These laws known as ohms law and kirchhoffs laws form

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Unformatted text preview: uits. These laws, known as Ohm’s law and Kirchhoff’s laws, form the foundation upon which electric circuit analysis is built. In this chapter, in addition to these laws, we shall discuss some techniques commonly applied in circuit design and analysis. These techniques include combining resistors in series or parallel, voltage division, current division, and delta-to-wye and wye-to-delta transformations. The application of these laws and techniques will be restricted to resistive circuits in this chapter. We will finally apply the laws and techniques to real-life problems of electrical lighting and the design of dc meters. 2.2 OHM’S LAW l i Material with resistivity r Cross-sectional area A (a) Figure 2.1 + v − R Materials in general have a characteristic behavior of resisting the flow of electric charge. This physical property, or ability to resist current, is known as resistance and is represented by the symbol R . The resistance of any material with a uniform cross-sectional area A depends on A and its length , as shown in Fig. 2.1(a). In mathematical form, R=ρ (b) (a) Resistor, (b) Circuit symbol for resistance. (2.1) A where ρ is known as the resistivity of the material in ohm-meters. Good conductors, such as copper and aluminum, have low resistivities, while insulators, such as mica and paper, have high resistivities. Table 2.1 presents the values of ρ for some common materials and shows which materials are used for conductors, insulators, and semiconductors. TABLE 2.1 Material Silver Copper Aluminum Gold Carbon Germanium Silicon Paper Mica Glass Teflon Resistivities of common materials. Resistivity ( ·m) Usage 1.64 × 10−8 1.72 × 10−8 2.8 × 10−8 2.45 × 10−8 4 × 10−5 47 × 10−2 6.4 × 102 1010 5 × 1011 1012 3 × 1012 Conductor Conductor Conductor Conductor Semiconductor Semiconductor Semiconductor Insulator Insulator Insulator Insulator | v v The circuit element used to model the current-resisting behavior of a material is the resistor. For the purpose of constructing circuits, resistors are usual...
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