ICSE Class 8 Physics Chapter 10 Static Electricity

10 Static Electricity

In your earlier classes, you have learnt that electricity is very useful in our day-to-day life. Electricity is used to light homes, schools, offices, etc. A number of appliances like fan, T.V., refrigerator, room heater, washing machine, etc. are used by most of us. All these appliances run on electricity. These appliances have made human life much comfortable. All these are examples of current electricity or charges in motion.

In this chapter, we shall confine our studies to the charges at rest i.e. static electricity.

The word electricity was derived from the greek word elektron which means amber (a kind of resin produced by certain plants).

Looking back at the history of electricity, it is found that electricity was initially produced by friction. It is believed to have been discovered by Thales of Miletus - one of the seven wise men of ancient Greece about 600 B.C. He found that after rubbing a piece of amber with wool, the amber would attract light objects like tiny pieces of dry paper, grass, feather, etc. This phenomenon was not very clear and was confusing till around 1600 AD, when father of electricity, William Gilbert gave a theory about electricity. He was able to explain satisfactorily about attraction of certain tiny substances by amber rubbed with wool.

If you bring a plastic comb near a few small pieces of paper, comb will not attract the pieces of paper. If the same comb is first rubbed with dry hair and then brought over the pieces of paper, then it will attract them.

Let us take the following examples:

Take a glass rod and a piece of silk cloth. Hold them close to small pieces of paper. None of them will attract the pieces of paper [Fig. 10.2(a)].

Now rub the glass rod with the silk cloth [Fig. 10.2(b)]. Bring the glass rod and the cloth slightly above the bits of paper, separately. You will observe that both glass rod and silk cloth are attracting the pieces of paper [Fig. 10.2(c)].

Teacher's Note

When you rub a comb through your hair on a dry day, the comb becomes electrically charged and can attract small pieces of paper - this is static electricity in action in our daily lives.

Take an inflated balloon. Put it against a dry wall. You will observe that the balloon is sliding down. Now rub it with a woollen material and place it against the wall. The balloon will stick to the wall [Fig. 10.3(a)]. Now take two such balloons and rub both of them with woollen material. Bring the balloons close to each other. Both will move away from each other i.e. both will repel each other [Fig. 10.3(b)].

From the examples given above, we find that rubbing a comb against dry hair or a balloon against woollen material produces in them a kind of charge which helps them to attract as well as repel objects. The developed force so created is called the electrostatic force. The comb, balloon, glass rod and silk cloth are said to be electrically charged when they are rubbed against hair or woollen clothes i.e. they acquire electrical charge. If this charge is not allowed to flow, it is termed as static electricity.

Teacher's Note

Balloons rubbed with wool and stuck to walls demonstrate how static electricity works when charges don't flow - the same principle that makes your clothes cling together after being in a dryer.

Modern Electronic Theory Of Static Electricity

To understand the static electricity and its nature, we must understand the basic construction of matter. Following are the characteristics of matter and its constituent particles:

(a) Each matter is made up of tiny particles known as atoms. An atom consists of three main particles - protons, electrons and neutrons.

(b) A proton carries a positive charge, an electron carries a negative charge while a neutron has no charge i.e. it is electrically neutral. Numerically, the charges on proton and electron are equal. In other words, we can say that charges on proton and electron are equal and opposite.

(c) The central part of the atom is called its nucleus and it contains protons and neutrons. The electrons revolve round the nucleus along circular paths called orbits or shells.

(d) The electrons present in the outermost orbit of an atom are called valence electrons. Electrons present in an orbit beyond the valence orbit are very weakly attracted by the nucleus. Therefore, these electrons can easily be removed from an atom. They are called free electrons.

(e) The transfer of electrons is responsible for the charging of bodies.

(f) In an atom, the number of protons are equal to the number of electrons. Hence, the total positive charge in it is equal to the total negative charge, thus showing that an atom is electrically neutral.

(g) If an atom gains electrons, the number of electrons in it becomes more than the number of protons i.e. total negative charge in the atom becomes more than the total positive charge. Thus, the atom becomes negatively charged.

(h) If an atom loses electrons, the number of electrons in it becomes less than the number of protons i.e. total negative charge in the atom becomes less than the total positive charge. Hence, the atom becomes positively charged.

(i) Because of their electric field, the charged particles exert force on each other, even when they are not in physical contact.

Electron Transfer - Main Reason For Frictional Electricity

When we rub a glass rod with a piece of silk, the glass rod loses electrons to the silk piece. It happens because the free electrons in the glass rod are less tightly bound as compared to those in the silk cloth. As a result, the glass rod has a deficiency of electrons and hence acquires a positive charge. Silk cloth has excess of electrons, so, it becomes negatively charged.

Similarly, when an ebonite rod is rubbed with fur, the fur loses electrons to the ebonite rod because the electrons in the outermost orbit of in case of fur are loosely bound as compared to those in an ebonite rod. Now the ebonite rod has excess of electrons, so it becomes negatively charged, while the fur has a deficiency of electrons, so it acquires a positive charge.

Laws Of Electrostatic Attraction And Repulsion

According to the laws of electrostatic attraction and repulsion,

- Two like charges repel each other.

- Two unlike charges attract each other.

To understand these laws, let us take the following examples:

(1) Suspend a glass rod rubbed with silk cloth, using a silk thread. Bring another glass rod rubbed with silk cloth close to the glass rod (Fig. 10.5). We observe that the suspended glass rod moves or tends to move away from the second glass rod. Both the glass rods are positively charged so they repel each other.

(2) Suspend, using silk thread, an ebonite rod rubbed with fur to a stand. Bring another ebonite rod rubbed with fur near the suspended rod (Fig. 10.6). We observe that ebonite rod moves or tends to move away from the suspended rod. Both the ebonite rods have negative charges so they repel each other.

(3) Suspend, using silk thread, a glass rod rubbed with silk cloth to a stand. Bring an ebonite rod rubbed with fur near the suspended glass rod. We see that the suspended glass rod moves towards the ebonite rod (Fig. 10.7). In this case, on rubbing the glass rod has acquire positive charge while the ebonite rod has acquired a negative charge. As a result, two rods attract each other.

Conclusions: From this activity the following conclusions are drawn -

(1) The charge produced on the ebonite rod when rubbed with a fur is opposite to that produced on the glass rod when robbed with a piece of silk.

(2) Two similarly charged rods repel each other, whereas two rods having opposite charges attract each other. Thus, we find that, like-charges repel each other, whereas unlike-charges attract each other.

Teacher's Note

When you walk across a carpet and get shocked touching a metal doorknob, you're experiencing the repulsion of like charges - the electrons built up on your body repel those in the metal, creating a spark.

Good Conductors And Insulators

The substance which allows electric charge to flow through it is called a good conductor of electricity. All metals are good conductors. Silver, copper and aluminium are some of the very good conductors of electricity. Mercury, earth and human body are also good conductors of electricity.

Some substances hardly allow electric charge to flow through them; they are called bad conductors of electricity. Most of the non-metals are bad conductors of electricity. Some exceptions are graphite, tap water, etc. However, some of the substances do not allow electric charge to pass through them at all. They are called insulators. Examples: Sulphur, rubber, oxygen, wood and hydrogen.

Why some substances are conductors and some are insulators of electricity depends upon the number of free electrons they possess. The substances which have a large number of free electrons are known as good conductors. The substances that have a very small number of free electrons or which do not have any free electrons are called bad conductors and insulators respectively.

Methods Of Charging A Conductor

We may charge an uncharged conductor by the following two methods:

1. Charging by conduction:

When we touch an uncharged conductor with a charged conductor, the charged conductor shares the charge with uncharged conductor. This process is called charging by conduction.

Take an uncharged metal rod A mounted on an insulating stand as shown in Fig. 10.8. Now, touch a positively charged conductor B, with an insulating handle, with the rod A. It will be observed that the uncharged metal rod gets positively charged. If B is a negatively charged conductor, then conductor A acquires a negative charge.

2. Charging by induction:

When an uncharged conductor is placed very close to a charged conductor, without touching it, the nearer end of the uncharged conductor acquires a charge of opposite nature as compared to the charge on a charged conductor. This process is known as charging by induction.

Take an uncharged conductor A mounted on an insulating stand, and bring a charged conductor B very close to it. It will be observed that an opposite charge develops on the end of the conductor A which is nearer to conductor B. At the same time, same kind of charge as that on B is developed on the farther end of A. Thus, if charge on B is positive, the charge developed on the nearer end of A will be negative and its farther end will acquire a positive charge. Conversely, if charge on B is negative, the charge developed on the nearer end of A will be positive and its farther end will acquire a negative charge.

Teacher's Note

When you comb your hair and the comb picks up a negative charge, it can attract uncharged bits of paper through induction - the paper develops opposite charges on its nearer surface.

Differences between charging by conduction and by induction: Following are the points of differences between charging by induction and by conduction:

(1) The process of charging a body by actually touching a charged body is called charging by conduction, whereas the process of charging a body by keeping it near (without touching) a charged body is called charging by induction.

(2) In charging by conduction, charge moves from the charged body to the uncharged body, with the result, the strength of charge on the charged body reduces. Now, on separating the two bodies, they retain their charges.

Whereas, in charging by induction, no charge flows from the charged body to the uncharged body. Infact, if the charged body is positively charged, it attracts the electrons in the uncharged body making nearer end of uncharged body surplus of electrons i.e. negatively charged and making the farther end of it with deficit of electrons i.e. positively charged. Similarly, if the charged body is negatively charged, it repels the electrons in the charged body making near end of uncharged body with deficit of electrons or positively charged and farther end of it with surplus of electrons i.e. negatively charged. Now, on removing the charged body, the body charged by induction will become neutral.

(3) In charging by conduction, some charge is lost (given) by charged body but in charging by induction, no charge is lost (given) by the charging body.

Electroscope

An electroscope is a device with the help of which we detect whether the body is charged or uncharged and if it is charged, what kind of charge does it carry, i.e., positive or negative.

In general, the electroscopes are of the following two types:

(i) Pith ball electroscope

(ii) Gold leaf electroscope

(i) Pith ball electroscope:

A pith ball electroscope is shown in Fig. 10.10. It consists of a small pith ball suspended with the help of a dry silk thread from an insulating stand.

To find out whether the body is charged or uncharged: The body is brought near the pith ball (without touching it). If the pith ball shows a movement towards the body, the body is charged, but if the pith ball remains static, the body is uncharged.

To find out whether the body has a positive charge or a negative charge: First of all, a pith ball electroscope is charged positively by conduction. Now the charged body is brought near the electroscope. If the pith ball moves away, the body has a positive charge, but if the pith ball moves towards the body, it has a negative charge.

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