电子电工技术英文课件 (10).pdf
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 is 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 resistor 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 concept 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 of 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 divided 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 1111RRRRRRR=+）(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)
