Topics Covered in Chapter 11 Function of the conductor Standard wire gage sizes Types of wire conductors Connectors Printed wiring
Switches Fuses Wire resistance Temperature coefficient of resistance Topics Covered in Chapter 11 (continued)
Topics Covered in Chapter 11 (continued) Ion current in liquids and gases Electrons and hole charges in semiconductors Insulators
Ideal conductors have no resistance. 12 V Cranking current = 150 A Vehicle cranking motor Real conductors have resistance. If the conductor resistance above is 0.03 , only 7.5 volts will be available at the motor! This is not acceptable (the I 2 R conductor loss is 675 W). Large diameter wire is needed in high current circuits. The larger the diameter, the lower the resistance.
Types of Electrical Materials Conductors Low resistance, conduct very well Insulators Very high resistance, conduct very poorly Semiconductors Higher resistance than conductors, but lower resistance than insulators Superconductors Virtually no resistance
Conductors Conductors have a very low resistance. All metals are good conductors. The resistance of pure metals increases with temperature.
Positive Temperature Coefficient ( ) Some devices show a large increase in resistance when energized. R t = R 0 + R 0 ( t) Tungsten filament ( = 0.005) R LAMP = 2 at 20 °C What’s the lamp’s resistance at 2020 °C? R t = 2 + 2 x 0.005 x 2000 = 22
Wire Conductors Sizes Sizes are specified by the American Wire Gage (AWG). 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.
# 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. Insulation 0.02535 inches Wire Size A mil is 0.001 inches. Circular mil area = [Diameter in mils] 2 = 25.35 2 = 643 Diameter in mils = = 25.35 0.02535 0.001 GageCMA 172048 181624 191288 201022 21810 22643 #22 wire
Wire Conductors Resistance The resistance of a conductor can be found by the formula: R = (l/A) Resistance is proportional to the length of the wire. Resistance is inversely proportional to the cross-sectional area of the wire.
Specific Resistance Resistance of a conductor = R = (length/CMA) Specific resistance = = CMA /ft GageCMA 172048 181624 191288 201022 21810 22643 Material Aluminum17 Copper10.4 Iron58 Nichrome676 Silver9.8 Tungsten33.8 Find R for 1000 ft. of #18 cu R = (length/CMA) R = (1000/1624) R =
Ions A negative ion is an atom that has an excess number of electrons. A positive ion is an atom that is missing one or more electrons. Ions are the electrical charge carriers in liquids and gasses.
Negative ions are formed when an additional electron is forced into the valence orbit. The overall charge of the atom is now negative.
Positive ions are formed when an electron is removed from the valence orbit. The overall charge of the atom is now positive.
Semiconductors There are two types of semiconductor materials, P and N. They are formed by doping otherwise pure silicon with trivalent and pentavalent impurities. A P-type material conducts by means of positive hole charges (doped with a trivalent impurity) An N-type material conducts by means of electron charges (doped with a pentavalent impurity)
Semiconductors (continued) The charge on a semiconductor hole or electron is 0.16 10 -18 C. Resistance decreases with increasing temperature. (The opposite of metals)
One Atom of Silicon Valence = 4 (unstable) 8 is a stable number
The covalent sharing satisfies the rule of 8 and the structure is stable. Silicon Crystal (Covalent Bonds) A poor conductor at room temperature
Doped Crystals (N-type and P-type) Phospherous has 5 valence electrons (extra electron) Aluminum has 3 valence electrons (missing electron) N - type P - type Ph Al
Insulators Insulators have very high resistance. Common insulator materials include: air and vacuum rubber and paper porcelain, and plastics Insulators are also called dielectrics.
Insulation isolates conductors from each other and from other objects, including people. Multi conductor cable Ribbon cable
MaterialDielectric strength (Volts/mil) Air20 Glass335 to 2000 Mica600 to 1500 Polystyrene500 to 760 Porcelain40 to 150 Hard rubber450 Insulation Materials Power transmission lines can operate as high as 1 million volts. What is the length of the arc path? 1 M/20 = 50,000 mils = 4.17 feet.