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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 deltatowye and wyetodelta 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
reallife problems of electrical lighting and the design of dc meters. 2.2 OHM’S LAW
l i Material with
resistivity r
Crosssectional
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 crosssectional 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 ohmmeters. 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 currentresisting behavior of
a material is the resistor. For the purpose of constructing circuits, resistors
are usual...
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 Spring '12
 bkav

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