Conductance and the siemens

Chapter Conductance and the siemens

Teach Yourself Electricity and Electronics Third Edition Book
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Summary of Contents

Teach Yourself Electricity and Electronics Third Edition Book

  • Resistance has another property in an electric circuit. If there is a current flowingthrough a resistive material, there will always be a potential difference across the resis-tive object. This is shown in Fig. 2-4. The larger the current through the resistor, thegreater the EMF across the resistor. In general, this EMF is directly proportional to thecurrent through the resistor. This behavior of resistors is extremely useful in the designof electronic circuits, as you will learn later in this book.28 Electrical units2-4Whenever a resistance carries a current, there is a voltage across it.Electrical circuits always have some resistance. There is no such thing as a perfectconductor. When some metals are chilled to extremely low temperatures, they losepractically all of their resistance, but they never become absolutely perfect, resistance-free conductors. This phenomenon, about which you might have heard, is called superconductivity. In recent years, special metals have been found that behave thisway even at fairly moderate temperatures. Researchers are trying to concoct sub-stances that will superconduct even at room temperature. Superconductivity is an active field in physics right now.Just as there is no such thing as a perfectly resistance-free substance, there isn’t atruly infinite resistance, either. Even air conducts to some extent, although the effect isusually so small that it can be ignored. In some electronic applications, materials are selected on the basis of how nearly infinite their resistance is. These materials make goodelectric insulators, and good dielectrics for capacitors, devices that store electric charge.In electronics, the resistance of a component often varies, depending on the condi-tions under which it is operated. A transistor, for example, might have extremely highresistance some of the time, and very low resistance at other times. This high/low fluc-tuation can be made to take place thousands, millions or billions of times each second.In this way, oscillators, amplifiers and digital electronic devices function in radio re-ceivers and transmitters, telephone networks, digital computers and satellite links (toname just a few applications).Conductance and the siemensThe better a substance conducts, the less its resistance; the worse it conducts, thehigher its resistance. Electricians and electrical engineers sometimes prefer to speak