That isn't true.
A non-sequitor.
If it varies, it's AC.
If there is such a think as "varying DC", connect a load to it... through a capacitor. Now, how do you describe the effect that load has on your "varying DC". The load see's *only* AC, even according to your definition. That AC came from somewhere, and it certainly was not generated by the capacitor.
That's because AC is *not* defined by any change in direction, but only by a rate of movement change.
It's not unusual to speak of the AC and DC _components_ of a waveform that does not readily satisfy the simplification.
One also speaks of _DC offset_ of an otherwise AC signal.
And, according to what you've said in other posts, if that were a
0.6 volt peak sinewave with 1.0 volt dc, it wouldn't be.But your definition of AC is faulty, because in fact they are the same thing, and *both* of them contain an AC component and a DC component, even if the general direction of electrons is always the same.
Except, polarity reversals are not significant to the definition of AC.
He's right.
Didn't you just say in your immediately previous post:
---------------------------------------------------------------- ""DC" is simply the first (or "offset" term in the Fourier expression of any repetitive waveform.
"AC" are all of the remaining components."
---------------------------------------------------------------- Did you intend to offer these descriptions knowing that they were "gross and meaningless oversimplifications"? Why bother in that case?
Where *do* you get this requirement for changing polarity? We don't call it "Alternating Polarity", we call it "Alternating Current". If the current is being altered, it's AC. You keep talking about AP, and it isn't the same.
Look, if the current's not alternating its direction, it's not alternating current. To be clear, just because its amplitude is changing does not mean it is alternating. In particular, a rectified AC waveform it isn't changing direction.
There are certainly AC components in the waveform. But the sum of all the components, including the DC component, never changes direction, so the total signal is a DC signal. Here, "DC" does NOT mean "constant"; it means unidirectional.
So you are saying that DC varying from 5 to 15 as the op referenced is AC? If you put a DC source across a capacitor and vary the source up and down, sometimes electrons are flowing into the capacitor, and sometimes they are flowing out of it. Same with an inductor.
For the record, I don't want to take one side or another in the debate about AC vs DC in this thread. The waters are muddy enough already. I view the op's scenario as DC with an AC signal imposed on it.
This whole discussion of whether it is AC or DC is a trap and diversion from the original. It does not matter whether it is AC or DC that the components see. For example, a capacitor operates the same on DC as it does on AC. If there is a path for it to charge, and a source sufficient to charge it, it charges. If there is a path for it to discharge, and no source applied sufficient to keep it charged, it discharges. Same thing for an inductor below saturation.
The op asked about a sinusoidal varying DC, but gave no info about frequency. He then asks about impedance of the (unknown) RLC circuit. The answer has to be arrived at by a consideration of how each component reacts. To say (not that you said it) the cap won't pass DC is crap. Connect a 15 V, 500 ohm relay coil to ground, and the other side to a 470 uF cap. Connect the other side of the cap to +12. The relay energizes briefly, proving that the cap did pass DC. Try the same thing with a supply that starts at 5 volts and increases to 15 volts at a rate of 1 cycle per hour, and it does not energize. But the relay coil DOES charge. For the op to understand the load impedance, he has to understand what each component does in his circuit. I see no other way to answer his question, in the absence of specifics.
Ed
You are the one with the requirements, assertions, and definitions, not me. Where are you coming up with them? If it's from the same place where
- zero current is not definable
- magnitude of current needs an outside reference
- voltage and current are for all practical purposes different expressions of the same thing
- alter is the same thing as alternate then I don't even want to know.
You need to come to realize there is no clear cut correct answer on this 'AC' vs 'DC' issue at this time. If there was one, there would be much more consensus between people on what the correct answer is. This big long thread would not have occurred. Now let's turn it around and look at it the other way. This big long thread did occur. We can plainly see that there is disagreement between groups on what exactly the precise meanings of AC and DC entail. Therefore there effectively is no single exact definition for "AC" or for "DC" that will allow us to resolve which is correct and which is not correct.
Picture my flashlight, battery powered. Generally this is considered a dc circuit. When I turn it on or off, there is 'change'. So is it in fact an AC flashlight? If the battery starts to die there is a change so is it in fact an AC battery? Etcetera. (These questions are rhetorical by the way). I know better than to try to pin a strict name on these things where there is not an (adequately) universal and strict definition.
On another note, how long are the days getting to be way up there? Do you get continuous sunshine?
j
Interesting! I thought John's response to the op was called for. The OP is going to get himself into trouble with the attitutde he's exhibited. In my opinion, John saw through the BS and called a spade a spade. I don't know whether the OP got it or not - but John made it clear that the BS wasn't fooling anybody.
I'll have to go back and read it again in light of your post.
Ed
True but pointless. We know what we mean. Even 'current' is a borrowed term used as an analogy as is 'potential' or even 'pressure'. If we have voltage surely we should only speak of amperage.
N
Oh, I agree. I just think it helps to be a bit patient. Not that I'm all that good at patience, myself. But we should try. We have all been rookies, once or twice.
That's going a bit far. "Meaningless" means no meaning, and that is not really an accurate description for the terms AC and DC. They have a pretty well understood meaning, despite some suggestions in this thread.
"quotes with no meaning, are meaningless" - Kevin Aylward
Kevin Aylward snipped-for-privacy@anasoft.co.uk
Since we are quibbling her on terms, lets get this bit straight shall we.
"Current flow" is wrong. Its simply "current" or "charge flow". "Current" already contains the notion of "flow".
Kevin Aylward snipped-for-privacy@anasoft.co.uk
Actually, the ones with the requirements, assertions, and definitions are codes and organizations such as the NEC and the IEEE, and the bothersome universities that teach the stuff.
On this point:
You are talking about transients, and if you intend for the questions to be rhetorical, then I think you should demonstrate some expertise in the subject matter that shows why the questions' answers must be obvious. I don't think they are, so I will answer the questions:
The behavior of the flashlight in your example is neither AC nor DC, it is transient. The first case is the instantaneous step function caused by the closing of a source to a circuit. The second case is a long-term curved ramp caused by the decay of a voltage source. AC and DC analyses are steady-state. AC analysis will never apply to the example. DC analysis must be performed prior to the transient analysis in order to provide a steady state model for the application of time-sensitive mathematics.
There is quite a bit of information available on the web about circuit analysis. Your curiosity is to be commended; you might consider a web crawling adventure, or even an education in the field.
And hey operator jay, what do you operate? Not electrical substations, I wouldn't guess.
No, both do not - only one of the 1 volt/.6 volt examples given has an _alternating_ direction component - both examples do have a _variation_ in their magnitude component. ( This is not a new discussion - and all of the dozen or so engineering and physics texts and training manuals I have researched on the matter adhere to the "alternating is reversing" definition of AC. It has been custom and practice for at least 40 years.)
1) the 1 volt dc with the .6 sine variation does not alternate its direction of flow. Its flow only varies in the magnitude of the charge flowing always in one direction. It has no alternating current ( i.e, it has no regularly reversing, i.e. _alternating_, charge flow direction) 2) the 1 volt sinewave with the .6 volt dc does reverse charge flow direction. It is alternating in its flow direction. It also varies in its magnitude.The direction of the description vector must alternate in order to have Alternating Current. If it does not change direction but only varies in magnitude, the descriptive vector is not alternating, it is merely varying in magnitude.
3) Impedance laws apply equally to varying DC and to AC.
'Alternating' is not the same as 'altering'. "Alternating current" is an electrical current where the magnitude and *direction* [emphasis added] varies cyclically.
daestrom
It is true by most definitions of 'Alternating Current'.
Only by your apparent definition. But your definition does not agree with the established industry.
By adding a capacitor in series, you have altered the circuit. The capacitor filters out the DC component of a the original varying DC voltage applied. The capacitor has a varying DC voltage across it, but it never changes polarity (you can use an electrolytic capacitor that is polarity sensitive without damage).
The current through the resulting series circuit *does* alternate in magnitude and *direction*, even though the voltage applied to the circuit varies in magnitude only. So yes, the 'AC came from somewhere'. But that doesn't mean the applied voltage is AC. Such logic is flawed. There is no 'law of conservation of AC' that says it can't be 'generated by the capacitor'.
Repeating yourself doesn't make you correct.
daestrom
Then you'll have a very difficult time explaining now a transistor amplifier works if it uses AC coupling that involves capacitors.
Which industry? You can't do AC circuit analysis with any other definition.
Capacitors don't generate voltage or current. The circuit alteration merely demonstrates that the voltage and current on one side meets all of your requirements, while the identical charge flow on the other side does not, which indicates a flaw in your specification.
Yes, which leaves the AC that was there all along. It's AC after, and it was AC before. If you do circuit analysis the treatment is exactly the same on both sides of the capacitor.
Exactly. Yet there *is* current through the capacitor, which only passes AC. That AC current isn't generated inside that capacitor. It comes out one side, so it *had* to be coming in the other side.
That's hilarious. DC applied to a capacitor generates AC????
I don't think so.
Won't help your point either. And it makes no difference how many places you find it ill defined either.
You were doing pretty good up to that point.
No. But that doesn't mean there is never any AC present in the circuit.
It happens that with a battery powered flashlight that is rare (but predictable too), and of no consequence whatever. It can be ignored in design and operation of the flashlight. But that doesn't mean there is never any AC in the circuit, or that there are no circuits where it is significant.
Every time you flip the switch on or off, there is AC in that circuit.
You can probably prove it too, relatively easy. Tune an AM receiver to a frequency where no station is being received, and hold the flashlight up close to the antenna. Flip it on and off a few times. I suspect, though I haven't actually tried this, that you'll hear a pop in the radio's speaker almost every time you flip the switch. That is because some of the AC produced by flipping that switch is RF.
It's been 24 hours of daylight for quite some time now. The sun hasn't actually gone down for a month (May 10th), but of course we had 24 hours of light long before that. The next time it gets below the horizon will be August 1, and it will be late August before it gets "dark".
The temperature is 30F right now, with a reported 18 mph wind and fog. It was gusting up to 30 mph last night. It probably won't get much warmer than maybe 36F today.
That is actually very comfortable weather, mostly because it is unlikely to rain. I hate getting wet... :-)
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