DC Wave Questions

"AC" are all of the rema '"AC" or "DC" are gross and meaningless oversimplifications.'

Does this comment apply to your own earlier definitions? Are you saying that even you can't give definitions to AC and DC that aren't "gross and meaningless oversimplifications"?

Exactly.

But look at all the people claiming it *isn't* AC! Very clearly anyone who claims the various "noise, trash, crap, spikes" etc are *not* AC, needs a reality check on their definition of AC.

On the other hand, while their definition and understanding of it is clearly invalid, there is no lack of wide spread belief that it is correct. Which I do find somewhat amazing...

think gold will

evaporate, though;

Or maybe just sublimation. And he didn't say the chamber was held at room temperature.

What's the vapor pressure of zinc at 20C? I once worked at a place where their product used a UHV bell jar - that's "Ultrahigh vacuum". They didn't even have an oil-based pump in the building. They started with an ordinary sorption pump, then they had ion pumps and molecular inertial pumps, and getter pumps, and the sexiest was the cryopump.

One day one of the vacuum engineers came into the shop from the line, fit to be tied. It seems someone had supplied feedthroughs with brass contacts. The zinc ruined some stuff, and wasted about a week from having to clean out the bell jar.

Cheers! Rich

In a case like this, "out-gas" would definitely be a better term, or even "sublimate", which means to go right from solid to gas. Only liquids can boil, and then only when the uneven heat causes bubbles of vapor to form. _That's_ what boiling is, regardless of the temp., material, or anything else. You should have seen the LN2 seethe when they opened the valve on the sorption pump!

Cheers! Rich

As with many historic terms, they may be off the mark by today's understanding, but they are not necessarily wrong.

For example, I have no problem using the term DC even when there is no current (flowing). Is that bad that I can assume DC is valid for static and dynamic states? It didn't cause me any problem when I first began to learn about electricity. How about we assume the term DC is a set with many subsets? That beats defining DC in a manner that says if there is a constant, never-ending load on a EMF, then it is DC, but if it is EVER interrupted, then it never was DC but was some form of AC?

The more I learn, the more I find fault with some definitions. I find more fault (pointing the finger nowhere specific), however, with people who want to redefine things before they have studied long enough to understand what they are doing.

Don (B)

think about

This is quite OT, but might be of interest, since we're so far OT anyway! Note the strange behavior of mercury. This makes it possible to use mercury vapor to make "a nice little vacuum pump". When I first started using vacuum pumps, mercury pumps were common. They were displaced by oil diffusion pumps and more recently by ion pumps. I don't know of any that would make cadmium "boil" at anywhere near "room temperature".

pressure of

value of

achieve

the actual

I said it was 40 years ago! And I make *no* claims about any absolute accuracy of any specific detail. I'm *not* making a point of what the specific material was, what the exact temperature was, what the exact pressure was.

I appreciate corrections to what would obviously have to be the correct parameters. Clearly at 70 some degrees C cadmium will out-gas if the pressure is down to 10^-8 Torr.

The point was about apparently solid things simply vaporizing without being raised to some significantly high temperature, because *pressure* is just as significant. And the results can come as an expensive surprise too.

The idea that water boils at 100C and freezes at 0C, without some mention of pressure, has little meaning. Water can "boil" at 0C too.

At least, not at

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How much less?

apparently you have not seen the equations governing reactance to make the statements you do about

deh said

zero-

to zero.

fld said

nor have you ever designed a switch carrying power loads, to say that in non-DC rated switches, arc quenching does not rely on current reversal. Or to ground fault design, I might add.

deh said

do

fld said

As I said, your position works only if you are in one corner of one part of all electrical phenomena, and if you use technicians tools rather than engineers and scientists tools.

You will just have to live with what the big boys > >"Floyd L. Davids> >> > Strictly speaking, I believe the reactance (part of impedance)

appropriate

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and

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direction

purpose.

have

you really don't know much about power switches, do you?

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Great isn't it ???

I have my own theories, but I'll keep them to myself for the time being. I regularly work on a piece of kit that outputs a varying DC voltage. Over a period of maybe 10-20 seconds it does often approximate a sine wave. With such a slow cycle time, I must admit I've never thought of an AC component, only of a varying DC one ;-)

- Steve back behind the asbestos screen.

Dr. Polemic wrote in news: snipped-for-privacy@4ax.com:

Yea, I'd have to call BS on that one...

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Your descriptions apply to *your* comments, not mine!

zero.

So your one single point of exposure defines the entire field?

I don't think so at all. You are trying to say that specific instance is the general case. I was saying that it isn't, and that the general case is *much* larger.

That describes *your* position very well!

Actually, getting too deep into either one, or into any single field in either one, is what causes these assumptions like yours, that specific instances are the entire general case.

One of the problems with academia, for example, is this specialization. Whereas someone who works in the field runs into whatever each project coughs up. The exposure is significantly greater. And no that does not mean that academics is bad, wrong, poor, unnecessary, less that useful or any other silly thing you are likely to try twisting it into. It does mean that it necessarily, for any one individual, has a narrower scope, and people with field experience have a broader view, generally (which also often lacks as much depth too). Both are necessary.

Well, I certainly do lack depth in that particular subject! But knowing about power switches doesn't define understanding AC vs DC, even if that is your field of expertise. In fact, it might be the cause of your confusion.

Power switches may in fact operate at the moment of direction reversal, but that is merely one specific example of the broader "relative to the static state" general description that I gave. The switch can happen at any time there is no current flow (or rather, when the phase angle and rate of change is appropriate) across the "contacts". But that can be at some DC potential which is equal on both sides of the switch, or at a zero voltage, either of which is *not* at a current direction reversal. Maybe those circumstances don't happen with AC distribution power switching, but they most certainly do in other fields.

The requirement is *not* that a direction reversal be taking place. That is just a collateral circumstance that happens to exist in AC power distribution systems at the same time the necessary phase relationship exists.

The very same general principle is used in video switching... except not at a time when there is any change in current direction!

Floyd L. Davidson wrote: (snip)

Unless it is a solid. Then it sublimates. Boiling only happens to liquids.

think gold will

evaporate, though;

What he said was: "Danged, several weeks of work shot because it just hadn't occurred to him that ni-cad would boil at room temperature." I suppose I could say that tungsten exaporates at room temperature and then say that I didn't say that the filament wan't held at room temperature; I just meant that the room was at room temperature.

--- As John Popelish remarked, boiling is only possible if the medium in which the boiling is occurring is a liquid, so if the water has turned into ice at 0°C, sublimation is the mechanism which water molecules will use to evaporate from their parent structure.

Since this a technical forum and we _do_ have ground rules, I believe we generally agree that, unless otherwise specified, standard pressure is defined as 760 millimeters of mercury and standard temperature is defined as zero degrees celcius. While the boiling point of water is dependent on pressure, the freezing point, I believe, is not.

At least, not to a great extent. I don't have any data to support that position, but I'd love to see some, if it's out there.

-- John Fields Professional Circuit Designer

Handbook of Chemistry and Physics, Twenty Fifth Edition, Table p.1696.

All you have to do is go ice skating! You skate on a thin layer of water, produced by the pressure of the skates. Water is also one of the few liquids that expands when it freezes, which is fortunate for life on this earth! :-)

I believe that in this forum we assume temperature is a variable that must be accommodated in design, unless otherwise stated. I cannot imagine why any unstated temperature would be assumed to be 0 oC. I suppose sea-level atmospheric pressure is often assumed, but where it matters, it should not be assumed at all.

You might want to consider the "triple point" of water, below which pressure "melting point" is meaningless and the "freezing point" varies considerably with pressure.

See: