1	Chapter 3 Resistance
2	Resistance of Conductors Resistance of material is dependent on several factors: Type of Material Length of the Conductor Cross-sectional area Temperature
3	Type of Material Atomic differences of materials cause variations in how electron collisions affect resistance Differences produce resistivity
4	Type of Material Represented by the symbol r (Greek letter rho) Units of r Ohms x meters (Ω∙m) or (circular mils x ohms)/feet (Ω∙CM/ft)
5	Length Resistance of a conductor Directly proportional to its length If you double the length of the wire, the resistance will double l = length In meters or feet
6	Area Resistance of a conductor Inversely proportional to cross-sectional area of the conductor If cross-sectional area is doubled Resistance will be one half as much
7	Area A = Cross-sectional area, in m² or circular mils (CM)
8	Resistance Formula At a given temperature (usually 20 ^oC) Formula can be used with both circular and rectangular conductors
9	Electrical Wire Tables American Wire Gauge is primary system to denote wire diameters The higher the AWG number, the smaller the diameter
10	Electrical Wire Tables A given length of AWG 22 wire will have more resistance than the same length of AWG 14 wire Larger gauge wires can handle more current
11	Circular Mils (CM) Length may also be in mils (0.001 inch) Area may be in circular mils (CM) 1 CM Area of a circle having a diameter of 1 mil
12	Circular Mils (CM) 1 square mil Area of a square having sides of 1 mil 1 CM = p/4 square mils
13	Temperature Effects For most conductors, a temperature increase causes an increase in resistance Increase is relatively linear In semiconductors and insulators Increase in temperature results in decrease in resistance
14	Temperature Effects Any material for which the resistance increases with temperature is said to have a positive temperature coefficient If it decreases, it has a negative coefficient
15	Temperature Effects Temperature coefficient Rate of change of resistance with respect to temperature It is represented by a (Greek letter alpha)
16	Temperature Effects Resistance at a specific temperature (R) may be calculated from resistance at a different temperature (R₁) by the formula:
17	Temperature Effects Where ΔT = T – T₁ is the difference between the two temperatures in Celsius degrees
18	Fixed Resistors Resistance of a fixed resistor is constant over a wide temperature range Rated by amount of resistance Measured in ohms (Ω) Also rated by power Measured in watts (W)
19	Fixed Resistors Different resistors for different applications Molded carbon composition Carbon film Metal film Metal Oxide Wire-Wound Integrated circuit packages
20	Variable Resistors Resistance may be changed (varied) Adjust volume, set level of lighting, adjust temperature Have three terminals Center terminal connected to wiper arm Potentiometers Rheostats
21	Color Code Colored bands on a resistor provide a code for determining Value Tolerance Reliability
22	Measuring Resistance Ohmmeter Remove all power sources to circuit Isolate component Connect probes across component No need to worry about polarity Ohmmeter determines shorts and opens
23	Thermistors Two-terminal transducer Resistance changes with temperature Applications include electronic thermometers and thermostatic control circuits for furnaces
24	Thermistors Most have negative temperature coefficients
25	Photoconductive Cells Two-terminal transducers Resistance determined by amount of light May be used to measure light intensity or to control lighting Used in security systems Linear response (negative slope)
26	Diodes Semiconductor devices Conduct in one direction only In forward direction, has very little resistance In reverse direction, resistance is very high Open circuit
27	Varistors Resistors sensitive to voltage High resistance when voltage is below breakdown value Low resistance when voltage is above breakdown value High power ratings When used in surge protectors
28	Conductance Measure of a material’s ability to allow flow of electrical current Conductance is reciprocal of resistance G = 1/R Unit is siemens (S)
29	Superconductors Low temperatures Resistance of some materials goes to almost zero Temperature is called critical temperature
30	Superconductors Meissner Effect Cooled below its critical temperature Magnetic fields may surround but not enter the superconductor