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Conductors and Insulators

Chapter. 11. Conductors and Insulators. Topics Covered in Chapter 11 11-2: Standard Wire Gage Sizes 11-8: Wire Resistance 11-9: Temperature Coefficient of Resistance. 11-2: Standard Wire Gage Sizes. Sizes are specified by the American Wire Gage (AWG) system (p. 317, Table 11-1).

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Conductors and Insulators

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  1. Chapter 11 Conductors and Insulators Topics Covered in Chapter 11 11-2: Standard Wire Gage Sizes 11-8: Wire Resistance 11-9: Temperature Coefficient of Resistance

  2. 11-2: Standard Wire Gage Sizes • Sizes are specified by the American Wire Gage (AWG) system (p. 317, Table 11-1). • Higher gage numbers mean thinner wire. • Typical sizes are 22 AWG for electronic hookup wire and 12 AWG for home electrical wiring. • The cross-sectional area of round wire is measured in circular mils. • The higher the gage number and the thinner the wire, the greater its resistance for any length.

  3. Diameter in mils = 25.35 11-2: Standard Wire Gage Sizes Wire Size p. 317 Table 11-1 Circular mil area = Diameter2 = 642.4 R / 1000ft of copper wire @ 25C = 16.46 

  4. 11-2: Standard Wire Gage Sizes • Wire Size • The circular area of the wire doubles for every three gage sizes. • # 19 is three gages larger than # 22 and has approximately twice the circular mil area. This is always the case when the gage number is decreased by 3.

  5. 11-2: Standard Wire Gage Sizes • Determine the resistance of a 30 feet section of #30 wire. • Determine the maximum length of a #25 cable if the wire resistance must be below 5 . • The resistance of a 200ft section cable must be less than 2 W, determine the required wire gage #.

  6. 11-8: Wire Resistance • Wire Resistance • Resistance is proportional to the length of the wire. • The resistance of a conductor can be found by the formula: ρ = specific resistance of the conductor R = ρ(l / A) cross-section of the wire length of the wire

  7. 11-8: Wire Resistance • Specific Resistance • Specific resistance = ρ = CMA [circular mil area] •Ω/ft • Resistance of a conductor = R = ρ (length/CMA) Find R for 1000 ft. of #18 cu R = ρ (length/CMA) R = 10.4 (1000/1624) R = 6.4 Ω

  8. 11-9: Temperature Coefficient of Resistance • Temperature coefficient of resistance indicates how much the resistance changes for a change in temperature. It is indicated by the alpha symbol (α). • A positive α value means R increases with temperature. • A negative α value means R decreases with temperature. • A value of 0 means R stays constant.

  9. 11-9: Temperature Coefficient of Resistance • α is generally positive for pure metals. • α is generally negative for semiconductors (silicon, germanium) and electrolyte solutions (sulfuric acid, water). • The increase in resistance may be calculated using the formula: Rt = R0 + R0(αΔt) • R0 = the resistance at 20 °C. • Rt = the resistance at the higher temperature • Δt = the temperature rise over 20° C.

  10. Positive Temperature Coefficient () Some devices show a large increase in resistance when energized. What’s the lamp’s resistance at 2020 °C? Rt = R0 + R0(t) Rt = 2 Ω + 2 Ω× 0.005 × 2000 = 22 Ω 11-9: Temperature Coefficient of Resistance

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