| 5/All substances have some ability of conducting the electric current; however, they differ greatly in the ease with which the current can pass through them. Metals, for example, conduct electricity with ease while rubber does not allow it to flow freely. Thus, we have conductors and insulators. What do the terms conductors" and insulators mean? Substances through which electricity is easily transmitted are called conductors. Any material that strongly resists the electric current flow is known as an insulator. Let us first turn our attention to conductance that is the conductors ability of passing electric charges. The four factors conductance depends on are: the size of the wire used, its length and temperature as well as the kind of material to be employed. It is not difficult to understand that a large water pipe can pass more water than a small one. In the same manner, a large conductor will carry the current more readily than a thinner one. Fig. 4 illustrates this fact better than words alone. It is quite understandable, too, that to flow through a short conductor is certainly easier for the current than through a long one in spite of their being made of similar material. Hence, the longer the wire, the greater is its opposition, that is resistance, to the passage of current As mentioned above, there is a great difference in the conducting ability of various substances. For example, almost all metals are good electric current conductors. Nevertheless, copper carries the current more freely than Iron; and silver, in its turn, is a better conductor than copper. Generally speaking, copper is the most widely used conductor. That is why the electrically operated devices in your home are connected to the wall socket by copper wires, Indeed, if you are reading this book by an electric lamp light and somebody pulls the metal wire out of the socket, * the light will go out at once. The electricity has not been turned off but it has no path to travel from the socket to your electric lamp. The flowing electrons cannot travel through space and get into an electrically operated device when the circuit is broken. If we use a piece of string instead of a metal wire, we shall also find that the current stops flowing. A material like string which resists the flow of the electric current is called an insulator. There are many kinds of insulation used to cover the wires. The kind used depends upon the purposes the wire | , . . ? . . . 4 : , . . . 4 . . . . . . . , . . . , . . . . . . |
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Conductors
| Conductors are materials having a low resistance, so that current easily passes through them. The lower the resistance of the material the more current can pass through it. The mst common conductors are metals, and silver and copper are the best of them. The advantage of copper is that it is much cheaper than silver. Thus copper is widely used to produce wire conductors. One of the common functions of wire conductors is to connect a voltage source to a load resistance. Since copper wire conductors have a very low resistance a minimum voltage drop is produced in them. Thus, all of the applied voltage can produce current in the load resistance. It should be taken into consideration that most materials change the value of resistance when their temperature changes. Metals increase their resistance when the temperature increases while carbon decreases its resistance when the temperature increases. Thus, metals have a positive temperature coefficient of resistance while carbon has a negative temperature coefficient. The smaller is the ternperature coefficient or the less the change of resistance with the change of temperature, the more perfetct is the resistance material. | , . . , . , . . - . . , . . . . |
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| 6.ON SEMICONDUCTORS A semiconductor is often defined as an electric conductor that has a conductivity intermediate between that of an insulator and that of metal. The more important semiconductors are: boron, germanium, silicon, selenium, phosphorus, gray tin and others. The mechanical properties of semiconductors vary greatly. However, in hardness, brittleness, and fracture strength, semiconducting crystals resemble insulating crystals more than they do metals. Besides their electric properties, which in themselves may be of great variety, semiconductors vary in such physical qualities as magnetism, specific heat and thermal conductivity. Semiconductors are widely used in electronics. They challenge vacuum tubes in many applications in the electronic industry. Engineers and physicists are going to solve many engineering problems by means of semiconductors. | . , , , , , . . , . , , - . . . . |
| SEMICONDUCTORS A transistor is an active semiconductor device with three or more electrodes. By active we mean that the transistor is capable of current gain, voltage, amplification and power gain. A transistor is an electron device in which electronic conduction takes place within a semiconductor. A semiconductor is an electric conductor with resistivity in the range between metals and insulators, in which the electrical charge carrier concentration increases with increasing temperature over some temperature range. The resistivities of semiconductors and insulators decrease rapidly with rising temperatures, while those of metals increase relatively slowly. Unlike metals and insulators, the resistivity of semiconductors depends upon the direction of current flow. The direction of easiest current flow or lowest resistivity is called the forward direction, the direction of restricted current flow or highest resistivity is known as the reverse or back direction. Semiconductors, such as the elements germanium and silicon, possess two types of current carriers, namely, negative electrons and positive holes. A hole is a mobile vacancy in the electronic valence structure of a semiconductor which acts like a positive electronic charge with a positive mass. . | 3 . , - , . , , , , . . , () () ( ). . |
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A circuit
| This is a circuit. Its elements are a voltage source, a resistor and a conductor. The circuit consists of a voltage source, a resistor and a conductor. A voltage source supplies current. A resistor reduces current. A conductor connects the elements of the circuit. Compare circuit a with circuit b. What is the difference between them? Current passes through circuit a while no current passes through circuit b. Circuit b has an open. No current through circuit b results from an open. An open and a short are troubles in a circuit. A trouble in a circuit may result in no current in it. An Open in a Parallel Circuit An open in any circuit results in no current. In parallel circuits there is a difference between an open in the main line and an open in a branch: an open in the mainline of a circuit of this type results in no current in the whole circuit while an open in a parallel branch results in no current in that branch only. Let us take 2 parallel circuits a and b and compare them. Circuit a and circuit b are used to light bulbs. The first circuit consists of the main line and three parallel branches. No bulb connected across this circuit can light since it has an open in the main line. Circuit b consists of the same elements as circuit a. It has an opennot in the mainline but in one of the branches. Since the open branch has no current, a bulb connected across it cannot light. In all the other branches current passes, since each branch is connected to the voltage source. Bulbs connected across these branches can light. This shows the advantage of a parallel circuit, since an open in one element opens only one branch while all the other branches can operate. | . - , . . . . . ? . . - . . . . . . . . . . . . . , . , , .. . . , , |
