Electrical Engineering ⇒ Topic : Conductors , insulators, semiconductors and superconductors
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Gopal
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CONDUCTORS AND INSULATORS Conductors are substances that permit free motion of a large number of electrons. Copper wire is a good conductor of electricity because it has many free electrons. Electrical energy is transported through conductors by the movement of free electrons inside the conductor. The greater the number of electrons that move in a material, the better are its conducting properties. A good conductor will have a large number of free electrons and will offer a very low resistance to the current (electron) flow. Some good conductors of electricity which are in use are copper, aluminium, etc. Some substances like rubber, glass and wood are not good conductors of electricity. In these materials, large amount of energy is required to be spent to make the electrons come out of the atom. Such substances which have very few free electrons are called insulators. Good conductors are used as wires to carry current and good insulators are used to prevent current from flowing into unwanted areas. Some of the best conductors and best insulators are listed in Table (1) on the next page. TABLE (1) Some of the best conductors and best insulators Although, generally, all matter can be divided into conductors an insulators, it should be realised that these are only relative terms. No material is a perfect insulator and no material is a perfect conductor. In general, copper and silver are very good conductors, whereas paper, rubber, glass, etc. are good insulators.During recent years, semiconducting materials, such as silicon and germanium,whose electrical properties lie between those of conductors and insulators have become very important. Semiconductors in suitable form are widely used in transistors, rectifiers and integrated circuits | |
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Sonali
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Conductors are materials in which large number of free electrons are available, i.e., the electrons in outermost orbit of the atoms are not tightly bound to the nucleus. Almost all metals belong to this category. In practice, copper and aluminium are widely used as conductors. All materials with resistivity less than 10-3 Ωm come under this category. Insulators are the materials in which free electrons are not available, i.e., the electrons in outer orbits are tightly bound to the nucleus. It is very difficult to dislodge the electrons from their outermost orbit. So, current cannot flow through an insulator. Materials with resistivity above 105 Ωm fall in this category Other materials like germanium and silicon are neither good conductors nor good insulators.They are known as semiconductors. In these materials, the electrons in the outermost orbit areheld by nucleus but can be taken out by adding impurity in these materials. The resistivity of semiconductors has values in between 10-3 Ωm and 105 Ωm. The resistance decreases rapidly at temperatures near the absolute zero and the substance becomes a superconductor. For example, a current in a ring of lead may be kept flowing for several days without the support of any external driving source at a temperature close to absolute zero | |
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