True power doesn’t travel

Chapter True power doesn’t travel

Teach Yourself Electricity and Electronics Third Edition Book
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Summary of Contents

Teach Yourself Electricity and Electronics Third Edition Book

  • Imaginary powerIf an ac circuit contains reactance, things get interesting. The rate of energy expendi-ture is the same as the VA power in a pure resistance. But when inductance and/or ca-pacitance exists in an ac circuit, these two definitions of power part ways. The VApower becomes greater than the power actually manifested as heat, light, radio waves,or whatever. The extra “power” is called imaginary power, because it exists in the re-actance, and reactance can be, as you have learned, rendered in mathematically imagi-nary numerical form. It is also known as reactive power.Inductors and capacitors store energy and then release it a fraction of a cycle later.This phenomenon, like true power, is expressible as the rate at which energy ischanged from one form to another. But rather than any immediately usable form ofpower, such as radio or sound waves, imaginary power is “stashed” as a magnetic orelectric field and then “dumped” back into the circuit, over and over again.You might think of the relationship between imaginary and true power in the sameway as you think of potential versus kinetic energy. A brick held out of a seventh-storywindow has potential energy, just as a charged-up capacitor or inductor has imaginarypower.Although the label “imaginary power” carries a connotation that it’s not real or im-portant, it’s significant indeed. Imaginary power is responsible for many aspects of accircuit behavior.True power doesn’t travelAn important semantical point should be brought up concerning true power, not only inac circuits, but in any kind of circuit or device.A common and usually harmless misconception about true power is that it “trav-els.” For example, if you connect a radio transmitter to a cable that runs outdoors to an True power doesn’t travel30717-2Peak versus effective power for a sine wave.