Magic capacitors!

Because circuit designers use "charge" in a somewhat different way than physicists. Call it "charge separation" of "differential charge" if you want to. We call it "charge" and measure it as CV = time integral of I. If you actually design electronics, you probably do too. I doubt you design timers or integrators entirely in terms of stored joules.

John

Quit stealing my insults. Invent your own.

John

As I said, I don't have to design timers or integrators by thinking in terms of stored energy, I can use the Q=CV equation and it works, because it relates the charges moving in and out of a capacitor to the voltage, and to the current in and out of a capacitor at any point in time. I can design anything you can using the same Q=CV equation, but I don't need to have any special cases or violate any conservation law.

Coulombs of charge are Coulombs of charge. You have invoked a special case of 'electrical charge' that doesn't seem to be conserved, and yet you still relate it to the current (which consists of electrons or the lack of them) by integration, and voltage. Are you saying that the engineer's view of what makes a current is different to an physicist's view? Is the 'charge' flowing per second for an engineer somehow a different, non-conserved charge flow to the physicist's charge flow? If not, then explain why, if the same current is flowing in and out of the capacitor, it doesn't violate the Law of Conservation of Charge.

Engineers and circuit designers are simply people who apply the laws of physics to create real world applications, they are bound by the same laws of physics - no exceptions.

Mark.

Go away, you modern day, retarded little wuss.

From:

"Work must be done by an external agent to charge a capacitor. Starting with an uncharged capacitor, for example, imagine that?using ?magic tweezers??you remove electrons from one plate and transfer them one at a time to the other plate. The electric field that builds up in the space between the plates has a direction that tends to oppose further transfer. Thus, as charge accumulates on the capacitor plates, you have to do increasingly larger amounts of work to transfer additional electrons. In practice, this work is done not by ?magic tweezers? but by a battery, at the expense of its store of chemical energy."

Yes, nice article. From this very same article (page 5):

"When a capacitor is charged, its plates have equal but opposite charges of +q and -q. However, we refer to the charge of a capacitor as being q, the absolute value of these charges on the plates. (Note that q is not the net charge on the capacitor, which is zero.)"

So a capacitor does not store net electrical charge. And the q in the q=CV equation relates to the magnitude (i.e. absolute) value of the charges on the plates, which are equal and opposite.

I'm going to conceed a point here, we do indeed speak of a 'charge of a capacitor as being q'. I apologise for suggesting that 'charging a capacitor' refers only to energy (it can depending on context, but it can also mean charging the plates to +/-q respectively, or more usually to a voltage). However in that respect we are talking the charge of a capacitor as being the absolute value of charge on each plate, but those plates have equal and opposite values so we *don't* talk about q as being the net stored charge, which is zero.

Mark.

Simply because the strict interpretation you argued for is useless for practical electronics design ;)

There's rules and rules and one picks what's convenient, sometimes one must follow physics more closely, but the loose interpretation is what works day-to-day by simplifying our models.

Grey area, not black & white.

Grant.

But I use Q=CV too, where Q is the absolute charge stored on each plate (one negative, one positive). The integral of the current going into the capacitor over time Q and is stored on one plate. Since the same current is coming out, the integral over time is -Q and is stored on the other plate. The energy is stored is Q^2/(2*C), or (C*V^2)/2. I use exactly the same equations!

But that 'simplification' lead directly to John claiming that charge wasn't conserved, and also claiming that capacitors store charge. It was a misinterpretation of exactly what Q=CV really means.

(BTW, Q=CV doesn't hold if the capacitor stores net charge, quote: "C= Q/V does not apply when there are more than two charged plates, or when the net charge on the two plates is non-zero":

Mark.

With zero inductance it oscillates at infinite frequency. The single stored photon probably buds off into a daughter universe.

It seems I owe John a BIG apology. Looking back at the posts, I see that whenever he talks about charge being stored in a capacitor he is talking about the convention of what is stored on a capacitor, which is actually

John actually said in a post to me "We say that a capacitor stores charge, the amount being C*V in coulombs, and it works. My whole point, which has evoked such ranting, is that when you use this convention, be careful about designing using the concept that (this kind of) charge is always conserved."

He is right. I assumed the phrase 'this kind of' charge meant a different type of charge that wasn't conserved, he actually meant be careful of using the q=CV charge definition, which is actually +/-q on the plates. It was actually ME who misinterpreted what was being said. The 'charge on a capacitor' by this definition is not conserved. The total net charge is.

So I acknowledge John actually really understands this, and I was in the wrong to assume he meant net charge.

Sorry John!

Mark.

Hey, I took two years of college physics, and got As. But as an engineer, it's not prudent to say that a capacitor has zero charge when it would actually knock you dead if you touched it.

Whatever term a physicist uses for "the differential charge on a capacitor" or "the integral of all the current that has ever gone through a capacitor" or "the charge on one plate", circuit designers just call "charge", which happens to be C*V, in coulombs. I have no idea how a typical physicist describes this in everyday English. The few physicists I know wouldn't correct me for saying that a 15 pF cap charged to 4 volts stores 60 picocoulombs.

Given that this is how EEs design electronics, one must be careful about basing conclusions on conservation of C*V. That's all I said.

In the case of the ancient "connecting the capacitors" riddle, the explanation almost always includes the phrase "since charge is conserved..." and uses C*V as the definition of "charge." It works in this circuit. In some circuits it doesn't. Using an inductor, I can transfer all the energy from one cap into another of a different C value, and C*V will not be conserved. No electrons will be created or destroyed.

Don't apologize. Just recognize that we use the word "charge" in a way than a physicist might get legal-picky about. (Unless that physicist designs circuits, too.)

John

I'm afraid a real physicist would, as far as their interpretation of what 'electrical charge' is, because to them this would require more charged particles to be present on the capacitor than were before. If they know you are using the electrical convention of 'charge stored on a capacitor', where q=CV and the plates have +/-q on them, then maybe not. What has happened in reality is you have taken charge (in the form of electrons) from one side of the plate to the other, via the external circuit, in the process doing work. The total number of electrons is the same before as it is afterwards. The net storage of electrical charge in a capacitor is therefore zero. What you have done, though, is created an electric field between the plates, and it is the electric field that stores the energy (equal to the work done needed to move the electrons from one plate to the other in the first place) - the belt you get is due to that energy discharging (and hence causing a current to flow, which moves the electrons in the plate with the abundance of electrons back into the plate with the depletions of electrons).

Well that's my point, a physicist *will* get picky, unless they understand you are using the electrical convention of q=CV, and they (and you!) understand that the capacitor has +/-q on its plates.

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Well! Well! Well! You finally got it right. I can actually demonstrate how the charge disparity (now "C*V :-) comes about. I'll put it on LTspice where my efforts won't be wasted. ...Jim Thompson

The occasional apology is a nice change from the Godzilla-Versus-Rodan style of dialogue usually employed here. (Oh, no! There goes Tokyo!) The very fact that Mark didn't actually keel over dead might be reassuring to some of The Usual Suspects.

Just recognize that we use the word "charge" in a way

Most of us have actual work to do, and recognize the working vocabulary of other fields. It's the Fourier transform sign conventions that really screw up communications. ;)

Cheers

Phil "somewhere in the grey zone" Hobbs

Hey, Word thinks that rotating a picture 90 degrees is *clockwise*

John

...

Okay, so what do you call that electric field? In my mind, that's the charge? I accept your argument about the equal but opposite quantity of electrons, but in moving 'charge' over to define that is what loses the point of energy stored, no?

And here you quite happily talk about 'discharging', you cannot have it both ways?

Sure, a capacitor's not going to do much with the other plate isolated, but it does do a little, because a body may carry charge relative to the environment.

Grant.

Physicists make this same mistake, and so do their undergrad textbooks. A few years ago this "capacitors store charge" error was discussed in detail on PHYS-L forum. If it wasn't such a widespread error and barrier to understanding, it wouldn't have attracted any discussion.

As for us techs and engineers, in early grades we were all taught power supplies and lowpass filters, where one capacitor plate is grounded and ignored. (Well, that's what happened to me, and that's where I got the misconception.) Or perhaps we started out with Leyden jars, where the outer foil is grasped by a grounded human, and only the inner foil is charged and dangerous. If capacitors only have one terminal, then obviously they can store positive or negative net charge. And therefore a capacitor is charged with coulombs injected into that one terminal. At least in my own case, the misconception was caused by "simplified one-wire capacitors."

But later in EE classes I sort of figured it out: charge *always goes through* all two-terminal components. Charge goes through resistors and inductors, but it also goes through capacitors in the form of displacement current in the dielectric which is exactly equal to the currents in both capacitor terminals. Mathematically, capacitors are like resistors, but where the voltage across the capacitor terminals is the time integral of the current plus K.

So capacitors store... "integrated current?" Yeah, I guess. But since the path for current in circuitry is always a network of loops, no charge builds up anywhere. It's not charge that gets stored, it's potential energy.

So now I imagine capacitors as being mechanical alarm clock windup springs, where the internal spring gets wound by a leather belt passing over the little wheel. If you remove all resistance and let the little wheel spin freely, that's a short circuit.

And then I carefully avoid telling kids that "capacitors store leather."

We're also using it in a way that might greatly confuse all literal- minded beginners who are trying like hell to figure out basic circuitry. The problem isn't with nitpickers, the problem is the same one described by CF Bohren:

"Lest you think that I am quibbling over minor points of language, I note that in my experience many of the misconceptions people harbor have their origins in imprecise language... Precise language is needed in science, not to please pedants but to avoid absorbing nonsense that will take years, if ever, to purge from our minds."

Bohren's own teaching philosophy leans towards misconception-removal rather than just "teaching true facts."

Others:

"The ill and unfit choice of words wonderfully obstructs the understanding." - Francis Bacon

"Many errors, of a truth, consist merely in the application of the wrong names of things." -Spinoza

"(language) becomes ugly and inaccurate because our thoughts are foolish, but the slovenliness of our language makes it easier for us to have foolish thoughts." - George Orwell

"The search for the MOT JUSTE is not a pedantic fad but a vital necessity. Words are our precision tools. Imprecision engenders ambiguity and hours are wasted in removing verbal misunderstandings before the argument of substance can begin." (from Roget's Thesaurus Webpage)

((((((((((((((((((((((( ( ( (o) ) ) ))))))))))))))))))))))) William J. Beaty Research Engineer beaty a chem washington edu UW Chem Dept, Bagley Hall RM74 billb a eskimo com Box 351700, Seattle, WA 98195-1700 ph206-762-3818

Cool. You must design integrators and linear ramps and timers and power supplies using only capacitor energies in joules. That will make the math much more interesting.

Do you think Spice uses joules for its internal representation of capacitor states? No wonder it's so slow.

John

Lol.

Nah, I just choose to recognize that "time integral of I" is a concept distinct from "charge stored on one capacitor plate." Even though their values are the same, and even though the former is the cause of the latter, they aren't measured the same. Why try to count the excess charges on a capacitor's internal plates when we can just measure the current in the lead wires?

Besides endless Newsgroup fights, really this stuff is only important when teaching basic physics/electronics to newbies and when writing electronics textbooks. And further, it's only important if we've decided to avoid filling students' heads with misconceptions like single-wire capacitors or "capacitors store charge."

Some educators say things like "what's good enough for me is good enough for them," and so proceed to infect their students with their own muddled thinking. This might work for most purposes, and the misconceptions in question might not act as very large learning barriers. But over decades and generations it's a "game of telephone." It's a recipe for filling textbooks and classrooms with increasing mistakes. Why not instead reverse the trend and turn your students into physicists who happen to specialize in electronics? It's not that hard ...just identify common misconceptions which violate basic physics rules, then avoid spreading those misconceptions to students. "Capacitors store charge" makes no sense to a student with a gut-level understanding of charge conservation. Or conversely, any student who truly believes that capacitors store charge, might forever afterward have troubles with basic physics. Remove the contradiction, and "Aha!" everything suddenly connects together in your brain and makes perfect sense. Ideas like "capacitors store charge" are bad because they prevent the wonderful Aha. That's why they need to be taken seriously as errors, and not just labeled as "nitpicking."

And you're certain that it doesn't just take the time integral of the current in the capacitor leads?

((((((((((((((((((((((( ( ( (o) ) ) ))))))))))))))))))))))) William J. Beaty Research Engineer beaty a chem washington edu UW Chem Dept, Bagley Hall RM74 billb a eskimo com Box 351700, Seattle, WA 98195-1700 ph206-762-3818