1 Hong Kong Institute of Vocational Education Electrical & Telecommunications Course Board Lecture Notes

2 Chapter 3 Resistance Part 1

3 Course materials Web Sites Address Internet Address: – –User ID : rlc –Password : cable

4 3.1 Resistance of Conductors (p. 60) The resistance of a material is dependent upon several factors: Type of materials Length of the conductor Cross-sectional area Temperature

5 The factors governing the resistance of a conductor at a given temperature: where  = resistivity, in ohms-meters (  -m )[Table 3-1] l = length, in meters (m) A = cross-sectional area, in square meters (m 2 )

6 Table 3-1Resistivity of Materials (p. 60) MATERIAL RESISTIVITY, , AT 20 o C (  -m) ___________________________________ Silver1.645 x Copper1.723 x Gold2.443 x Aluminum2.825 x Tungsten5.485 x Iron12.30 x Lead22 x Mercury95.8 x Nichrome x Carbon3500 x Germanium * Silicon  500* Wood Glass Mica Hard rubber Amber5 x Sulphur1 x Teflon1 x 10 16

7 Example 3-1 (p. 61) EXAMPLE 3-1Most homes use solid copper wire having a di-ameter of 1.63mm to provide electrical distribution to outlets and light sockets. Determine the resistance of 75 meters of a solid copper wire having the above diameter. SolutionWe will first calculate the cross-sectional area of the wire using equation 3-2. A= πd 2 /4 = π(1.63 x m) 2 /4 = 2.09 x m 2 Now, using Table 3-1, the resistance of the length of wire is found as R = ρl/A = (1.723 x Ωm)(75m)/2.09 x m 2 = Ω

8 Example (p. 61)

9 Question: Two copper wires with same volume has radius r 1 and r 2 respectively. Find the ratio of their resistance. Homework :Practice Problem I Learning Check I

10 Solution : Two copper wires with same volume has radius r 1 and r 2 respectively. Find the ratio of their resistance. [ In Forms of R 1, R 2 (ohms) ] Same vol=>A 1 L 1 = A 2 L 2 or Homework:Practice Problem I Learning Check I

Electrical Wire Table (p. 62) The American Wire Gauge (AWG) is the primary system used to denote wire diameters. In this system, each wire diameter is assigned a guage number. The higher the AWG number, the smaller the diameter of the cable or wire. Question: For a given length of aWG 22 gauge-wire and a given length of AWG 14 gauge-wire, which one has larger resistance?

12 Table 3-2 (p. 63)

13 3.3Resistance of Wires - Circular Mils (p. 65) 1 mil = inch The American Wire Gauge system for specifying wire diameterts was developed using a unit called the circular mil (CM), which is defined as the area contained within a circle having a diameter of 1 mil. The greatest advantage of using the circular mil to express areas of wires is the simplicity with which calculations may be made. A wire which has diameter d expressed in mils has an area in circular given as A CM = d mil 2 [ circular mils, CM ]

14 The cross-sectional area of a cable may be a large number when it is expressed I circular mils. The Roman numeral M is often used to represent If a wire has a cross-sectional area of CM, it is written as 250 MCM. Note that it is different from the SI unit, M represents 1 million. Assignment : (Ch.3) Problem 1, 5, 7, 11

Temperature Effects (p. 69) Resistance of a conductor is not constant at all temperature. As temperature increasers, more electrons will escaoe their orbits, causing additional collisions within the conductors. For most conducting materials, the increase in the number of collisions translates into an icnrease in resistance. The rate at which the resistance of a material changes with a variation in temperature is called the temperature coefficient of the material (  ). [Fig. 3.6]

16 Fig. 3-6Temp effects on the resistance of a conductor (p. 69)

17 Resistance of any material increases as temp increases - positive temp coefficient. For semiconductor materials, increases in temp allow electrons to escape their usually stable orbits and become free to move within the material - negative temp coefficient. [Table 3.4, p. 70] TABLE 3-4Temperature Intercepts and Coefficients for Common Materials  T( o C) -1 ( o C) -1 ( o C)AT20 o CAT 0 o C _____________________________________________________________ Silver Copper Aluminum Tungsten Iron Lead Nichrome Brass Platinum Carbon Germanium Silicon-0.075

18 From Fig. 3-6, slope of the line m =  R/  T = (R 1 - 0) / (T 1 - T) [unit :  / o C] Define temp. coefficient as  1 = m / R 1 [unit: ( o C) -1 ] Question: What is the sign (+ve or -ve) of the slope, m, for a) copper? b) silicon?

19 The temp coefficient will not be cosntant everywhere but is dependent upon the resistance R 1 and temp. R 2 = R 1 + m  T =R 1 + m (T 2 - T 1 ) = R 1 [ 1 + (m/R 1 )(T 2 - T 1 )] R 2 = R 1 [ 1 + (  1 )(T 2 - T 1 )]

20 EXAMPLE 3-8 (p. 71) An aluminum wire has a resistance of 20  at room temperature (20  C). Calculate the resistance of the same wire at temperatures of –40  C, 100  C, and 200  C. Solution At –40  C: From Table 3-4, we see that aluminum has a temperature coefficient of  = The resistance at –40  C is now determined to be `R –40  C = (20  ){1 + [ (  C) –1 ] [–40  C –20  C]} = 15.3  [Learning Check IV]