chapter 3 resistance. 2 resistance of conductors resistance of material is dependent on several...
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Chapter 3
Resistance
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Resistance of Conductors• Resistance of material is dependent on
several factors:– Type of Material– Length of the Conductor– Cross-sectional area– Temperature
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Type of Material• Atomic differences of materials cause
variations in how electron collisions affect resistance
• Differences produce resistivity
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Type of Material• Represented by the symbol
– (Greek letter rho)
• Units of – Ohms x meters (Ω∙m) or (circular mils x
ohms)/feet (Ω∙CM/ft)
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Length
• Resistance of a conductor– Directly proportional to its length– If you double the length of the wire, the
resistance will double
• = length– In meters or feet
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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
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Area
• A =– Cross-sectional area, in m2 or circular mils
(CM)
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Resistance Formula• At a given temperature (usually 20 o C)
• Formula can be used with both circular and rectangular conductors
AR
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Electrical Wire Tables
• American Wire Gauge is primary system to denote wire diameters
• The higher the AWG number, the smaller the diameter
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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
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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
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Circular Mils (CM)• 1 square mil
– Area of a square having sides of 1 mil
• 1 CM = /4 square mils
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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
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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
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Temperature Effects• Temperature coefficient
– Rate of change of resistance with respect to temperature
• It is represented by (Greek letter alpha)
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Temperature Effects• Resistance at a specific temperature (R)
may be calculated from resistance at a different temperature (R1) by the formula:
TRR 11
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Temperature Effects• Where ΔT =
– T – T1 is the difference between the two temperatures in Celsius degrees
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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)
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Fixed Resistors• Different resistors for different applications
– Molded carbon composition– Carbon film– Metal film– Metal Oxide– Wire-Wound– Integrated circuit packages
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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
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Color Code• Colored bands
on a resistor provide a code for determining– Value– Tolerance– Reliability
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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
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Thermistors
• Two-terminal transducer – Resistance changes with temperature
• Applications include electronic thermometers and thermostatic control circuits for furnaces
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Thermistors
• Most have negative temperature coefficients
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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)
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Diodes• Semiconductor devices
– Conduct in one direction only– In forward direction, has very little resistance– In reverse direction, resistance is very high
• Open circuit
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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
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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)
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Superconductors• Low temperatures
– Resistance of some materials goes to almost zero
• Temperature is called critical temperature
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Superconductors• Meissner Effect
– Cooled below its critical temperature– Magnetic fields may surround but not enter the
superconductor