Basics of Electricity

BASICS OF ELECTRICITY

Matter has mass and takes up space. Atoms are basic building blocks of matter and can’t be chemically subdivided by ordinary means. ‘Atom’ is derived from the Greek word ‘atom’, which means indivisible.

STRUCTRURE OF ATOM:


Atoms are composed of three types of particles: protons, neutrons and electrons. These particles have different properties. An atom has a positive centre called the nucleus. The central nucleus consists of protons, neutrons and the electrons revolve round the nucleus. Electrons are very tiny, very light particles having negative electrical charge. Protons are much larger than electrons and have positive electronic charge. Neutrons are large and heavy like the protons but no electrical charge. In any atom, the number of positive charges present in the nucleus is equal to the number of negatively charges electrons revolving round it. Thus, the atom is electrically neutral on the whole.

CHARGE:

When positive or negative charge builds up in fixed positions on objects, certain phenomena can be observed. The charge can be built by rubbing objects together, such as silk and glass or ebonite and fur. The friction between the objects causes electrons to be transferred from one to the other-from a glass rod to a silk cloth or from fur to an ebonite rod-with the result that the object that has lost the electrons has a positive and the object that has gained them has an equal negative charge. An electrically neutral object can be charged by bringing it in contact with a charged object, if the charged object is positive, the neutral object gains a positive charge when some of its electrons are transferred on to the positive object. (When the glass rod is rubbed with silk, the glass rod becomes positively charged.)
If the charged object of negative, the neutral object gains a negative charge when some electrons are attracted on to it from the negative object. (When an ebonite rod is rubbed with fur, the ebonite rod becomes negatively charged.)

CHARGING OF BODIES:

Charging by friction

When a glass rod is rubbed with silk or an ebonite rod is rubbed with fur, the glass rod becomes positively charged and the ebonite rod becomes negatively charged due to friction. Friction between the objects causes the transfer of electrons. The glass rod loses some of its electrons to the silk cloth and becomes positively charged due to excess of positively charged particles. Similarly, when the ebonite rod is rubbed with fur, the rod gets extra electrons from the fur and becomes negatively charged. Charging a body by friction if possible when two bodies of opposite nature (i.e.) one having a tendency to give away free electrons and the other having a tendency to accept the same are made to rub against each other.
In this process, both the bodies, which were not charged, acquire opposite charges.


Charging by conduction


When charges are transferred to an uncharged body from a charged by making physical contact between them, it is called charging by conduction. The undercharged conductor B is mounted on an insulating stand. A positively charged conductor A is brought in contact with the conductor B. Since the conductor A is positively charged, it has a less number of free electrons and hence when B touches A, a few electrons move from B to A. Thus B gets positive charge due to loss of free electrons.
Charges by conduction are possible when one of the bodies is already charged. Here, both the bodies acquire the same type of charge as they share the excess or deficit of electrons.

Charging by induction


When charges are transferred to an uncharged body from a charged body without physical contact between them, it is called charging by induction.
For charge of given sign, it is necessary to start with an inducing charge of the opposite kind. To charge the uncharged body AB positively, a negatively charged rod has been brought near to the end A of the uncharged body AB. Some of the free electrons in the conductor are repelled towards B by the excess of electrons on the negative rod, thereby creating a negative induced charge at B and a positive induced charge at A. The conductor is now earthed by touching it with the finger, as a result some of the free electrons are repelled to earth. The finger is then removed and the charged rod is taken away. The positive charges redistribute themselves uniformly throughout AB. Thus, AB is positively charged.

Conductors and Insulators

Though all materials are made of atoms and all atoms are made of electrons and nuclei, the electrical properties of substances vary widely. According to their electrical properties substances are grouped into conductors and insulators.
Conductors contain free electrons, which can move throughout the material. Metals are good conductors. Silver, copper and aluminum are some of the best conductors of electricity and acidified water and electrolytes are no-metallic conductors.
Insulators or dielectrics have tightly bound electrons, which are not free to move through the material. Hence, electricity cannot move through them. E.g. ebonite, mica, glass, rubber, etc.

GOLD LEAF ELECTROSCOPE

The electroscope is a device used for detecting the presence of charge and testing the given electric charge. It was invented by Nollet in the 18th century.
The gold leaf electroscope consists of a glass bell jar resting on a wooden base. The mouth of the bell jar is closed with an ebonite stopper or a rubber cork having a central hole, through which passed a brass rod. At the lower end of the brass rod are two narrow strips of very thin leaves of gold and the upper end is fitted to a metal disc with a spherical knob. A small cup of calcium chloride may be placed inside the jar to keep the air inside dry. There are two strips of tin foil on the inner wall of the bell jar and facing the gold leaves. The jar protects the leaves from air currnts. When any charged body is dept in contact with the metal disc, the leaves acquire the charges of the same nature, so the leaves repel each other. The divergence of the two leaves indicates the presence of the charge. The increase of the leaves is the only sure test of the electrification of a body. (Brief picture of Gold Leaf Electroscope is available at wwww.encarta.com)

(i) To detect the presence of charge on a body

If a rod of some suitable material is charged by friction and then brought near the disc of the electroscope, the leaves diverge. A charge has been induced on the leaves and consequently the leaves diverge. On removing the charged rod, the leaves collapse, showing that the induced charge on the electroscope is only temporary.
Very small charges may be detected by this method.

(ii) To charge the electroscope by induction

(a) A negatively charged ebonite rod is held near the disc of the electroscope. A positive charge is induced in the disc and a negative charge is induced in the leaves. This happens because the negatively charged ebonite rod repels the electrons from the disc to the leaves. The leaves diverge.
(b) When the disc is connected to the earth by touching it with your finger, electrons from the leaf flow to the earth and the leaves collapse.
(c) Keeping the ebonite rod near the disc, if you remove your finger, the leaves remain in the collapsed position because they do not have any charge in them.
(d) If you remove the ebonite rod, the leaves diverge again. The positive charge flows to the leaves also. Thus, the electroscope is positively charged by induction.

To charge the electroscope negatively, a positively charged glass rod is used.

(iii) Detecting the type of charge present in a body using electroscope

Having charged the electroscope positively as described above, bring a charged rod near the disc of the electroscope. If the divergence of the gold leaves increases, then the rod is negatively charged. If the divergence decreases, then the rod is positively charged.

CHARGE DENSITY

Whenever a conductor is charged, the charges reside on the outer surface of the conductor. The charges are not distributed uniformly over the surface. The distribution depends on the shape of the surface and it is found to be inversely proportional to the radius of curvature. The amount of charge per unit area of the surface of the conductor is known as charge density.

In case of a spherical conductor, since the radius is uniform, the charge distributes uniformly and so the charge density will be equal everywhere. But in the case of a pear shaped conductors, the charge density is greater at the pointed end than at the places. Thus a pointed end acquires a large amount of charge.

As a pointed end acquires a large amount of charge, it sets up a field in its neighborhood. Air molecules entering into the field take the same charge from the pointed end become ionized. Since the pointed end as well as the air molecules are of the same charge, the air molecules which is called the electronic wind. Ultimately, the pointed end loses its charge and it is discharged easily. A point also collects charges easily from its surroundings. This phenomenon is called ‘action of points’.

LIGHTNING ARRESTERS

Benjamin Franklin(1706-1790), an American scientist, proved that lightening and electricity are identical and subsequently invented the lightning arrester also called lightning conductor. It protects tall buildings from being damaged by lightning. The light conductor is based on the principle of action of points.

It consists of a thick copper strip to an outside wall, reaching above the highest part of the building and ending in several sharp spikes. The lower part is buried in the earth.
When a negatively charged thundercloud passes overhead, it acts inductively on the conductor, charging the sharp points and te earthed plate negatively. The negatively charge on the plate is, of course, immediately dissipated into the surrounding earth. At the same time, action of points occurs at the spikes. Negatively ions are attracted to the spikes and become discharged by giving up their electrons. These electrons pass down the conductor and escape to the earth.

The upward stream of positive ions spread out and form what is called a space charge, which reduces the powerful electric forces existing between the cloud and the building. Under these conditions lightening will nor normally strike a building but if it should do so, the discharge passes harmlessly to the earth through the thick copper conductor.

(The pictures and explanatory images of the corresponding topics are available at www.encarta.com. But the explanation which is given here is more understandable than Encarta.)

***This article is more useful for the students of 6th std(sixth std), 7th std(seventh std) and 8th std(eight std).


An article by,
Surenthiran