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Electrostatics - Dielectrics



Most of the substances in nature are divided in to two categories, namely conductors and insulators.

A substance which can be used to carry or conduct electric charge from one place to the other is called a conductor. Silver is one of the best conductors. Other examples of conductors are copper, iron aluminum mercury, coal etc. earth is a goods conductors. Human body is also a goods conductor of electricity. The liquid conductors include, salt solutions, acids, alkalis etc. in a metallic conductor, there are a large number of free electrons which act as carriers of charge. Infect, in a metal, the outer (valence) electrons part away from their atomic and are free to roam leave the natal under normal circumstances. The free electrons form a kind of electron gas, they collide with one another; and also with the ions; and move randomly indifferent directions. Inane external electric field, the free electrons drift against the direction of the field. The residual atoms made up of nuclei and the bound electrons remain held in their fixed positions. They constitute the bound charges in the conductors as charge carriers are both, the positive and negative ions.

The insulators are the materials which cannot conduct electricity they are poor conductors of electricity. Common examples of insulators are glass rubber, plastic ebonite mica, wax paper wood etc. they are called insulators because they prevent charge from going to places where it is not desired. Such substances posses a negligibly small number of free electrons. 

Infact, in an insulator, watch electron is attached or bound to a particular atom and is not free to move in the body of the insulator. As each electron stays near its parent nucleus or within its atom or molecule, and cannot move far away from it, therefore an insulator does not possess freely movable charges. Hence it fails to conduct electricity.

Note insulators are also called dielectrics. Obviously, dielectrics cannot conduct electricity however; when external electric fields applied induced charges appear on the surface of the dielectric. Hence we may define dielectrics as the insulating materials which transmit electric effects without conducting. 

When some charge is transferred to a conductor, it gets distributed readily over the entire surface of the conductor. In contract, if some charge is put on an insulator, it stays at the same place.

A nylon or plastic comb gets electrified on combing dry hair or on rubbing. But a metallic rod does not het electrified on rubbing. This is because charges on metal leak through our body to the ground as both are goods conductors of electricity.

When a charged body is brought in contact with the earth, all the excess charge on the body disappears by causing a momentary current to pass to the ground through our body. This process of sharing charges with the earth is called grounding or earthling. Earthling near the mains supply of a building is done by burying deep into earth, a thick metal plate. He electric wiring in our houses has three wires; live neutral and earth. The first two carry electric current from the power station and the third (earth) is connected to the buried metal plate. Metallic bodies of appliances like T.V, frig electric iron etc are connected to earth wire. If a live wire were to touch the metallic body, charge flows to earth without damaging the appliance and without causing injury to us. 

A gold leaf electroscope (GLE) is an instrument which is used for detecting the presence of electric charge and its polarity sign of charge. The instrument can also be used for measuring potential difference.

. LL are two extremely thin gold foils attached to lower end of a metal rod R fitted in a glass jar G through an insulating stopper S of cork or rubber etc. D is a metal disc at the free end of the metal rod. The sensitivity of the instrument is increased by pasting two tine foils F.F on the inside of glass jar opposite to the gelds leaves. 

As the gold leaves are extremely thin conducting foils which have low mass per unit area and are flexible, therefore, they respond very quickly to small electrostatic forces. Thin aluminum foils can also serve the same purpose. When a charged rod is touched with themetaldisc D, the same charge is transferred to the gold levees through the metal rod. The leaves repel eachother and diverge as shown in fig. 1 (a) 3. By measuring the divergence of leaves, the amount of charge on the body can be estimated. 


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