| Resistance |
| Resistance of material is dependent on several factors: | ||
| Type of Material | ||
| Length of the Conductor | ||
| Cross-sectional area | ||
| Temperature | ||
| Atomic differences of materials cause variations in how electron collisions affect resistance | |
| Differences produce resistivity |
| 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) | ||
| 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 | ||
| 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 | ||
| A = | ||
| Cross-sectional area, in m2 or circular mils (CM) | ||
| At a given temperature (usually 20 o C) | |
| Formula can be used with both circular and rectangular conductors |
| American Wire Gauge is 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 |
| 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 | ||
| 1 square mil | ||
| Area of a square having sides of 1 mil | ||
| 1 CM = | ||
| p/4 square mils | ||
| 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 | ||
| 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 |
| Temperature coefficient | ||
| Rate of change of resistance with respect to temperature | ||
| It is represented by a (Greek letter alpha) | ||
| Resistance at a specific temperature (R) may be calculated from resistance at a different temperature (R1) by the formula: | |
| Where ΔT = | ||
| T – T1 is the difference between the two temperatures in Celsius degrees | ||
| 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) | ||
| Different resistors for different applications | ||
| Molded carbon composition | ||
| Carbon film | ||
| Metal film | ||
| Metal Oxide | ||
| Wire-Wound | ||
| Integrated circuit packages | ||
| Resistance may be changed (varied) | ||
| Adjust volume, set level of lighting, adjust temperature | ||
| Have three terminals | ||
| Center terminal connected to wiper arm | ||
| Potentiometers | ||
| Rheostats | ||
| Colored bands on a resistor provide a code for determining | ||
| Value | ||
| Tolerance | ||
| Reliability | ||
| Ohmmeter | |
| Remove all power sources to circuit | |
| Isolate component | |
| Connect probes across component | |
| No need to worry about polarity | |
| Ohmmeter determines shorts and opens |
| Two-terminal transducer | ||
| Resistance changes with temperature | ||
| Applications include electronic thermometers and thermostatic control circuits for furnaces | ||
| Most have negative temperature coefficients |
| 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) | ||
| Semiconductor devices | |||
| Conduct in one direction only | |||
| In forward direction, has very little resistance | |||
| In reverse direction, resistance is very high | |||
| Open circuit | |||
| 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 | ||
| Measure of a material’s ability to allow flow of electrical current | |
| Conductance is reciprocal of resistance | |
| G = 1/R | |
| Unit is siemens (S) |
| Low temperatures | ||
| Resistance of some materials goes to almost zero | ||
| Temperature is called critical temperature | ||
| Meissner Effect | ||
| Cooled below its critical temperature | ||
| Magnetic fields may surround but not enter the superconductor | ||