Chapter Phase

Teach Yourself Electricity and Electronics Third Edition Book
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Summary of Contents

Teach Yourself Electricity and Electronics Third Edition Book

  • 12CHAPTERPhaseAN ALTERNATING CURRENT REPEATS THE SAME WAVE TRACE OVER AND OVER.Each 360-degree cycle is identical to every other. The wave can have any imaginableshape, but as long as the polarity reverses periodically, and as long as every cycle is thesame, the wave can be called true ac.In this chapter, you’ll learn more about the most common type of ac, the sine wave.You’ll get an in-depth look at the way engineers and technicians think of ac sine waves.There will be a discussion of the circular-motion model of the sine wave. You’ll see howthese waves add together, and how they can cancel out.Instantaneous voltage and currentYou’ve seen “stop motion” if you’ve done much work with a video-cassette recorder(VCR). In fact, you’ve probably seen it if you’ve watched any television sportscasts.Suppose that it were possible for you to stop time in real life, any time you wanted. Thenyou could examine any instant of time in any amount of detail that would satisfy yourimagination.Recall that an ac sine wave has a unique, characteristic shape, as shown in Fig. 12-1. This is the way the graph of the function ysin x appears on the coordinateplane. (The abbreviation sin stands for sine in trigonometry.) Suppose that the peakvoltage is plus or minus 1 V, as shown. Further imagine that the period is 1 second, sothat the frequency is 1 Hz. Let the wave begin at time t0. Then each cycle beginsevery time the value of t is a whole number; at every such instant, the voltage is zero andpositive going.If you freeze time at t446.00 seconds, say, the voltage will be zero. Looking at thediagram, you can see that the voltage will also be zero every so-many-and-a-half sec-onds; that is, it will be zero at t446.5 seconds. But instead of getting more positive atthese instants, the voltage will be swinging towards the negative.215Copyright © 2002, 1997, 1993 by The McGraw-Hill Companies. Click here for terms of use.