Static Electricity

We know that every matter consists of protons and electrons. Do you know that there are more than 1028 protons and almost equal number of electrons in your body? The basic property of these protons and electrons is charge. The charge of proton is considered as positive and of electron is negative. The charged objects that apply force on each other are known as electric force. The electric force is a basic and important force of nature. In this chapter we will see how a body is charged. We will also learn detect the presence of charge and measure the force between them. As the discussed charges will remain static we will call the chapter Static Electricity. Finally we will discuss the usage of this static charge and some of its danger and the ways to remain safe from them.

Static Charge

In a winter morning John took his plastic comb to comb his hair. Before combing his hair he rubbed the comb with his woolen pullover for a while. While combing his hair he observed surprisingly that he cannot do it because all the hair was becoming straight and repels away each other. As soon as John brings the comb close to the tale he observes the comb attracting some pieces of paper on the table. Some of you may have the same kind of experience like John. In our daily life we observe that many objects around us behave like the comb of John.

Do it yourself: Rub your dry hair with your plastic scale for a while and hold it near some pieces of paper.

We see that an object attracts another object in a special condition or becomes charged that is electricity produced in the object. Their charge remains static in the place of its production. So it is called static electricity. Now let us see what we mean by being charged.

We know that each matter is consisted of small particles. The atom of every matter consists of electrons and which revolves around the nucleus. In the nucleus there are two type of particles-proton and neutron. The fundamental and special property of primary particles (electron and proton) of which the matter consist of is electrical property, which is determined by charge. The charges of electrons are negative and the charges of protons are considered to be positive. Neutron is electrically neutral that is it has no charge. The amount of charge in a proton is equal to the charge of an electron. Naturally an atom has an equal number of protons and electrons. As a result in the atom whole atom there is no electrical property to be detected. The number of electrons and protons in different matters is different.

As long as the number of electron and proton are equal in any atom, it is electrically neutral. But if their number is not equal in any atom, then the atom will be charged. If the number of electron decreases, the number of proton increases. This situation is said to be charged positively. Again, if these scattered electrons are joined with any atom, the number of its electron increases, as a result it is charged negatively. The shortage or excess of electrons in any atom is considered to be charged.

The substances through which electricity or electric charge can pass easily are called conductors, e.g. metal, soil, human body etc. Generally the metals are good conductors. Copper, silver, aluminum etc. are good conductors. On the other hand the substances through which electricity or electric charge cannot pass easily are called nonconductors or insulator, such as wood, paper, glass etc.

Electrification by Friction Experiments

Experiment: Suspend a light pith ball freely with a string from a stand or a hook. Rub one end of a glass rod with a piece of dry silk cloth very well. It will be more convenient if the glass rod and the silk are dried to make them warm. Now bring the rubbed end of the glass rod near the freely suspended pith ball.

In normal condition the number of protons and electrons in any atom is equal. But every atom has got affinity for getting excess electrons. This affinity for excess electron is different in different substances. That is why when two bodies are brought in contact with each other, the body which has greater affinity for electron collect electrons from other body and gets charged negatively this happens when a glass rod is rubbed with silk.

Figure 1

[Fig: 1] Silk has more electron affinity than glass and as such these two are rubbed together, the electron of glass goes to silk. As a result, the silk gets charged negatively and the glass rod becomes positively charged.

Figure 2

This is why glass rod attracts pith ball [Fig: 2].

Figure 3

Again when a rod of ebonite or polythene is rubbed with flannel, polythene rod gets charged negatively and the flannel becomes positively charged. Because polythene has more affinity for electrons than that of flannel and so when they are rubbed together free electrons of flannel moves to ebonite or polythene and gets charged negatively [Fig: 3].

Electrical Induction

We know that when two bodies are rubbed together electric charge is produced. Again when a charged body is brought in contact with another neutral body later is charged. When the neutral body is placed not in contact with the charged body but close to the charged body, the former becomes charged. This happens due to induction. The process of charging a neutral body by bringing it very near to the charged body is called electrostatic induction. With a simple experiment the electrostatic induction has been explained below.

Electric Induction

Experiment: A dry glass rod is rubbed with silk. One end of the rod is brought very near to the neutral conductor rod AB keeping the other end of the glass rod in hand [Fig: Electric Induction (a)]. As a result the free electrons of the conductor are attracted by the positive charges of the glass rod and move the end A. Therefore, the end B falls in short of electrons and is charged positively. The end A is charged negatively. With a charge collector [a small metal sheet attached to a nonconductor handle] if some charges are collected from end B and their nature is detected with the help of an electroscope then the above discussion will be proved.

Here no new charge is produced. Due to the presence of the charges glass rod equal amount of opposite charges are separated only and move towards the end o the conductor. As long as the glass rod is present near the conductor AB, the opposite charges are separated and remain at the end of the conductor.

In the above experiment the positive charges in the glass rod which has created induction in the conductor AB is called inducing charge. The charges which are accumulated in the conductor A are called induced charge.

Extended activities: Rub a balloon full of air with your shirt. Then press the balloon with the wall of your house for a while and then release it. What did you observe? The balloon becomes attached to the wall.

Figure: 1

Extended activities: Rub a plastic object with your shirt. Then hold it near a narrow flow of water falling from a tap. The flow of water will bend towards the plastic object.

Figure: 2

The negative charge of the balloon produces electrostatic induction in the wall. The induced positive charge in the wall attracts the negatively charged balloon [Fig: 1]. Same case occurs with the narrow flow of tap water [Fig: 2].

Electroscope

The instrument which detects the presence and nature of charge in any body is called electroscope. A metallic circular disk is fixed on the top of brass or any metallic rod R [Fig: Charging the electroscope] and at lower end of the rod two light leaves of gold are attached. Instead of gold aluminum or any other light metal can also be used. The lower end of the rod with the leaves is placed in a glass jar passing through a cork of nonconductor matter C. As the apparatus is kept inside a glass jar, the humidity and the wind cannot damage the instrument.

Charging the electroscope

Charging the electroscope

If a glass rod is rubbed with silk, it becomes positively charged. If that charged rod is attached to the disk or sphere of the electroscope, some charge is transferred from the rod to the disk. This charge reaches to the gold leaves through conducting metal rod. As the gold leaves receive similar charges, they repel each other and move away from each other or explode. In this situation, if the glass is removed but the gap between the leaves is not reduced, then it can be decided that the electroscope is charged with positive ions. To charge the electroscope with negative ions, an ebonite rod should be rubbed with flannel and this negatively charged ebonite rod be allowed to touch the disk in the above mentioned process. As a result, the gold leaves of the electroscope will be charged negatively and they will repel each other and will remain in that position. The more is the quantity of charge the more will be the gap between the leaves.

Detection of existence of electric charges

To determine the presence of charge in any body, the body may be brought to an unchanged electroscope. If the two leaves move away from each other then it is understood that there is an existence of charge in the body but if they don’t move away from each other then it is understood that the body has no charge.

Detection of the nature of charge

To know the nature of charge in any charged body, the electroscope should be charged either positively or negatively. Suppose the electroscope is charged positively. In this position, the leaves having positive charge will stay apart. Now if the experimental body is brought in contact with the disk of the electroscope and if the gap between the leaves decreases, then it is understood that the body is charged negatively. On the other hand if the gap increases due to touching the disk with the experimental body, then it is understood that the body is positively charged.

Static Electricity Uses

Electrostatic Painting Spray:

Nowadays painting spray is used to color car, cycle, cupboard or other things with help of static electricity. Spray gun is made in such a way that it produces very small charged particles of color. The sharp edge of the painting spray gun is connected to a terminal of the static electrical generator. The other terminal of the generator is connected to the metal plate to be colored which must be connected with earth. In case of coloring a car the charged small particle emitted from the spray gun attracts the outer frame of the car. As a result a uniform layer of color is formed on the outer surface of the car. Moreover these small particles move along the electric lines of force and reach the narrower places of its surface and color it.

Electrostatic Painting Spray

Ink Jet Printer:

This is the most ordinary printer which remains connected to the computer. An ink-gun with its narrow mouth projects very small particles of ink. These small particles are positively charged. These ink particles move through the space between two plates. These positively charged ink particles are repelled by the positive plate and attracted by the negative plate.

Ink Jet Printer

A computer controls the voltage of the plates in such a way that the plates are sometimes positively charged and sometimes negatively charged. And the ink particles fall on moving paper scattered in different places to give the necessary shape of letters and pictures. For color print four kinds of color are used.

Photocopier:

Nowadays photocopier or photocopy machine is very essential and thus it has become very popular machine. Not only the educational institutions and other offices but also general people use this machine to photocopy any kind of necessary papers and documents. Static energy is also used in this machine. There is a rotating dram inside the dark part of the photocopier. Positive charge is sprayed over the dram. A bright light lights up the page to be photocopied. The white part of the page reflects light but the dark or printed part does not reflect light. The reflected light centers on the dram. Charges releases from the part of the dram where the reflected light from white paper falls. Only the dark part of the dram is charged positively. Negatively charged ink of carbon powder (toner) is sprayed over the dram. Negatively charged ink particles get stick to the positively charged part of the dram. A piece of white paper is positively charged and is kept pressed with the dram. This paper picks up the pattern of carbon powder on its surface from the dram. Positively charged paper attracts the negatively charged toner. Then the paper is passed through the heated roller. As a result the ink of the toner gets melted and mixed with the paper and makes a permanent copy.

Static Electricity Dangers

Sometimes the presence of static electricity is harmful and may cause danger.

Loading of fuel in aero plane:

When aero plane flies in the air it may electrify due to the friction with air. If the charge increases continuously the potential difference between the aero plane and the earth increases simultaneously. Due to this high potential difference when the fuel is loaded there is possibility of releasing some charge to the ground which may create spark. This spark may lead to a huge explosion. This is why the wheels of aero plane are made of conductor rubber so that the stored charges in the aero plane can be released safely to the ground when it lands.

The solution of the problem is that a conductor is to be connected to the aero plane and to the ground as soon as it lands and just before the loading is started.

Loading of Fuel into Tanker:

Sparking or explosion may occur when a truck or tanker etc. carries fuel from one place to another. To prevent this type of danger the tanker should be connected to the ground by a conductor.

Television and Monitors of Computer:

During the operating period television and monitor of computer, electrostatic charges are produced. These charges attract the uncharged dust particles thus they become dirty soon.

Change of clothes:

Sometimes our wearing clothes may be charged due to the friction with us. When we change our clothes, there is a possibility of getting light shock as the charges pass to the ground through our body.

Operation Theater:

Necessary measures have to be taken to keep the surgeons, concerned people and treatment equipment’s of the operation theater in a hospital free from electric charges as they attract dirt and germs. This is why they have to wear conductor shoes of rubber and use rubber gloves. So that electron can pass through them to the ground.

Hanging metal chain with petrol transporting truck:

A metal chain has to be hung touching the road with the trucks that carries petrol, diesel or other liquid fuels. When a truck moves along the road, the petrol in the tank dashes against the inner surface of the tank and oscillates to and fro. As a result of this, friction charge is stored in petrol. If any spark takes place from the edge of the tank it may cause serious accident and may set fire. So the charges in petrol are not safe. Therefore a chain is connected at the back of the tank so that the charge can pass through it to the ground easily as metal is a good conductor.

No direct connection between metal pillar and electric line:

The metal pillars on the road have any direct connection with electric lines pulled over the pillars. As metals are good conductors, the electricity of the wire may pass through the pillar to the ground if the pillar has connection with wires. If anyone touches the pillar he could immediately be electrified causing serious accident. So the wires are connected to the pillars by a porcelain cup.

Thunder bolt and lightning conductors:

We know that there is water vapor in atmosphere. This water vapor being condensed on the charged ions on the atmosphere and form water droplets and become charged with electricity. When these water droplets gather together cloud is formed. Cloud can be charged either positively or negatively. When two oppositely charged clouds come close to each other than electrical discharge takes place between them and makes huge spark. This is called lightning or electric flash.

During electric flash the air around the cloud expands suddenly by getting heated. Due to sudden expansion the pressure of the air lowers much. Then neighboring air at higher pressure contracts the expanded air. Due to this sudden expansion and contraction of air, violent sound is produced. This is known as roaring of thunder. If there is too much charge on a cloud then it includes opposite charge on the surface of the earth and electric discharge takes place. This is known as thunder bolt.

Lightning conductor or lightning arrester:

In order to protect buildings from the ravage of lightning the lightning conductor is used. The metallic rod R (Fig: Lightning conductor or lightning arrester) with several sharp points at the top is fixed in such a way that its upper end extends several feet above the roof of the building and lowers end runs down along the outer surface of the building and buried well inside the moist earth.

Lightning conductor or lightning arrester

When a charged cloud passes above the building then the charge of opposite kind is induced in the conductor rod R. The accumulation of charge is at maximum at the pointed ends of lightning conductor and pointed ends discharges their charge to the air particles around it. The air particles around the sharp points are charged by conduction and are attracted by the opposite charges of cloud and moves towards the cloud is neutralized. As a result the probability of thunder bolt decreases.

Electricity always passes following the shortest path through a conductor. The charge produced in the clouds tends to reach the earth through the high standing objects. During storm and rain it is rather good to wet in rain than to stand under an umbrella, any tree, near any conductor, iron bridge or fence of sharp iron wire.

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