Physics Class 12 Electrostatics notes 1 Learn CBSE

Physics Class 12 Electrostatics notes 1 Learn CBSE. CBSE are prepared for CBSE and RBSE students both. These notes are so much useful for the board exam. These notes are in a short form for your exam point with all derivations. Physics Notes Class 12. The notes are available in pdf form.

Electrostatics notes 2

Frictional electricity:-

If a glass rod is rubbed with a silk cloth, or a fountain-pen with a coat-sleeve, it is able to attract small pieces of paper, straw, lint, light feathers, etc. Similarly, a plastic comb passed through dry hair can attract such light objects. In all these examples, we can say that the rubbed substance has become electrified or electrically charged. It is because of friction that the substances get charged on rubbing.

The property of rubbed substances due to which they attract light objects is called electricity. The electricity developed by rubbing or friction is called frictional or static electricity. The rubbed substances which show this property of attraction are said to have become electrified or electrically charged.

Historical view of frictional electricity. In 600 B.C., Thales of Miletus, one of the founders of Greek science, first noticed that if a piece of amber is rubbed with a woollen cloth, it then acquires the property of attracting light feathers, dust, lint, pieces of leaves, etc.

In 1600 A.D., William Gillbert, the personal doctor to Queen Elizabeth -1 of England, made a systematic study of the substances that behave like amber. In his book De Magnete- (on the magnet), he introduced the name electrical for such substances. In fact, the Greek name for amber is electron which is the origin of all such words : electricity, electric force, electric charge and electron.

Amber is a yellow resinous (gum like) substance found on the shores of the Baltic sea.

Both electric and magnetic phenomena can be derived from charged particles. Magnetism arises from charges in motion. The charged particles in motion exert both electric and magnetic forces on each other. Hence electricity and magnetism are studied together as electromagnetism.Physics Notes Class 12

ELECTRIC CHARGE

Electric charge. Electric charge is an intrinsic property of the elementary particles like electrons, protons, etc., of which all the objects are made up of. It is because of these electric charges that various objects exert strong electric forces of attraction or repulsion on each other.

Electric charge is an intrinsic property of elementary particles of matter which gives rise to electric force between various objects.

Electric charge is a scalar quantity. Its SI unit is coulomb (C). A proton has a positive charge (+ e) and an electron has a negative charge (- e), where

e = 1.6 × 10^-19 coulomb

Physics Class 12 Electrostatics notes Learn CBSE

Large-scale matter that consists of equal number of electrons and protons is electrically neutral. If there is an excess of electrons, the body has a negative charge and an excess of protons results in a positive charge.

ELECTROSTATICS

Electrostatics. Electrostatics is the study of electric charges at rest. Here we study the forces, fields and potentials associated with static charges.

Applications of electrostatics. The attraction and repulsion between charged bodies have many industrial applications. Some of these are as follows :

1. In electrostatic loudspeaker.

2. In electrostatic spraying of paints and powder coating.

3. In flyash collection in chimneys.

4. In a Xerox copying machine.

5.In the design of a cathode-ray tube used in television and radar.

TWO KINDS OF ELECTRIC CHARGES

1. There are only two kinds of electric charges – positive and negative.

2. Like charges repel and unlike charges attract each other.

What are vitreous and resinous charges ? What was wrong with this nomenclature ?

Vitreous and resinous charges. Charles Du Fay used the terms vitreous and resinous for the two kinds of charges.

1. The charge developed on glass rod when rubbed with silk was called vitreous charge (Latin virtum = glass).

2. The charge developed on amber when rubbed with wool was called resinous charge (amber is a resin).

But later on, these terms were found to be misleading. For example, a ground glass rod develops resinous electricity while a highly polished ebonite rod develops vitreous electricity.

Physics Class 12 Electrostatics notes part 1 Learn CBSE

What are positive and negative charges ? What is the nature of charge on an electron in this convention ?

Positive and negative charges. Benjamin Franklin (1706-1790), an American pioneer of electrostatics introduced the present-day convention by replacing the terms vitreous and resinous by positive and negative, respectively. According to this convention :

1. The charge developed on a glass rod when rubbed with silk is called positive charge.

2. The charge developed on a plastic rod when rubbed with wool is called negative charge.

The above convention is consistent with the fact that when two opposite kinds of charges are brought in contact, they tend to cancel each other’s effect. According to this convention, the charge on an electron is negative.

Table 1.1 gives a list of the pairs of objects which get charged on rubbing against each other. On rubbing, an object of column I will acquire positive charge while that of column II will acquire negative charge.

Table 1.1 Two kinds of charges developed on rubbing

Obviously, any two charged objects belonging to the same column will repel each other while those of two different columns will attract each other.

For Your Knowledge

Benjamine’s choice of positive and negative charges is purely conventional one. However, it is unfortunate that the charge on an electron (which is so important to physical and chemical properties of materials) turns out to be negative in this convention. It would have been more convenient if electrons were assigned positive charge. But in science, sometimes we have to live with the historical conventions.

Different substances can be arranged in a series in such a way that if any two of them are rubbed together, then the one occurring earlier in the series acquires a positive charge while the other occurring later acquires a negative charge :

Thus glass acquires a positive charge when rubbed with silk but it acquires negative charge when rubbed with flannel.

Physics Class 12 Electrostatics notes part 1 Learn CBSE

1.5 ELECTRONIC THEORY OF FRICTIONAL ELECTRICITY

Electronic theory of frictional electricity. All matter is made of atoms. An atom consists of a small central nucleus containing protons and neutrons, around which revolve a number of electrons. In any piece of matter, the positive proton charges and the negative electron charges cancel each other and so the matter in bulk is electrically neutral.

The electrons of the outer shell of an atom are loosely bound to the nucleus. The energy required to remove an electron from the surface of a material is called its ‘work function’. When two different bodies are rubbed against each other, electrons are transferred from the material with lower work function to the material with higher work function. For example, when a glass rod is rubbed with a silk cloth, some electrons are transferred from glass rod to silk. The glass rod develops a positive charge due to deficiency of electrons while the silk cloth develops an equal negative charge due to excess of electrons. The combined total charge of the glass rod and silk cloth is still zero, as it was before rubbing i.e., electric charge is conserved during rubbing.

Electric origin of frictional forces. The only way by which an electron can be pulled away from an atom is to exert a strong electric force on it. As electrons are actually transferred from one body to another during rubbing, so frictional forces must have an electric origin.

For Your Knowledge

The cause of charging is the actual transfer of electrons from one material to another during rubbing. Protons are not transferred during rubbing.

The material with lower work function loses electrons and becomes positively charged.

As an electron has a finite mass, therefore, there always occurs some change in mass during charging. The mass of a positively charged body slightly decreases due to loss of some electrons. The mass of a negatively charged body slightly increases due to gain in some electrons. Physics Class 12 Electrostatics notes part 1 Learn CBSE

1.6 CONDUCTORS AND INSULATORS

Conductors. The substances through which electric charges can flow easily are called conductors. They contain a large number of free electrons which make them good conductor of electricity. Metals, human and animal bodies, graphite, acids, alkalies, etc. are conductors.Physics Notes Class 12

Insulators. The substances through which electric charges cannot flow easily are called insulators. In the atoms of such substances, electrons of the outer shell are tightly bound to the nucleus. Due to the absence of free charge carriers, these substances offer high resistance to the flow of electricity through them. Most of the non- metals like glass, diamond, porcelain, plastic, nylon, wood, mica, etc. are insulators.

An important difference between conductors and insulators is that when some charge is transferred to a conductor, it readily gets distributed over its entire surface. On the other hand, if some charge is put on an insulator, it stays at the same place. We shall discuss this distinguishing feature in the next chapter.

A metal rod held in hand and rubbed with wool does not develop any charge. This is because the human body is a good conductor of electricity, so any charge developed on the metal rod is transferred to the earth through the human body. We can electrify the rod by providing it a plastic or a rubber handle and rubbing it without touching its metal part.

What is meant by earthing or grounding in household circuits ? What is its importance ?

Earthing and safety. When a charged body is brought in contact with the earth (through a connecting conductor), its entire charge passes to the ground in the form of a momentary current. This process in which a body shares its charges with the earth is called grounding or earthing.

(a) Positively charge (b) Negatively charge, earthed body.

The electricity from the mains is supplied to our houses using a three-core wiring : live, neutral and earth wires. The live wire red in colour brings in the current. The black neutral wire is the return wire. The green earth wire is connected to a thick metal plate buried deep into the earth. The metallic bodies of the electric appliances such as electric iron, refrigerator, TV, etc. are connected to the earth wire. When any fault occurs or live wire touches the metallic body, the charge flows to the earth and the person who happens to touch the body of the appliance does not receive any shock.

ELECTROSTATIC INDUCTION

Electrostatic induction. As shown in Fig. 1.4, hold a conducting rod AB over an insulating stand. Bring a positively charged glass rod near its end A. The free electrons of the conducting rod get attracted towards the end A while the end B becomes electron deficient. The closer end A acquires a negative charge while the remote end B acquires an equal positive charge. As soon as the glass rod is taken away, the charges at the ends A and B disappear.

Electrostatic induction.

Electrostatic induction is the phenomenon of temporary electrification of a conductor in which opposite charges appear at its closer end and similar charges appear at its farther end in the presence of a nearby charged body.

The positive and negative charges produced at the ends of the conducting rod are called induced charges and the charge on the glass rod which induces these charges on conducting rod is called inducing charge.

Physics Class 12 Electrostatics notes 1 Learn CBSE

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