Download Resistance - Pui Chor Wong

Chapter 3
Resistance
Resistance of Conductors
• Resistance of material is dependent
on several factors
• Type of Material
• Length of the Conductor
• Cross-sectional area
• Temperature
Type of Material
• Differences at the atomic level of
various materials will cause variations in
how the collisions affect resistance.
• These differences are called the
resistivity.
• We use the symbol .
• Units are ohm-meters.
Length
• The resistance of a conductor is directly
proportional to the length of the
conductor.
• If you double the length of the wire, the
resistance will double.
•  = length, in meters.
Area
• The resistance of a conductor is
inversely proportional to the crosssectional area of the conductor.
• If the cross-sectional area is doubled,
the resistance will be one half as much.
• A = cross-sectional area, in m2.
Resistance Formula
• At a given temperature,

R
A
• This formula can be used with both
circular and rectangular conductors.
Electrical Wire Tables
• The American Wire Gauge is the primary
system to denote wire diameters.
• The higher the AWG number, the smaller the
diameter.
• 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.
Circular Mils
• Diameter is expressed in circular mils.
• 1 CM is defined as area of a circle
having a diameter of 1 mil (0.001 inch).
• A square mil is the area of a square
having sides 1 mil long.
• 1 CM = /4 square mils
Temperature Effects
• For most conductors, an increase in
temperature causes an increase in
resistance.
• This increase is relatively linear.
• In semiconductors, an increase in
temperature results in a decrease in
resistance.
Temperature Effects
• The rate of change of resistance with
temperature is called the temperature
coefficient ().
• Any material for which the resistance
increases as temperature increases is said to
have a positive temperature coefficient. If it
decreases, it has a negative coefficient.
R  R1 1  T 
Fixed Resistors
• Resistances essentially constant.
• Rated by amount of resistance,
measured in ohms.
• Also rated by power ratings, measured
in watts.
Fixed Resistors
• Different types of resistors are used for
different applications.
• Molded carbon composition
• Carbon film
• Metal film
• Metal Oxide
• Wire-Wound
• Integrated circuit packages
Variable Resistors
• Used to adjust volume, set level of
lighting, adjust temperature.
• Have three terminals.
• Center terminal connected to wiper arm.
• Potentiometers
• Rheostats
Color Code
• Colored bands on a resistor provide a code
for determining the value of resistance,
tolerance, and sometimes the reliability.
Measuring Resistance
• Remove all power sources to the circuit.
• Component must be isolated from rest
of the circuit.
• Connect probes across the component.
• No need to worry about polarity.
• Useful to determine shorts and opens.
Thermistors
• A two-terminal transducer in which the
resistance changes with change in
temperature.
• Applications include electronic
thermometers and thermostatic control
circuits for furnaces.
• Have negative temperature coefficients.
Photoconductive Cells
• Two-terminal transducers which have a
resistance determined by the amount of
light falling on them.
• May be used to measure light intensity
or to control lighting.
• Used as part of security systems.
Conductance
• The measure of a material’s ability to
allow the flow of charge.
• Conductance is the reciprocal of
resistance.
• G = 1/R
• Unit is siemens.
Superconductors
• At very low temperatures, resistance of
some materials goes to almost zero.
• This temperature is called the critical
temperature.
• Meissner Effect - When a
superconductor is cooled below its
critical temperature, magnetic fields
may surround but not enter the
superconductor.