (+) charge

Nylon & wool  

Lead A surprise that lead would collect as much static electricity as cat fur

Silk  

Aluminum Gives up some electrons

Paper  

Neutral

There are very few materials that do not tend to readily attract or give up electrons when brought in contact or rubbed with other materials.

Cotton Best for non-static clothes

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Steel Not useful for static electricity

Become negative in charge

Amber  

Hard rubber Some combs are made of hard rubber

Nickel, Copper Copper brushes used in Wimshurst electrostatic generator

Brass, Silver  

Gold, Platinum It is surprising that these metals attract electrons almost as much as polyester

Polyester Clothes have static cling

Styrene (Styrofoam) Packing material seems to stick to everything

Cling film You can see how cling film will stick to things

Polyethylene (like Scotch Tape)

Pull Scotch Tape off surface and it will become charged

Polythene (PVC) Many electrons will collect on PVC surface

Silicon  

Teflon Greatest tendency of gathering electrons on its surface and becoming highly negative (-) in charge

The following materials will tend to attract electrons when brought in contact with other materials. They are listed from those with the least tendency to attract electrons to those that readily attract

Best Combinations

The best combinations of materials to create static electricity would be one from the positive charge list and one from the negative charge list.

Skin and polyester clothes

A common complaint people have in the winter is that they shoot sparks when touching objects. This is typically caused because they have dry skin, which can become highly positive (+) in charge, especially when the clothes they wear are made of polyester material, which can become negative (-) in charge.

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People that build up static charges due to dry skin are advised to wear all-cotton clothes, which is neutral. Also, moist skin reduces the collection of charges.

Combing your hair

Human hair becomes positive (+) in charge when combed. A hard rubber or plastic comb will collect negative (-) charges on its surface. Since similar charges repel, the hair strands will push away from each other, especially if the hair is very dry. This is called "fly-away" hair. Since the comb is negatively charged, it will attract object with a positive charge--like hair. It will also even attract material with no charge--like small pieces of paper.

Fur and polythene strip

Rubbing a polythene rod with rabbit fur or wool will give the rod a negative charge. Although the rod can be used to pick up scraps of paper, the fur and wool quickly lose their charge.

Cling Film

Unrolling a piece of Cling Film or similar plastic wrap creates negative charges on the sheet. It will tend to stick to neutral items.

In conclusion

Various materials have a tendency of either giving up electrons and becoming positive (+) in charge or attracting electrons and becoming negative (-) in charge.

CONSERVATION OF CHARGE

When we charge something with static electricity, no electrons are made or destroyed. No new protons appear

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or disappear. Electrons are just moved from one place to another. The net, or total, electric charge stays the same. This is called the principle of conservation of charge.

COULOMB'S LAW

Charged objects create an invisible electric force field around themselves. The strength of this field depends on many things, including the amount of charge, distance involved, and shape of the objects. This can become very complicated.

Coulomb's Law

 (1) Charges may be either positive (like protons) or negative (like electrons).

(2) Like charges repel; unlike charges attract.

Charge is Quantized

•In the early part of the 20th century Robert Millikan performed an experiment to determine the smallest possible charge in nature.This basic charge is 1.6 x 10-19 Coulombs.

•This was later found to be the charge on every proton and electron (negative for electrons).

•Every experiment since then has observed the basic electron charge or some integral multiple of it.

•Current subnuclear physics suggests that there may be a smaller basic charge on particles called quarks.

Electric Charge

 •Electric charge is an intrinsic property of the fundamental particles that make up matter.

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•The atoms that make up most matter are electrically neutral. This is because they normally have equal numbers of positively charged protons in the nucleus and negatively charged electrons surrounding the nucleus.

•When there is a charge imbalance (i.e. an excess of positive or negative charges), an object is said to contain a net charge. Only the net charge matters. The effect of a large number of positive charges and a large (but equal) number of negative charges is small because of cancellation on the atomic level.

•The rules for electric charges are simple. Like charges repel; unlike charges attract.

•Note that positive charges are on the protons that are bound in the atoms and are not mobile. Electrons are responsible for many of the properties of electromagnetism since they may be made free to move.

Charges on surface

Note that the charged atoms are on the surface of the material. Static electricity is different than regular electricity that flows through metal wires. Most of the time the materials involved in static electricity are nonconductors of electricity.

If electrical charges build up on the outside of a metal, most of them will dissipate into the metal, similar to an electrical current.

Prefers dry air

When the air is humid, water molecules can collect on the surface of various materials. This can prevent the buildup of electrical charges. The reason has to do with the shape of the water molecule and its own electrical forces.

Thus, static electricity is formed much better when the air is dry or the humidity is low.

Picking up tissue with a comb

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Strong forces hold electrons

The reason the electrons can't leave their present material is because of strong molecular forces that keep them where they are. If there are enough positive (+) charges attracting them, and the distance is not too great, some of the electrons will break loose and fly across the gap to the (+) side.

Once a few electrons start to move across the gap, they heat up the air, such that more and more will jump across the gap. This heats the air even more. It all happens very fast, and the air gets so hot that it glows for a short time. That is a spark.

The same thing happens with lightning, except on a much larger scale, with higher voltages and current.

Ben Franklin

Ben Franklin proved that lightning was static electricity by flying a kite in a storm and detecting static electricity by seeing the hairs on the kite string stand on end and creating a spark with a metal key. This dangerous experiment showed that static electricity was being formed in the clouds by the rain.

In conclusion

Rubbing certain materials together can cause the buildup of electrical charges on the surfaces. Opposite charges attract and same charges repel. Either charge will be attracted to something of neutral charge. Sparks are an extreme case of electrons being attracted to an object that has a positive charge.

Stealing electrons

According to Solar System Model (or Bohr Model) of the atom, electrons are in orbits or shells around the nucleus. A maximum number of electrons are allowed in each orbit. Forces in each atom seek to reach that maximum number, such that if an element is just one electron short of the maximum amount in its outer orbit, it would try to "steal"

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an electron from another element that may be just starting its outer orbit. This is the basis of chemical reactions.

Adhesive force takes electrons

That force will also tend to hold two different materials together. In that situation, the force is called the adhesive molecular force. When different materials are pressed together and then pulled apart, the adhesive molecular force pulls electrons from material unto the other. This creates the static electricity.

You can see this effect with a piece of Scotch tape or similar tape. First verify that it is not attracted to your finger. Then stick it to some surface and then pull it off. Put you finger near the tape and it will now be attracted to your finger, showing that there is an excess of charges on the tape.

Uses for Static Electricity

(Although static electricity can be a nuisance--like getting shock when you tough a doorknob or static cling on your clothes--it has a number of beneficial uses. The forces of attraction caused by static electricty are used in air pollution control, automobile painting, and xerography or copy machines.

Pollution control

By charging dirt particles in the air with a static charge, they can be made to be collected, thus acting as an effective air pollution control. They are often called electrostatic precipitators.

Smokestacks

Factories use static electricity to reduce pollution coming from their smokestacks. They give the smoke an electric charge. When it passes by an electrode of the opposite charge, most of the smoke particles cling to the electrode.

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This keeps the pollution from going out into the atmosphere.

Air fresheners

Some people purchase what are called air ionizers to freshen and purify the air in their homes. They work on a similar principle as the smokestack pollution control. These devices strip electrons from smoke molecules, dust particles, and pollen in the air, just as what happens in creating static electricity.

These charged dust and smoke particles are then attracted to and stick to a plate on the device with the opposite charge. After a while, much of the pollution is drawn from the air.

Since charged particles will also stick to neutral surfaces, some of them can stick to the wall near the ionizer, making it very dirty and difficult to clean.

Xerography

Your photocopier or Xerox machine uses static electricity to copy print to a page. This is done through the science of xerography.

One version gives ink an electrical charge so that it will stick to the paper in the designated areas. Another version of a photocopier uses charges to stick the ink to a drum, which then transfers it to the paper.

Painting cars

Some automobile manufacturers use static electricity to help them paint the cars they make. The way this works is first they prepare the car's surface and put it in a paint booth, next they give the paint an electrical charge, and then they spray a fine mist of paint into the booth.

The charged paint particles are attracted to the car and stick to the body, just like a charged balloon sticks to a wall. Once the paint dries, it sticks much better to the car and is smoother because it is evenly distributed.

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In conclusion

Uses of static electricity include pollution control, Xerox machines, and painting. Can you think of any other uses for static electricity? Think of its properties of attraction, repulsion and causing sparks.

Static Electric Sparks and Lightning

A spark is a stream of electrons jumping across an air gap, heating the air until it glows and expands. Certain conditions can cause enough static electricity buildup to cause a spark or lightning. A spark often requires both a conductor and non-conductor. Lightning is an extreme example of a spark.

Conditions for sparks

Sparks do not happen easily. They are violent occurrences that require special conditions. They need both non-conductors and conductors to occur. The way this happens can get complex. These conditions include walking on a carpet on a very dry day or the rapid movement of tiny water particles in a summer storm.

In the clouds

Normally water inhibits static electricity, but in the case of thunderstorms, there is so much movement of air and water droplets within the clouds that charges collect on the surface of the droplets. Enormous amount of charges can collect in the clouds, some positive ( + ) and some negative ( - ).

Sparks require conductors

You know that static electricity collects on the surface of non-conductors. But you seldom—if ever—see a spark fly from one non-conductor to another. The reason is that sparks need conductors, so that the electrons can freely move about and gather enough charges together to be able to jump from one material to another.

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If you took a charged piece of plastic and put it next to some metal, there would be no spark. The charges are held on the surface of the plastic, so that they won't jump the air gap

Another good example of this concerns how you can get shocked with a spark. You are a conductor of electricity—although not as good as a piece of metal. The reason you conduct electricity is because the of salt in your blood and your cells. Now if you notice, you usually see sparks when you start to touch something metal—like a doorknob—or another person or animal.

So static electricity is formed and gathered on the surface of a non-conductor, but it must be then transferred to a conductor to cause a spark.

Charges move in conductor

When a conductor—like a metal rod—is brought near a charged non-conductor, the free electrons in the conductor will move to one end or the other of the rod, depending on whether the non-conductor surface is positive or negative.

Opposite charges in conductor move toward non-conductor

When the conductor is brought into contact with the non-conductor, the electrical charges on the surface of the non-conductor are "sucked" into the conductor. In other words, if negative charges are on the surface of the non-conductor, these electrons will move into the conductor. If positively charged atoms are on the surface, electrons from the metal or conductor will neutralize those atoms, resulting in an excess of positive charges in the conductor.

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Now, if another conductor is brought near the first conductor, the same thing will happen. Since electrons can move so freely in a conductor, many may collect near the surface and actually jump across the air gap as a spark.

Anatomy of a spark

Air is a non-conductor of electricity and resists the movement of electrons through it. When the attraction or electrical pressure is great enough between objects with positive ( + ) and negative ( - ) charges--or even between a charged object and a neutral one--some electrons are able to overcome the resistance and jump the air gap. This electrical pressure is also called potential difference or voltage difference.

Heats up the air

Since air is a non-conductor of electricity, it does not readily let electrons pass through it. But if the attraction is great enough, some electrons will leave their material and fly to the other object. While they move through the air, may smash into and bounce off molecules or atoms that are in their way. This heats up the air.

Lower resistance

Now, the hotter the air is, the less resistance it gives the electrons. So as the air gets heated, more and more electrons start jumping over to the other side. This only heats the air even more, until it actually gets white-hot. That is the spark or bolt of lightning that you see and feel.

Electrons stop jumping

Once enough electrons have made the jump, the attraction is reduced and the flow stops. The spark quickly cools down and the air stops glowing. It is all over in a fraction of a second. Since this happens for such a short time, so you may only feel a slight discomfort from the heat of the spark. But if the spark is a bolt of lightning, it can cause an enormous amount of damage.

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Lightning is a real big spark

Lightning works the same way as a little spark, except that it happens on a massive scale. Some lightning bolts are several miles long. Compare that to the tiny 1/4 inch or 1 centimeter length of the spark that comes off your finger.

Lightning is created when water drops are churning around in a thunder cloud. They gather either positive or negative electrical charges, so that soon one cloud may be positive and another cloud may be negative. Or perhaps some object on the earth may have an excess of opposite charge.

Has high electrical pressure

The electrical pressure builds up, the same way as it does for a spark. Since the distances are so much greater between the clouds, the electrical pressure must be extremely high for lightning to start. But once it does and a lightning bolt jumps from one cloud to another, it is a tremendous spectacle.

Most go from cloud to cloud

Most lightning bolts are from cloud to cloud, but sometimes there are no positive charged clouds nearby, so the negative cloud, sends its electrons to the ground or any object that may have a slight positive charge.

Thunder

Air expands when it is heated and contract when it cools. Since the spark happens so fast, the air expands and contracts very rapidly. When it contracts, the air slaps together, just like when you clap your hands or pop a balloon. The noise you hear from a spark is just a snap, because it is so small.

On the other hand, the noise of thunder is a tremendous crash, because the size of lightning is so large. The snap

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of a spark and the crash of thunder are caused by the same effect. The only difference is in the size of the spark.

In conclusion

Static electricity is caused when friction causes electrical charges to build up on a surface of a non-conduction material. Its explanation comes from the Atomic Theory of Matter. Static electricity can cause sparks and other problems, but it also is useful in pollution control.

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