电子电工技术英文课件 (10).pdf

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1、1.6 Resistive Circuits 1.6.1 Resistor and Ohms Law A resistor shown in Fig.1 is a circuit element that can resist current.VIR Figure 1:Circuit model of a resistor The resistor obeys Ohms law:the voltage V across a resistor is directly proportional to the current I flowing through the resistor,that i

2、s VIR=(1)where R is the constant of proportionality,which is defined as the resistance of the resistor.We may also derive from(1)that VRI=(2)VIR=(3)1.6.2 Power of Resistors The power of a resistor is 2VPVIR=(4)It is shown that the larger the voltage,the larger power of the resistor.The power of a re

3、sistor can also be written as 2PVII R=(5)It is obviously the larger the current,the larger power of the resistor.1.6.3 Resistors in Series Fig.2 shows two resistors in series.V2V1R2R1VseriesI Figure 2:Two resistors in series According to KVL ()series1series2seri12es12IRIRVRRVIV=+=+(6)An important co

4、ncept for the resistor is equivalent resistance.We define the equivalent resistance as eq12seriesVRRRI=+(7)It shows that the equivalent resistance of two resistors in series is equal to the sum of the two resistances.Moreover,the equivalent resistance of two resistors in series is larger than that o

5、f the individual one,i.e.,eq12inseriesandRRR(8)From Fig.2,R1 and R2 divide the source voltage.The divided voltages of R1 and R2 are 11series111212VRVIRRVRRRR=+(9)22series221212VRVIRRVRRRR=+(10)We call resistors in series as voltage divider.From(9)and(10),the larger the resistance,the larger the divi

6、ded voltage.1.6.3 Resistors in Parallel Fig.3 shows two resistors in parallel.V2I1R2R1IparallelI Figure 3:Two resistors in parallel According to KCL 121212parallel11VVVRIIRRIR=+=+(11)We define the equivalent resistance as eqparallel12111VRIRR=+(12)It is obvious that eq121212111in parallel(and 1111RR

7、RRRRR=+）(13)This means that the equivalent resistance of two resistors in parallel is smaller than the individual resistance one.The inverse of the resistance R is defined as the conductance G:1GR=(14)It is often more convenient to use conductance rather than resistance when dealing with resistors in parallel.For example,(12)may be simplified as eq12GGG=+(15)