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In all of the sciences, it is common to minimize the variables to facilitate understanding and simplify calculation, if only for the moment. With that in mind, consider impedance as the full-color version of resistance. The series parallel circuit examples in this article consisted mostly of a battery and resistors—certainly not the real world, so black and white that there wasn’t even a power switch! AC voltages were treated like DC, except when calculating peak and RMS power.
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Next month’s math takes time into consideration, but don’t get nervous about it. A demo version of “Micro-Cap 6” is available at www.spectrumsoft.com as a free download. Just draw a schematic and it does the ciphering. Meanwhile, let’s look at the basic concepts comparing resistance to impedance.
SPECIFIC HEIGHTS
Resistance is a scalar quantity measured in ohms, just as the term “height” is defined by its magnitude (inches, feet, centimeters, meters) and no more. By contrast, temperature is defined by its magnitude—degrees—which alone does not tell the whole story. Wind can make a 20° day feel like 10° (wind chill), while humidity can make 70° feel like 80° (the yuck factor). Like the temperature example, impedance is a vector quantity defined by its magnitude in ohms, but instead of wind chill, there is a phase angle, a manipulation of time.
I-C-E is N-I-C-E
Remember that DC can be stored over long periods of time in batteries and to a lesser extent in capacitors. Add a switch to the series or parallel resistor examples. At the moment, the voltage E is applied to the resistance R no matter whether the source is AC or DC, the current will be instantaneous.
Reactive components—capacitors and inductors—manipulate the time relationship between voltage and current. A fully discharged capacitor appears as a dead short at the moment it is connected to a DC voltage source, the current leading the voltage by 90°. In engineering school, simple expressions such as “E-L-I the I-C-E man” helped students remember that voltage-leads-current by 90° in an inductor L (a coil of wire).
SPARK!
Have you ever plugged or unplugged a device when its switch was in the On position? At the exact moment the plug and socket made or broke the connection, a sizeable spark most likely occurred. Connecting AC power to a reactive device such as a transformer is one of the reasons lights dim but don’t stay dim when a device is turned on.
ABSOLUTE ZERO
Getting back to the temperature analogy for a moment, the materials used for making or plating wire—copper, silver, gold or aluminum—all have a defined resistance at room temperature. Absolute zero on the Kelvin scale (0 K) is the lowest temperature theoretically possible—at approximately -273.16° C (-459.69° F). Wire that cold becomes a more perfect conductor. That’s why audiophiles love winter here in Minnesota!
In reality, the slight amount of resistance per foot becomes cumulative with extremes of distance or of thinness. For example, speaker cable consists of a pair of conductors separated by insulation, a capacitor by definition that becomes a contributing factor as the series resistance increases. At high frequencies, the wire also has some inductance. Hold that thought…
Next month, the circuit examples will include capacitors and inductors in real-world examples of signal corruption and failing components. In the meantime, drop by www.tangible-technology.com for a visit.
This past winter, Eddie shoveled 65 inches of snow. By the time you read this, he’ll be planting a vegetable garden. Got any seeds?
Reprinted with permission from
Magazine, May, 2001© 2000, Intertec Publishing, A Primedia Company All Rights Reserved
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