NCERT Class 10: Science Learning Concepts

Science: Chapter 12-Electricity-Concepts

12.1. The Electric Current and Circuit

Without electricity it would be all dark and dull in this age which we live in this world.
The existence of electricity has been known since about 600 BC when Greek philosopher Thales observed that when amber (a type of tree resin, similar to modern day plastic) is rubbed vigorously with fur, the amber attracts light objects such as feathers. This mysterious phenomena was named electricity. The term electricity derives from the word amber which means electron in Greek. Similarly, when a plastic comb is run through dry hair, it can attract small bits of paper. When a glass rod is rubbed with silk fabric, the glass also attracts bits of paper. Again, two charged glass rods (both rubbed with silk) were observed to push each other whereas the charged plastic rod can be observed to attract the suspended charged glass rod. From similar experiments and observations it was therefore evident that there are two kinds of electrical charges, and that the like charges repel each other and unlike charges attract each other. American statesman and philosopher Benjamin Franklin finally gave these charges their name. The type of charge that appears on glass rod when rubbed with silk was labelled positive charge and the type of charge that appears on plastic comb when ran through dry hair or rubbed with fur was named negative charge.
Electricity begins with electric charge. Charges, like mass, occur naturally and are present in some particles inside the tiny atoms from which the visible matter is constructed. In the early twentieth century, around 250 years after Franklin named the charges, a series of spectacular discoveries revealed the structure of the atoms. It became clear that atoms are made of three elementary particles-electron, proton and neutron. The electron and proton carry an equal but opposite amount of charges, while the neutron has no charge. In an atom, there are as many protons as electrons. So normally an object is said to be electrically neutral. It is only when an excess amount of net charge-or charge imbalance-occurs on the object, only then the object can interact electrically with other objects. Each charge has its own electric field, the space around it in which it exerts its electrical influence.When another charged particle moves into this field, it experiences a force-the electrostatic force. The strength of this force depends on:
Two factors
• the amount of electrical charge they charges carry-the more the charge, the larger the force
• the distance between the charged particles-the closer they are, the larger the force.

The electrons revolve around the central core of nucleus while the protons are locked up within the nucleus along with neutrons. The electrons are bound inside the atoms by electrostatic force of attraction with the protons. While the protons fiercely repel each other with electrostatic force of repulsion, they are, however, bound together by the stronger nuclear force. The electrostatic forces between the charges hold together not only the atoms but also the matter around us. An object can acquire a net charge when rubbed (or brought into contact) with another appropriate object, and become electrically charged. Actually, at the points of contact, the objects transfers a tiny amount of charge-some electrons, and slightly upset the charge neutrality. Rubbing only increases the number of contact points and gives enormous opportunity for more charges to transfer. Therefore, when the glass rod is rubbed with the silk, the glass loses some of its electrons and therefore has a small unbalanced positive charge. Similarly, when the plastic rod is rubbed with fur, the rod gains some electrons and has a net unbalanced negative charge.
An electrically charged object can induce opposite charges on other neutral objects, without any rubbing or actual contact. This is how the charged glass rod or charged plastic rod attract light objects such as feathers or bits of paper, or a balloon when rubbed with rug or carpet can cling to the wall. Actually, the charged object induces some opposite amount of charge on the neutral object and therefore attract each other. This method of charging is called charging by induction. Please note that no charge transfer occurs in this case.
The phenomena of electricity that occurs on the materials such as glass, plastic, silk, paper (dry) as discussed above is static electricity, static meaning the charges stay at the surface. You often observe this phenomena in your daily life. For example, you often have found yourself struggling to throw away the plastic wrapper removed from a gift that won’t leave your hand. When you peel off an adhesive tape from a glass window, the tape still gets attracted towards the spot on the glass it left behind. Lightning during a thunderstorm in a rainy day is also a phenomena of static electricity on a very large scale in which huge amount of charge on the clouds discharge to the earth, through the air.
The laser printer or the xerographic copier machine works on the phenomena of static electricity. Electrostatic painting is also good method painting bodies of cars. In chimneys of industries charged pollutant particles from the smoke is separated by a electrostatic scrubber.
In a petrol refuelling station, when a tanker unloads fuel, a metallic wire is always connected from the tanker to the earth. Can you answer why? Static charges on a surface attract nearby opposite charges in air or other objects. When static charge is large enough, it fiercely attracts the nearby electrons in air or on any other surface. The electrons then rush through the air toward the charged object, collide with the air molecules, and produce more electrons and ions in its way. This process produce light, heat and sound, causing a spark. Therefore any build-up of static charge can be fatal and may cause fire, usually in cold and dry days.
Unit of charge: Like any other physical quantity, charge is also measured in certain units. The standard (SI) unit of charge is coulomb (denoted by symbol C) given in honour of French physicist Charles-Augustin de Coulomb. From careful experiments, it was established that the electron and proton each have a extremely small amount of electric charge, called the elementary unit of charge (e), the magnitude of which is about
$ 1.6*10^{-19} C$. Any amount of charge that occurs must be an integer multiple of this elementary charge, that is, Q = ne, where n is an integer. For example, the charge on an object can be of 625e but not 6.25 e. The electron carries -1 unit of elementary charge while the proton carries +1 unit of this elementary charge. 1 coulomb of charge carries an enormous amount of elementary unit charges, actually, about $6.25*10^{18}$. Therefore, if an object has one coulomb of positive charge, then it means it has lost $6.25 × 10^{18}$ number of electrons. And if the object has one coulomb of negative charge, then it has gained $6.25 × 10^{18}$ number of electrons.
From the above discussion, it is concluded that:
o Electric charge occurs naturally, and there are two types of charge: positive (+ sign) charge and negative (-sign) charge.
o Each charge has an electric field associated with it. Charges interact through this field.
o Charges exert force on one another. Two like charges (charges with equal algebraic signs) repel each other, that is, push a part directly away from each other. Two unlike charges (charges with opposite signs) attract, that is, pull directly towards each other.
o Electric charges appear as integer multiple of an elementary unit of charge (e), the magnitude of

which is about $1.6 × 10^{-19} C$. An electron carries -1 unit of this elementary charge and a proton carries +1 unit of this elementary charge.
o Like mass, momentum and energy, charge is always conserved. That is, charge can not be created or destroyed. The total charge of an electrically isolated system always remains the same.

Conductors and Insulators:
In static electricity, any unbalanced charge stays at the surface. If you pull out some electrons from a part of the glass, electrons from other parts do not move in quickly to replace them, and the surface stays positively charged. Similarly, if you add some electrons on the surface of a plastic rod, the electrons do not flow away to other parts. These type of substances in which electrons cannot move freely are called insulators. Glass, plastic, rubber, silk, nylon, dry paper are all examples of insulators.

Other substances such as metals, tap water or human body allow rather easy flow of electrons. These substances are called conductors.

Most metals are good conductors of electricity. Human body is a fairly good conductor of electricity. Ionic liquids, ionised gases are also conductors of electricity. The difference between conductors and insulators will be more clear in the following sections.
If, for example, in place of the plastic rod, you take a copper rod, and rub vigorously with silk cloth while holding in your hand, the rod will not be charged. The unbalance charges that appear on copper on rubbing, will move quickly through the length of the rod, through your body, to the floor. This method of neutralising or removing any unbalanced charge from an object is called discharging. When the object is discharged by connecting a conductor to the earth, the method is called grounding. Earth always acts as a source or a sink of electric charge.

You can charge a copper rod while holding with yours hands using rubber or plastic gloves. But any unbalanced charge that appears on the copper rod quickly spreads out over its entire surface.

Two metallic spheres A and B at some distance apart, insulated from the earth by two insulating stands. Sphere A is given an unbalanced positive charge by contacting with a positively charged glass rod while sphere B is charged equally negative by contacting with a negatively charged plastic rod. If spheres A and B are connected by a copper wire, electrons immediately flow from sphere B to sphere A until both the spheres become neutral. Note that it is only the electrons, with their negative charges, that move, as protons are locked up inside the nucleus. If you repeat the experiment with a nylon or silk thread in place of a copper wire, you will find that no charge flow takes place and the spheres still remain charged. Therefore, copper is a good conductor of electricity whereas nylon or rubber are good insulators of electricity. The flow of any net charge constitutes current electricity. Also note that it does not matter which of the charges-positive or negative-move along the wire, because the end result is the same, the unbalanced charges on both the spheres are neutralised.
Conductors permit easy flow of charges whereas insulators do not. Most metals are good conductors of electricity. This is because, one or two outermost or valence electrons of, a copper atom, are loosely bound to the nucleus. When a large number of copper atoms form a solid, these valence electrons detach from the atom and become free to move about within the material, like the molecules of air move within a closed container. These electrons are called free electrons or conduction electrons. The other electrons remain bound to the nuclei of individual atoms which are fixed in their position within the solid. Note that the free electrons remain bound to the surface of the solid, and the solid is neutral. Metals have generally enormous amount of conduction electrons, for example, in the numbers of about $10^{22}$ in only one $cm^{2}$ of area. Insulators have no, or very few, of these conduction electrons, and charges are not free to move through the materials. Therefore, insulators are useful in confining of electricity only to the conductor and are very useful for insulation of electrical appliances.
In ionic liquids, such as salt dissolved in water, or aqueous solution of an acid or a base, both positive and negative ions are mobile and can conduct electricity. In ionised gases, such as in a fluorescent tube light, both mobile positive ions as well as electrons conduct electricity. In metals, it is only the mobile conduction electrons that conduct electricity through the substance.
From the above discussion we learned that
• The substances which permit the flow of charges rather freely through them are called conductors of electricity. Example: metals, ionic solutions, ionised gases, tap water, human body.
• The substances which do not permit the flow of charges through them are called non-conductors or insulators. Example: glass, rubber, plastic, dry paper, silk, and so on.
• Conductors such as metals contain a very large number of free electrons that are in random motion within the substances. Insulators contain no, or very few conduction electrons.