Consider a voltage source with output Eo = 2 + sin(w t) driving a capacitor as its load. The voltage does not alternate but the current will.
-- --Larry Brasfield email: donotspam_larry_brasfield@hotmail.com Above views may belong only to me.
words,
No, rather the problem is that many of the fundamental physical sciences and most of electrical engineering use the concept, and it is not used merely by a small corner of circuit analysis. The definition has to work for all the sciences where it may be used. E.g., many switches use the "AC as reversing" concept for quenching contact arcs during switching (as the current passes thru zero as direction reverses) and the defintion of AC as varying DC falls flat for that purpose. Install an AC designed switch on a varying DC circuit, and you may well have a safety switch contacts welded shut. Here, AC DEFINITELY means reversing direction.
I believe the equations are not DC-AC specific - the "AC" term drops to zero if the change in magnitude drops to zero. Your rationale of using the equations does not hold up.
changing in direction or magnitude?
You suggest we use AC is defined as: "regularly or irregularly varying uni- or bi- directional current if it varies at a frequency that could have an effect for that application; unless it is digital, where then the one-time rise time and the fall time of each state change is calculated as AC, too, even though it 'alternates' once for each pulse"
vs.
AC is defined as:
charge flow that changes direction.
which leaves the calculations for reactance out of the definition.
attached
We define air, and black holes, and impracticality. The circular logic that has your AC/DC missing half the paramters of the phenomena also has no use for defining air since we only feel wind, for not defining black holes which have no practical value since we have never seen one, and no use for the definition of impracticality because by definition it has no practical value.
If
not if we consider the two paramters that make up the phenomena - direction and magnitude. And if memory serves me correctly, the "AC" equations are rigorous, and apply equally well to your one-voltage DC when the frequerncy drops to zero- the reactance term of the changing magnitude goes to zero.
Since the analysis is the
no, just two - reversing flow direction, and varying magnitude.
Because the reactance equations only apply to varying magnitude, and they do not apply to reversing direction.
If one has never heard of the ocean, one finds the ocean meaningless. And for the memebrs of that society, they also would find ocean-going boats useless. because they have to define the ocean thru their own experience. As I understood, scienctific method is designed to remove personal views from science. Thus the definition,must stand alone, and since we can't see all that is ahead, science has to fall in behind a definition of that phenomena in pure terms.
imho.......
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Alternating-direction Current, aka Alternating Current
Direction-specific Current, aka Direct Current.
-- True, but in the context of the post from which the excerpt came capacitive loads had not yet been introduced into the discussion.
You might want to look into the basis of Fourier analysis. It all falls out of a very simple mathematical property of the sine wave.
If you take any periodic waveform, and multiply its value at every point in time with the value of any frequency of sine wave at the same points in time, over all time and add up (integrate) all the products and divide by the total time (an infinite amount of time), only sine waves that fit an integral number of cycles within the period of the waveform will produce nonzero results (infinite integral divided by infinite time). In fact, it can be shown that you get the same quotient for harmonics if you use any integral number of periods of the waveform, including one period. Testing an infinite number of waves is only necessary to show that non harmonics always produce a zero contribution. For instance, if you test a sine wave that fits
1.000001 cycles into a cycle of the waveform, you don't reach the first zero result till you include a million periods of the waveform (and you get more zeros at every integer multiple of a million cycles, with a smaller and smaller cycle of results between those millions as the number of cycles increases because you are dividing by larger and larger times).Harmonics (sine waves that fit an integral number of cycles within the waveform) will produce a finite result representing that frequencies contribution to the waveform. (Actually you have to test both the sine and cosine against the waveform to cover all possible phase shifted versions of the sine. Any phase shifted sine can be broken sown into sine and cosine components. Another nice property of sine waves.) Since only harmonics contribute to the total wave shape, you can skip all the other frequencies, and just evaluate the part each harmonic contributes to making the total waveform.
That is Fourier analysis.
The rest is about making the math more efficient.
Reality check! Capacitors are passive devices. They do *NOT* generate signals.
All that has happened is the capacitor does not pass DC. You haven't generated AC on one side, you've merely removed the DC.
I don't see how that could be any more obvious. You did take a high school physics class, didn't you? *Use* what you learned!
Okay, so you not only need to restudy high school physics, but differential equations too.
If you had read the definition you posted, you might have noticed that it perfectly described the remark that I was commenting on. It had nothing to do with the discussion.
You are the one stooping to spelling flames.
-- Floyd L. Davidson Ukpeagvik (Barrow, Alaska) floyd@barrow.com
Oh, my. And you said what about Emily Post.
Nothing I said was a flame. And I'd suggest you go practice (a *lot*) before you try me on for a flame war. Especially if you think *that* is a flame.
Apparently I read a lot better than you write.
You write a lot of things that are not valid.
Do you understand that is not significant? The reactance of circuit components, the fundamental significance of AC circuit analysis, does not depend upon polarity alternation in any way. What else is there to talk about? How many chocolate drops should be in each chocolate chip cookie? I await your essay on *something* of significance.
But please, that is the *end* of discussion on your confusion about AC.
-- Floyd L. Davidson Ukpeagvik (Barrow, Alaska) floyd@barrow.com
There is no contradiction, so what is wrong with understanding both statements?
-- Floyd L. Davidson Ukpeagvik (Barrow, Alaska) floyd@barrow.com
Bad example. That does *not* require a direction reversal. All it requires is understanding that it is relative to the static state.
It does happen that the static state in that specific case is when a polarity reversal takes place, but in the general case it is not required. In other examples both sides of the switch might well be at some DC potential, that happens to be equal on both sides at the time the switch is made, even though there is no direction reversal.
Everything concerned with reactance is AC specific. Nothing concerned with reactance requires a polarity reversal. Reactance is the essence of the difference between DC and AC, not some notion of reversing polarity.
Which means it is worthless. Reactance *is* the significance.
All of which *does* have practical value.
That is an hilarious idea! If the magnitude is zero all the way around... we aren't talking about AC or DC... maybe about blown breakers or taking a coffee break, but not about current.
Oh? DC doesn't exist? What about "steady AC"? (That's two exactly equal signals 180 degrees out of phase, combined in that capacitor which can generate AC mentioned by John Fields, perhaps???)
Then why would we be concerned at all about this reversing direction, and give it a specific name and have a whole separate field of study for it? Sounds like we need to be concerned with varying magnitude, *not* with reversing direction. (Which is what I've been saying...)
A nice goal.
Except that alternating direction has no significance. Changing magnitude does. Why bother with alternating-direction at all, it is just an insignificant, though interesting, part of the more general case of changing magnitude. All of the same equations apply.
And if you claim that only alternating direction current is AC, then you have to have two sets of equations for DC, one for non-varying magnitude and one for varying magnitude.
That doesn't make a lick of sense.
-- Floyd L. Davidson Ukpeagvik (Barrow, Alaska) floyd@barrow.com
-- I said nothing about Emily Post. What I alluded to was that you have bad manners and could use a little training in etiquette.
That's easy for you to say!
I think you've shown me something. When I hear "sine wave" I imagine one cycle. I guess that's wrong, and a wave is a train of cycles.
Musical harmony is in a sustained interaction between trains of cycles. The interaction won't be simple enough to hear unless the quotient between the frequencies is a small integer.
When they talk about harmonics in an electrical wave, I guess they're talking about the potential for energy transfer. In that case, only odd multiples of the fundamental will stay in phase to tap the energy from the distortion. Where a wave is flattened it may resemble part of a sine curve with a longer period than the fundamental, but that doesn't count because you can't tap energy from the flat part.
If there's any truth in what I've said, I'll forget in a flash. In 1975 I was working in a repair facility. We'd use Bird Wattmeters to see forward and reflected power in antenna feeds. We knew the jargon and how to use the meters, but one day it struck me that none of us understood why they worked. I had a flash of insight and everybody stopped work to listen to me explain. Their faces lit up with comprehension. I felt pretty smart. The next day I couldn't remember whatever it was I'd figured out.
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One statement (quoted in full above) that (even though wrong) at least has something to do with the topic, Six out of seven comments are piddly attempts a childish and gratuitous insults.
No discussion John. I don't waste time teaching basics to grown men who have temper tantrums in public.
-- Floyd L. Davidson Ukpeagvik (Barrow, Alaska) floyd@barrow.com
You're confusing a temper tantrum with getting a dressing down you thoroughly deserve. You're a poser pretending to knowledge and skills you don't have, for the purpose of elevating yourself to a station which you yearn to occupy, but can't.
If you wish to end the discussion or exit the thread, then just do it. There's really no need for parting shots unless you find it necessary to have the last word before you run away.
-- John Fields Professional Circuit Designer
^^^^^^^^^^^^
Posting public articles like the above may well cost you employment in the future.
-- Floyd L. Davidson Ukpeagvik (Barrow, Alaska) floyd@barrow.com
The problem with that definition is that it is unnecessarily limiting. You can find other sources where the definition reads "magnitude *or* direction," the latter which I believe to be more correct. If the signal is steady state, then the current that changes magnitude but never direction is simply an AC signal with a DC component greater in positive amplitude than the negative peak of the AC component.
-- Al Brennan "If you only knew the magnificence of the 3, 6 and 9, then you would have a key to the universe." Nicola Tesla
(snip)
True mathematical sine waves extend from infinite negative time to infinite positive time. Practical sine waves last long enough for things to respond to their frequency. How long that is, depends o what is reacting to it. A frequency counter operating in period mode needs only a single cycle to make its measurement. An ear needs several cycles to several dozen cycles, depending on exactly what part of the audible spectrum being detected (this property of ears is part o the MP3 music encoding scheme). A quartz lattice filter may need thousands of cycles to of a pure frequency before it develops a nearly steady state output.
Something like that. Each frequency component in the signal has to last long enough for the time response of that frequency of the ear's sorting system to respond to it. If two frequencies fall within a single reception band, they are not heard as two tones, but as a beat addition and cancellation) as a single tone at about the average of the two frequencies and an AM modulation at the difference of the two frequencies. Obviously, if the beat is very long period, you have to hear the two beating tomes for a cycle or two of the beat period to detect that effect. Harmonically related tones just produce a repeating pattern at some integer multiples of each of the component frequencies. This can produce a very pleasing effect. You hear sound from one musical source as a fundamental and several harmonically related frequencies. If a second musical source (a harmonizing voice, for example) has its fundamental at one of the harmonics of the other signal, your brain recognizes this simple multiple relationship as a pleasing musical harmony. For some ratios. This page shows some of the approximate ratios between notes that sound interesting together:
Not really. since linear circuit components react to many frequencies by the addition if the effect of each frequency, it is a very powerful analytical procedure to break a signal down into its harmonics and evaluate the response of a circuit to each of those harmonics, and add all the effects together to get the total response.
Symmetrical distortion of a sine wave (shape of positive half cycle is a mirror image of that on the negative half cycle) can be shown to be made up of only the fundamental and odd harmonics (3 times. 5 times, etc.). If the distortion peaks up one half cycle and flattens the other or shifts the zero crossing so that one half cycle lasts longer than the other, there are even harmonics in the wave shape. There may also be odd ones, too. Got to do that Fourier analysis to quantify that.
You can with a resistor. From a Fourier perspective, that flat part just represents a time when the curve of some frequencies is nearly canceled by the curve from other frequencies. You need an infinite number of harmonics to make a truly flat square wave with perfectly square corners.
I hate it when that happens.
-- I see. Since you can\'t impugn my technical credentials and you can\'t have your way with me in terms of controlling the direction of the thread to your advantage, you think that some contrived reference to my livelihood being affected by the way I handle the likes of you is going to, somehow, influence my actions? Think again.
[snip]
Well, "intrinsic" may be a bit to strong of a word. Boiling and freezing points very considerably with pressure.
--Mac
me.
If you have definitions of AC and DC handy from IEEE or someone, stick them on here. I'd say that the (apparent) widespread disagreement means that, functionally, there is no single pervasive definition for these terms, but it would be interesting to see if some of these bodies have published definitions. It would be really interesting if they had definitions, and they didn't quite agree with one another, or if they were "wishy-washy".
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"show why the answers must be obvious" sounds like a peculiar concept. An obvious answer inherently needs no explaining.
You feel that neither AC nor DC is correct as a description for the flashlight behavior. I wonder whether there is a sufficiently definitive (and also agreeable) meaning of "AC" or of "DC" that would merit this position. I infer from other posts that there are people who would say it is DC. There may be others who would say it is AC. So a statement of fact that it is neither AC nor DC is suspect. By the way, my rhetorical questions were actually whether it is an AC flashlight and whether it is an AC battery. The point of this was (I thought obviously) to illuminate the difficulty in declaring some things to be AC or to be DC. There is a certain ridiculousness (I thought obviously) in calling a flashlight AC or in calling a battery AC. Yet it would be awkward (I thought obviously) in adhering to calling it DC if one's description of DC was that the (voltage / current) would essentially remain constant. Thus my (possibly obvious) point and my rhetoricals. I'll work on them. Your response did not seem to conradict my point.
The web has lots of information and lots of misinformation, I think you'd agree. I'm not sure that I have displayed curiosity in these posts. An education in the field of circuit analysis? No, thanks, you go ahead.
That's remarkably funny. What do you operate? Not your brain I wouldn't guess.
j
I simply feel it's prudent to carefully consider all responses, as I've grown rather un-fond of the taste of my foot in my mouth. I agree that the poster's attitude is obtuse at best, but flinging off-hand insults is risky. I should know.
Besides, right now I'm the new guy in a new job after 30 years in electronics, so I have to practice restraint. Practice, practice, practice! :-)
-- Al Brennan "If you only knew the magnificence of the 3, 6 and 9, then you would have a key to the universe." Nicola Tesla
And purity and other factors.
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