Magic capacitors!
in [Design]
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From: markp on 7 Aug 2010 06:52 "markp" <map.nospam(a)f2s.com> wrote in message news:8c3jbcFr8rU1(a)mid.individual.net... > > "Grant" <omg(a)grrr.id.au> wrote in message > news:ko2p56p8k3pdimdkkdnp627vi2q6eqfdm1(a)4ax.com... >> On Fri, 6 Aug 2010 20:16:38 +0100, "markp" <map.nospam(a)f2s.com> wrote: >> >>> >>>"VWWall" <vwall(a)large.invalid> wrote in message >>>news:_M-dnTT-O9YTzcHRnZ2dnUVZ_vOdnZ2d(a)earthlink.com... >>>> markp wrote: >>>>> "John Larkin" <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote in >>>>> message news:258o56l0k2k1id6bm27oevicacm6ifd5ft(a)4ax.com... >>>>>> On Fri, 6 Aug 2010 15:05:38 +0100, "markp" <map.nospam(a)f2s.com> >>>>>> wrote: >>>>>> >>>>>>> "John Larkin" <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote in >>>>>>> message >>>>>>> news:u4sm56lncllfueedvo2ahi4ihr8j00gu3p(a)4ax.com... >>>>>>>> On Thu, 5 Aug 2010 18:43:28 +0100, "markp" <map.nospam(a)f2s.com> >>>>>>>> wrote: >>>>>>>> >>>>>>>>> "John Larkin" <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote >>>>>>>>> in >>>>>>>>> message >>>>>>>>> news:7tjl5615o4lklftqq34fncd86soc75forh(a)4ax.com... >>>>>>>>>> On Thu, 5 Aug 2010 13:20:12 +0100, "markp" <map.nospam(a)f2s.com> >>>>>>>>>> wrote: >>>>>>>>>> >>>>>>>>>>> "John Larkin" <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote >>>>>>>>>>> in >>>>>>>>>>> message >>>>>>>>>>> news:889k5654o0h9qfgs3cej7gfe99ahsg42am(a)4ax.com... >>>>>>>>>>>> On Thu, 5 Aug 2010 02:40:12 +0100, "markp" <map.nospam(a)f2s.com> >>>>>>>>>>>> wrote: >>>>>>>>>>>> >>>>>>>>>>>>> "John Larkin" <jjlarkin(a)highNOTlandTHIStechnologyPART.com> >>>>>>>>>>>>> wrote >>>>>>>>>>>>> in >>>>>>>>>>>>> message >>>>>>>>>>>>> news:ci3k56d0kga1776gghosaq09q2e0i2ahhq(a)4ax.com... >>>>>>>>>>>>>> On Wed, 4 Aug 2010 16:55:16 +0100, "markp" >>>>>>>>>>>>>> <map.nospam(a)f2s.com> >>>>>>>>>>>>>> wrote: >>>>>>>>>>>>>> >>>>>>>>>>>>>>> "John Larkin" <jjlarkin(a)highNOTlandTHIStechnologyPART.com> >>>>>>>>>>>>>>> wrote in >>>>>>>>>>>>>>> message >>>>>>>>>>>>>>> news:2vge46h4sragrk4jdn6sasde6hg2r52nos(a)4ax.com... >>>>>>>>>>>>>>>> On Wed, 21 Jul 2010 12:17:41 -0500, "George Jefferson" >>>>>>>>>>>>>>>> <phreon111(a)gmail.com> wrote: >>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>> "John Larkin" <jjlarkin(a)highNOTlandTHIStechnologyPART.com> >>>>>>>>>>>>>>>>> wrote >>>>>>>>>>>>>>>>> in >>>>>>>>>>>>>>>>> message >>>>>>>>>>>>>>>>> news:dj7e465sga7fe3nq7hfl3f0uk601pvrem8(a)4ax.com... >>>>>>>>>>>>>>>>>> On Wed, 21 Jul 2010 11:19:31 -0500, "George Jefferson" >>>>>>>>>>>>>>>>>> <phreon111(a)gmail.com> wrote: >>>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>>>> "John Larkin" >>>>>>>>>>>>>>>>>>> <jjlarkin(a)highNOTlandTHIStechnologyPART.com> >>>>>>>>>>>>>>>>>>> wrote >>>>>>>>>>>>>>>>>>> in >>>>>>>>>>>>>>>>>>> message >>>>>>>>>>>>>>>>>>> news:s43e46la1p1vt11527eg3ptl9ulm44dfrj(a)4ax.com... >>>>>>>>>>>>>>>>>>>> On Wed, 21 Jul 2010 07:54:03 -0500, "George Jefferson" >>>>>>>>>>>>>>>>>>>> <phreon111(a)gmail.com> wrote: >>>>>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>>>>>> Suppose you have two capacitors connected as >>>>>>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>>>>>> --*-- >>>>>>>>>>>>>>>>>>>>> | | >>>>>>>>>>>>>>>>>>>>> C1 C2 >>>>>>>>>>>>>>>>>>>>> | | >>>>>>>>>>>>>>>>>>>>> ----- >>>>>>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>>>>>> where * is a switch. >>>>>>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>>>>>> What is the total energy before and after the switch >>>>>>>>>>>>>>>>>>>>> is >>>>>>>>>>>>>>>>>>>>> closed(in >>>>>>>>>>>>>>>>>>>>> general). >>>>>>>>>>>>>>>>>>>> Energy is conserved, so it's the same, if you account >>>>>>>>>>>>>>>>>>>> for >>>>>>>>>>>>>>>>>>>> all >>>>>>>>>>>>>>>>>>>> the >>>>>>>>>>>>>>>>>>>> manifestations of energy. >>>>>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>>>> You didn't answer the question. I assume this because >>>>>>>>>>>>>>>>>>> you >>>>>>>>>>>>>>>>>>> don't >>>>>>>>>>>>>>>>>>> know. >>>>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>>> State the question unambiguously and I will. >>>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>>> As I said, the puzzle is both ancient and trivial, so >>>>>>>>>>>>>>>>>> probably >>>>>>>>>>>>>>>>>> JT >>>>>>>>>>>>>>>>>> invented it. There are web sites and even academic papers >>>>>>>>>>>>>>>>>> devoted >>>>>>>>>>>>>>>>>> to >>>>>>>>>>>>>>>>>> it. Given all that, how could I not understand it? >>>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>> Um you don't get it. Your ignorance in basic electronics >>>>>>>>>>>>>>>>> amazes >>>>>>>>>>>>>>>>> me. >>>>>>>>>>>>>>>> That's funny. But people can choose to be amazed in all >>>>>>>>>>>>>>>> sorts >>>>>>>>>>>>>>>> of >>>>>>>>>>>>>>>> ways. >>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>> Michael >>>>>>>>>>>>>>>>> got it(although he didn't explain where the energy went >>>>>>>>>>>>>>>>> but I >>>>>>>>>>>>>>>>> think >>>>>>>>>>>>>>>>> gets >>>>>>>>>>>>>>>>> it). >>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>> Assume the second cap is initially "uncharged" and has the >>>>>>>>>>>>>>>>> same >>>>>>>>>>>>>>>>> capacitance >>>>>>>>>>>>>>>>> as the first. >>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>> Then the initial energy is >>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>> Wi = 1/2*C*V^2 >>>>>>>>>>>>>>>>> Wf = 2*1/2*C*(V/2)^2 = 1/4*C*V^2 = 1/2*Wi >>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>> Hence the final energy of the system 1/2 what we started >>>>>>>>>>>>>>>>> with. >>>>>>>>>>>>>>>> Miraculous calculation. Yours and about 300 web sites that >>>>>>>>>>>>>>>> admire >>>>>>>>>>>>>>>> this >>>>>>>>>>>>>>>> puzzle. >>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>> You didn't wxplain where the energy went - see those 300 >>>>>>>>>>>>>>>> web >>>>>>>>>>>>>>>> sites - >>>>>>>>>>>>>>>> but you are assuming losses. Another solution is that no >>>>>>>>>>>>>>>> energy is >>>>>>>>>>>>>>>> lost, and it rings forever, in which case the final state >>>>>>>>>>>>>>>> that >>>>>>>>>>>>>>>> you >>>>>>>>>>>>>>>> cite never happens. The exact waveforms are actually >>>>>>>>>>>>>>>> interesting. >>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>> I'd really like to hear your explanation but I know thats >>>>>>>>>>>>>>>>> impossible(as >>>>>>>>>>>>>>>>> you'll steal someone elses). After all your the one that >>>>>>>>>>>>>>>>> believes >>>>>>>>>>>>>>>>> charge >>>>>>>>>>>>>>>>> isn't conserved... heres your change to *prove* it. >>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>> Check my previous posts. I noted the exact waveform across >>>>>>>>>>>>>>>> a >>>>>>>>>>>>>>>> resistive >>>>>>>>>>>>>>>> switch, for any values of C1 and C2, and an independent way >>>>>>>>>>>>>>>> to >>>>>>>>>>>>>>>> compute >>>>>>>>>>>>>>>> the energy lost in that switch. >>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>> Given an inductor, one can move all the energy from one >>>>>>>>>>>>>>>> charged >>>>>>>>>>>>>>>> cap >>>>>>>>>>>>>>>> to >>>>>>>>>>>>>>>> another, uncharged one. If the C values are unequal, the >>>>>>>>>>>>>>>> C*V >>>>>>>>>>>>>>>> (charge) >>>>>>>>>>>>>>>> on the first cap obviously becomes a different C*V on the >>>>>>>>>>>>>>>> second >>>>>>>>>>>>>>>> one. >>>>>>>>>>>>>>>> I noted that here some weeks ago, too. >>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>> This is all EE101 stuff. >>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>> John >>>>>>>>>>>>>>> Yes, Q=CV equation is somewhat misleading in this context. A >>>>>>>>>>>>>>> capacitor >>>>>>>>>>>>>>> doesn't store electrical charge, it stores energy. This is a >>>>>>>>>>>>>>> very >>>>>>>>>>>>>>> common >>>>>>>>>>>>>>> misconception, when we say 'charge a capacitor' we don't >>>>>>>>>>>>>>> mean >>>>>>>>>>>>>>> put >>>>>>>>>>>>>>> electrical >>>>>>>>>>>>>>> charge into it, we mean put energy into it. The plates are >>>>>>>>>>>>>>> equal and >>>>>>>>>>>>>>> opposite in electrical charge due to an abundance of >>>>>>>>>>>>>>> electrons >>>>>>>>>>>>>>> on >>>>>>>>>>>>>>> one >>>>>>>>>>>>>>> plate >>>>>>>>>>>>>>> and an equal and opposite charge on the other. The total >>>>>>>>>>>>>>> stored >>>>>>>>>>>>>>> electrical >>>>>>>>>>>>>>> charge in a capacitor is zero, and the Q=CV equation relates >>>>>>>>>>>>>>> to >>>>>>>>>>>>>>> how >>>>>>>>>>>>>>> much >>>>>>>>>>>>>>> charge flowed *in and out* of the capacitor (in fact since >>>>>>>>>>>>>>> electrons >>>>>>>>>>>>>>> can't >>>>>>>>>>>>>>> cross the barrier between the plates, it actually describes >>>>>>>>>>>>>>> the >>>>>>>>>>>>>>> *modulus* >>>>>>>>>>>>>>> of >>>>>>>>>>>>>>> the abundance of charge on each plate, one abundance is >>>>>>>>>>>>>>> positive and >>>>>>>>>>>>>>> the >>>>>>>>>>>>>>> other is negative). >>>>>>>>>>>>>>> >>>>>>>>>>>>>>> Mark. >>>>>>>>>>>>>>> >>>>>>>>>>>>>> That's not what they taught us in college, and that's not the >>>>>>>>>>>>>> way we >>>>>>>>>>>>>> do engineering. 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. >>>>>>>>>>>>>> >>>>>>>>>>>>>> John >>>>>>>>>>>>>> >>>>>>>>>>>>> With all due respect, we don't, and shouldn't, say a capacitor >>>>>>>>>>>>> 'stores >>>>>>>>>>>>> charge'. >>>>>>>>>>>> What do you mean by "we"? Electronic design engineers do this >>>>>>>>>>>> all >>>>>>>>>>>> the >>>>>>>>>>>> time, with reference to capacitors and batteries. You pump X >>>>>>>>>>>> coulombs >>>>>>>>>>>> into a cap; it becomes stored, coincidentally as C*V. You can >>>>>>>>>>>> extract >>>>>>>>>>>> those coulombs later, and the accounting is correct. The math >>>>>>>>>>>> works. >>>>>>>>>>>> The gear works. >>>>>>>>>>>> >>>>>>>>>>>> The misconception comes from the use of the word charge when >>>>>>>>>>>>> talking about putting energy into a capacitor, and more >>>>>>>>>>>>> explictly >>>>>>>>>>>>> the >>>>>>>>>>>>> significant lack of clarification given on this when being >>>>>>>>>>>>> taught. >>>>>>>>>>>>> This >>>>>>>>>>>>> is >>>>>>>>>>>>> compounded by a confusion of the q=C*V equation which actually >>>>>>>>>>>>> relates >>>>>>>>>>>>> to >>>>>>>>>>>>> the charge on the plates, but one of the plates is of the same >>>>>>>>>>>>> value >>>>>>>>>>>>> but >>>>>>>>>>>>> opposite in polarity, so the sum of those is zero. This is an >>>>>>>>>>>>> extremely >>>>>>>>>>>>> popular misunderstanding unfortunately, and leads to >>>>>>>>>>>>> conclusions >>>>>>>>>>>>> that >>>>>>>>>>>>> electrical charge is not conserved. In fact, in a closed >>>>>>>>>>>>> system >>>>>>>>>>>>> where >>>>>>>>>>>>> no >>>>>>>>>>>>> electrical charge can get in or out, within that system >>>>>>>>>>>>> electrical >>>>>>>>>>>>> charge >>>>>>>>>>>>> *is* conserved, it's actually a fundamental law of physics >>>>>>>>>>>>> (along >>>>>>>>>>>>> with >>>>>>>>>>>>> conservation of energy and momentum, again for closed >>>>>>>>>>>>> systems). >>>>>>>>>>>>> >>>>>>>>>>>>> The same current flows in and out of a capacitor when it is >>>>>>>>>>>>> being >>>>>>>>>>>>> 'charged' >>>>>>>>>>>>> (I assume you are not going to deny that). Note I said the >>>>>>>>>>>>> same >>>>>>>>>>>>> current, >>>>>>>>>>>>> but >>>>>>>>>>>>> they are not made of the same electrons because those can't >>>>>>>>>>>>> cross >>>>>>>>>>>>> the >>>>>>>>>>>>> plate >>>>>>>>>>>>> barrier. The same amount of electrical charge that goes in >>>>>>>>>>>>> comes >>>>>>>>>>>>> right >>>>>>>>>>>>> out >>>>>>>>>>>>> again. How can the capacitor possibly end up with a net charge >>>>>>>>>>>>> in >>>>>>>>>>>>> it? >>>>>>>>>>>>> If >>>>>>>>>>>>> it >>>>>>>>>>>>> can, where has the electrical charge come from? Have electrons >>>>>>>>>>>>> just >>>>>>>>>>>>> been >>>>>>>>>>>>> conjured up out of nowhere? >>>>>>>>>>>> It's a different convention. Words. But the units work and the >>>>>>>>>>>> numbers >>>>>>>>>>>> work, so we use it. Call our kind of charge "charge separation" >>>>>>>>>>>> or >>>>>>>>>>>> "plate charge differential" if it makes you happier. >>>>>>>>>>>> >>>>>>>>>>>> Do you design electronics? Do mosfet data sheets refer to >>>>>>>>>>>> stored >>>>>>>>>>>> gate >>>>>>>>>>>> energy, or stored gate charge? >>>>>>>>>>>> >>>>>>>>>>>> John >>>>>>>>>>> Yes, I'm actually an electronics design consultant. >>>>>>>>>>> Take a look at this: >>>>>>>>>>> www.irf.com/technical-info/appnotes/mosfet.pdf >>>>>>>>>>> >>>>>>>>>>> Note the equivalent circuits, which show capacitance between the >>>>>>>>>>> gate, >>>>>>>>>>> source and drain. They talk about 'gate charge' as being a >>>>>>>>>>> conveient way >>>>>>>>>>> of >>>>>>>>>>> relating the capacitance charging and discharging (energy, not >>>>>>>>>>> electrical >>>>>>>>>>> charge) with current, and hence time. Again, confusion can >>>>>>>>>>> arrise >>>>>>>>>>> because >>>>>>>>>>> they use the word 'charge' in two contexts. >>>>>>>>>> They use "charge", in many places, the same way most electronics >>>>>>>>>> engineers use the word, namely C*V. >>>>>>>>>> >>>>>>>>>>> The fact is that since current flows in and out of these >>>>>>>>>>> capacitors >>>>>>>>>>> in >>>>>>>>>>> equal >>>>>>>>>>> amounts the net stored electrical charge on each one is zero. >>>>>>>>>>> However >>>>>>>>>>> the >>>>>>>>>>> Q=CV equation relates to the magnitude of charge that each of >>>>>>>>>>> the >>>>>>>>>>> plates >>>>>>>>>>> of >>>>>>>>>>> these capacitances carries, but for each capacitor there is >>>>>>>>>>> another >>>>>>>>>>> plate >>>>>>>>>>> with equal and opposite charge. >>>>>>>>>> Exactly. We say a cap is "charged" if C*V <> 0. In fact, C*V is >>>>>>>>>> the >>>>>>>>>> exact charge. We say that a cap integrates charge into voltage, >>>>>>>>>> and >>>>>>>>>> that it can return that same charge as we drain it down to zero >>>>>>>>>> volts. >>>>>>>>>> So it's handy to think that a cap can store that charge for us, >>>>>>>>>> which >>>>>>>>>> it actually does. >>>>>>>>>> >>>>>>>>>>> Here is a good derivation of the elecrostatic forces between the >>>>>>>>>>> plates >>>>>>>>>>> of >>>>>>>>>>> a >>>>>>>>>>> parallel plate capacitor. Note that the electrical charge on >>>>>>>>>>> each >>>>>>>>>>> plate >>>>>>>>>>> has >>>>>>>>>>> the same magnitude Q, but one is positive and the other >>>>>>>>>>> negative. >>>>>>>>>>> If you >>>>>>>>>>> think this is not correct maybe you should contact the >>>>>>>>>>> University >>>>>>>>>>> of >>>>>>>>>>> Pennsylvania and tell them :) >>>>>>>>>>> http://dept.physics.upenn.edu/~uglabs/lab_manual/electric_forces.pdf >>>>>>>>>> The plates need not gave the same absolute Q, because the overall >>>>>>>>>> cap >>>>>>>>>> can have a net charge, what we electronic guys would call an >>>>>>>>>> electrostatic charge. That ususlly doesn't matter to us, so we >>>>>>>>>> use >>>>>>>>>> the >>>>>>>>>> conviently short word "charge" to mean C*V, where V is the sort >>>>>>>>>> of >>>>>>>>>> potential difference we measure with a 2-terminal voltmeter. When >>>>>>>>>> we >>>>>>>>>> rarely refer to physics-type net charge, we say "electrostatic >>>>>>>>>> charge." >>>>>>>>>> >>>>>>>>>> To express the concept of "charge on a capacitor" any way other >>>>>>>>>> than >>>>>>>>>> the way we use it would be grammatically and numerically very >>>>>>>>>> messy. >>>>>>>>>> >>>>>>>>>> But when we use the term this way, we have to be careful to >>>>>>>>>> remember >>>>>>>>>> what it means to us, and we can't blindly say stuff like "charge >>>>>>>>>> is >>>>>>>>>> conserved" without thinking carefully. It's safe to say "energy >>>>>>>>>> is >>>>>>>>>> conserved." >>>>>>>>>> >>>>>>>>>> John >>>>>>>>> I'm giving up. Your concept of charge is obviously not Coulombs! >>>>>>>> Q=C*V is coulombs. The units work. If a cap stores 5 coulombs, I >>>>>>>> can >>>>>>>> load it at 2.5 amps for 2 seconds, and then the charge is gone. It >>>>>>>> works. >>>>>>>> >>>>>>>> John >>>>>>> Actually the capacitor stores + 5 Coulombs on one plate, and -5 >>>>>>> Coulombs on >>>>>>> the other plate. When you 'load it at 2.5 Amps for 2.5 seconds' the >>>>>>> negatively charged electrons move from the negatively charged plate, >>>>>>> through >>>>>>> the external circuit, and into the positively charged plate. At the >>>>>>> end >>>>>>> there are zero Coulombs stored on both plates. This is why the maths >>>>>>> works, >>>>>>> at no point does the capacitor as a whole ever have net stored >>>>>>> charge, >>>>>>> the >>>>>>> sum of the charges of both plates is always zero. In your system of >>>>>>> stored >>>>>>> charge you cannot explain why, if the same current is flowing in and >>>>>>> out of >>>>>>> the capacitor, it doesn't violate the Law of Conservation of Charge. >>>>>>> >>>>>>> >>>>>> 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 >>>>>> >>>>> >>>>> 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. >>>>> >>>> >>>> From: >>>> http://www.google.com/url?sa=t&source=web&cd=94&ved=0CB4QFjADOFo&url=http%3A%2F%2Furegina.ca%2F~benslamk%2Fteach%2Fphys201%2Flect201-6.pdf&ei=RkxcTP32IIH4swOZgZFB&usg=AFQjCNHh0gfLiX04UuCcC1Jjwdja7IvM6g >>>> >>>> "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." >>>> >>>> -- >>>> Virg Wall, P.E. >>> >>>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. >> >> Simply because the strict interpretation you argued for is useless for >> practical electronics design ;) > > 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! > >> >> 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. >> > > 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": > http://en.wikipedia.org/wiki/Capacitance) > > Mark. 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 +/-q on the plates, related to voltage by the q=CV equation. 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.
From: John Larkin on 7 Aug 2010 11:17 On Sat, 7 Aug 2010 11:52:23 +0100, "markp" <map.nospam(a)f2s.com> wrote: > >"markp" <map.nospam(a)f2s.com> wrote in message >news:8c3jbcFr8rU1(a)mid.individual.net... >> >> "Grant" <omg(a)grrr.id.au> wrote in message >> news:ko2p56p8k3pdimdkkdnp627vi2q6eqfdm1(a)4ax.com... >>> On Fri, 6 Aug 2010 20:16:38 +0100, "markp" <map.nospam(a)f2s.com> wrote: >>> >>>> >>>>"VWWall" <vwall(a)large.invalid> wrote in message >>>>news:_M-dnTT-O9YTzcHRnZ2dnUVZ_vOdnZ2d(a)earthlink.com... >>>>> markp wrote: >>>>>> "John Larkin" <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote in >>>>>> message news:258o56l0k2k1id6bm27oevicacm6ifd5ft(a)4ax.com... >>>>>>> On Fri, 6 Aug 2010 15:05:38 +0100, "markp" <map.nospam(a)f2s.com> >>>>>>> wrote: >>>>>>> >>>>>>>> "John Larkin" <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote in >>>>>>>> message >>>>>>>> news:u4sm56lncllfueedvo2ahi4ihr8j00gu3p(a)4ax.com... >>>>>>>>> On Thu, 5 Aug 2010 18:43:28 +0100, "markp" <map.nospam(a)f2s.com> >>>>>>>>> wrote: >>>>>>>>> >>>>>>>>>> "John Larkin" <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote >>>>>>>>>> in >>>>>>>>>> message >>>>>>>>>> news:7tjl5615o4lklftqq34fncd86soc75forh(a)4ax.com... >>>>>>>>>>> On Thu, 5 Aug 2010 13:20:12 +0100, "markp" <map.nospam(a)f2s.com> >>>>>>>>>>> wrote: >>>>>>>>>>> >>>>>>>>>>>> "John Larkin" <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote >>>>>>>>>>>> in >>>>>>>>>>>> message >>>>>>>>>>>> news:889k5654o0h9qfgs3cej7gfe99ahsg42am(a)4ax.com... >>>>>>>>>>>>> On Thu, 5 Aug 2010 02:40:12 +0100, "markp" <map.nospam(a)f2s.com> >>>>>>>>>>>>> wrote: >>>>>>>>>>>>> >>>>>>>>>>>>>> "John Larkin" <jjlarkin(a)highNOTlandTHIStechnologyPART.com> >>>>>>>>>>>>>> wrote >>>>>>>>>>>>>> in >>>>>>>>>>>>>> message >>>>>>>>>>>>>> news:ci3k56d0kga1776gghosaq09q2e0i2ahhq(a)4ax.com... >>>>>>>>>>>>>>> On Wed, 4 Aug 2010 16:55:16 +0100, "markp" >>>>>>>>>>>>>>> <map.nospam(a)f2s.com> >>>>>>>>>>>>>>> wrote: >>>>>>>>>>>>>>> >>>>>>>>>>>>>>>> "John Larkin" <jjlarkin(a)highNOTlandTHIStechnologyPART.com> >>>>>>>>>>>>>>>> wrote in >>>>>>>>>>>>>>>> message >>>>>>>>>>>>>>>> news:2vge46h4sragrk4jdn6sasde6hg2r52nos(a)4ax.com... >>>>>>>>>>>>>>>>> On Wed, 21 Jul 2010 12:17:41 -0500, "George Jefferson" >>>>>>>>>>>>>>>>> <phreon111(a)gmail.com> wrote: >>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>>> "John Larkin" <jjlarkin(a)highNOTlandTHIStechnologyPART.com> >>>>>>>>>>>>>>>>>> wrote >>>>>>>>>>>>>>>>>> in >>>>>>>>>>>>>>>>>> message >>>>>>>>>>>>>>>>>> news:dj7e465sga7fe3nq7hfl3f0uk601pvrem8(a)4ax.com... >>>>>>>>>>>>>>>>>>> On Wed, 21 Jul 2010 11:19:31 -0500, "George Jefferson" >>>>>>>>>>>>>>>>>>> <phreon111(a)gmail.com> wrote: >>>>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>>>>> "John Larkin" >>>>>>>>>>>>>>>>>>>> <jjlarkin(a)highNOTlandTHIStechnologyPART.com> >>>>>>>>>>>>>>>>>>>> wrote >>>>>>>>>>>>>>>>>>>> in >>>>>>>>>>>>>>>>>>>> message >>>>>>>>>>>>>>>>>>>> news:s43e46la1p1vt11527eg3ptl9ulm44dfrj(a)4ax.com... >>>>>>>>>>>>>>>>>>>>> On Wed, 21 Jul 2010 07:54:03 -0500, "George Jefferson" >>>>>>>>>>>>>>>>>>>>> <phreon111(a)gmail.com> wrote: >>>>>>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>>>>>>> Suppose you have two capacitors connected as >>>>>>>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>>>>>>> --*-- >>>>>>>>>>>>>>>>>>>>>> | | >>>>>>>>>>>>>>>>>>>>>> C1 C2 >>>>>>>>>>>>>>>>>>>>>> | | >>>>>>>>>>>>>>>>>>>>>> ----- >>>>>>>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>>>>>>> where * is a switch. >>>>>>>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>>>>>>> What is the total energy before and after the switch >>>>>>>>>>>>>>>>>>>>>> is >>>>>>>>>>>>>>>>>>>>>> closed(in >>>>>>>>>>>>>>>>>>>>>> general). >>>>>>>>>>>>>>>>>>>>> Energy is conserved, so it's the same, if you account >>>>>>>>>>>>>>>>>>>>> for >>>>>>>>>>>>>>>>>>>>> all >>>>>>>>>>>>>>>>>>>>> the >>>>>>>>>>>>>>>>>>>>> manifestations of energy. >>>>>>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>>>>> You didn't answer the question. I assume this because >>>>>>>>>>>>>>>>>>>> you >>>>>>>>>>>>>>>>>>>> don't >>>>>>>>>>>>>>>>>>>> know. >>>>>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>>>> State the question unambiguously and I will. >>>>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>>>> As I said, the puzzle is both ancient and trivial, so >>>>>>>>>>>>>>>>>>> probably >>>>>>>>>>>>>>>>>>> JT >>>>>>>>>>>>>>>>>>> invented it. There are web sites and even academic papers >>>>>>>>>>>>>>>>>>> devoted >>>>>>>>>>>>>>>>>>> to >>>>>>>>>>>>>>>>>>> it. Given all that, how could I not understand it? >>>>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>>> Um you don't get it. Your ignorance in basic electronics >>>>>>>>>>>>>>>>>> amazes >>>>>>>>>>>>>>>>>> me. >>>>>>>>>>>>>>>>> That's funny. But people can choose to be amazed in all >>>>>>>>>>>>>>>>> sorts >>>>>>>>>>>>>>>>> of >>>>>>>>>>>>>>>>> ways. >>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>> Michael >>>>>>>>>>>>>>>>>> got it(although he didn't explain where the energy went >>>>>>>>>>>>>>>>>> but I >>>>>>>>>>>>>>>>>> think >>>>>>>>>>>>>>>>>> gets >>>>>>>>>>>>>>>>>> it). >>>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>>> Assume the second cap is initially "uncharged" and has the >>>>>>>>>>>>>>>>>> same >>>>>>>>>>>>>>>>>> capacitance >>>>>>>>>>>>>>>>>> as the first. >>>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>>> Then the initial energy is >>>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>>> Wi = 1/2*C*V^2 >>>>>>>>>>>>>>>>>> Wf = 2*1/2*C*(V/2)^2 = 1/4*C*V^2 = 1/2*Wi >>>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>>> Hence the final energy of the system 1/2 what we started >>>>>>>>>>>>>>>>>> with. >>>>>>>>>>>>>>>>> Miraculous calculation. Yours and about 300 web sites that >>>>>>>>>>>>>>>>> admire >>>>>>>>>>>>>>>>> this >>>>>>>>>>>>>>>>> puzzle. >>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>> You didn't wxplain where the energy went - see those 300 >>>>>>>>>>>>>>>>> web >>>>>>>>>>>>>>>>> sites - >>>>>>>>>>>>>>>>> but you are assuming losses. Another solution is that no >>>>>>>>>>>>>>>>> energy is >>>>>>>>>>>>>>>>> lost, and it rings forever, in which case the final state >>>>>>>>>>>>>>>>> that >>>>>>>>>>>>>>>>> you >>>>>>>>>>>>>>>>> cite never happens. The exact waveforms are actually >>>>>>>>>>>>>>>>> interesting. >>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>>> I'd really like to hear your explanation but I know thats >>>>>>>>>>>>>>>>>> impossible(as >>>>>>>>>>>>>>>>>> you'll steal someone elses). After all your the one that >>>>>>>>>>>>>>>>>> believes >>>>>>>>>>>>>>>>>> charge >>>>>>>>>>>>>>>>>> isn't conserved... heres your change to *prove* it. >>>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>> Check my previous posts. I noted the exact waveform across >>>>>>>>>>>>>>>>> a >>>>>>>>>>>>>>>>> resistive >>>>>>>>>>>>>>>>> switch, for any values of C1 and C2, and an independent way >>>>>>>>>>>>>>>>> to >>>>>>>>>>>>>>>>> compute >>>>>>>>>>>>>>>>> the energy lost in that switch. >>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>> Given an inductor, one can move all the energy from one >>>>>>>>>>>>>>>>> charged >>>>>>>>>>>>>>>>> cap >>>>>>>>>>>>>>>>> to >>>>>>>>>>>>>>>>> another, uncharged one. If the C values are unequal, the >>>>>>>>>>>>>>>>> C*V >>>>>>>>>>>>>>>>> (charge) >>>>>>>>>>>>>>>>> on the first cap obviously becomes a different C*V on the >>>>>>>>>>>>>>>>> second >>>>>>>>>>>>>>>>> one. >>>>>>>>>>>>>>>>> I noted that here some weeks ago, too. >>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>> This is all EE101 stuff. >>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>> John >>>>>>>>>>>>>>>> Yes, Q=CV equation is somewhat misleading in this context. A >>>>>>>>>>>>>>>> capacitor >>>>>>>>>>>>>>>> doesn't store electrical charge, it stores energy. This is a >>>>>>>>>>>>>>>> very >>>>>>>>>>>>>>>> common >>>>>>>>>>>>>>>> misconception, when we say 'charge a capacitor' we don't >>>>>>>>>>>>>>>> mean >>>>>>>>>>>>>>>> put >>>>>>>>>>>>>>>> electrical >>>>>>>>>>>>>>>> charge into it, we mean put energy into it. The plates are >>>>>>>>>>>>>>>> equal and >>>>>>>>>>>>>>>> opposite in electrical charge due to an abundance of >>>>>>>>>>>>>>>> electrons >>>>>>>>>>>>>>>> on >>>>>>>>>>>>>>>> one >>>>>>>>>>>>>>>> plate >>>>>>>>>>>>>>>> and an equal and opposite charge on the other. The total >>>>>>>>>>>>>>>> stored >>>>>>>>>>>>>>>> electrical >>>>>>>>>>>>>>>> charge in a capacitor is zero, and the Q=CV equation relates >>>>>>>>>>>>>>>> to >>>>>>>>>>>>>>>> how >>>>>>>>>>>>>>>> much >>>>>>>>>>>>>>>> charge flowed *in and out* of the capacitor (in fact since >>>>>>>>>>>>>>>> electrons >>>>>>>>>>>>>>>> can't >>>>>>>>>>>>>>>> cross the barrier between the plates, it actually describes >>>>>>>>>>>>>>>> the >>>>>>>>>>>>>>>> *modulus* >>>>>>>>>>>>>>>> of >>>>>>>>>>>>>>>> the abundance of charge on each plate, one abundance is >>>>>>>>>>>>>>>> positive and >>>>>>>>>>>>>>>> the >>>>>>>>>>>>>>>> other is negative). >>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>> Mark. >>>>>>>>>>>>>>>> >>>>>>>>>>>>>>> That's not what they taught us in college, and that's not the >>>>>>>>>>>>>>> way we >>>>>>>>>>>>>>> do engineering. 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. >>>>>>>>>>>>>>> >>>>>>>>>>>>>>> John >>>>>>>>>>>>>>> >>>>>>>>>>>>>> With all due respect, we don't, and shouldn't, say a capacitor >>>>>>>>>>>>>> 'stores >>>>>>>>>>>>>> charge'. >>>>>>>>>>>>> What do you mean by "we"? Electronic design engineers do this >>>>>>>>>>>>> all >>>>>>>>>>>>> the >>>>>>>>>>>>> time, with reference to capacitors and batteries. You pump X >>>>>>>>>>>>> coulombs >>>>>>>>>>>>> into a cap; it becomes stored, coincidentally as C*V. You can >>>>>>>>>>>>> extract >>>>>>>>>>>>> those coulombs later, and the accounting is correct. The math >>>>>>>>>>>>> works. >>>>>>>>>>>>> The gear works. >>>>>>>>>>>>> >>>>>>>>>>>>> The misconception comes from the use of the word charge when >>>>>>>>>>>>>> talking about putting energy into a capacitor, and more >>>>>>>>>>>>>> explictly >>>>>>>>>>>>>> the >>>>>>>>>>>>>> significant lack of clarification given on this when being >>>>>>>>>>>>>> taught. >>>>>>>>>>>>>> This >>>>>>>>>>>>>> is >>>>>>>>>>>>>> compounded by a confusion of the q=C*V equation which actually >>>>>>>>>>>>>> relates >>>>>>>>>>>>>> to >>>>>>>>>>>>>> the charge on the plates, but one of the plates is of the same >>>>>>>>>>>>>> value >>>>>>>>>>>>>> but >>>>>>>>>>>>>> opposite in polarity, so the sum of those is zero. This is an >>>>>>>>>>>>>> extremely >>>>>>>>>>>>>> popular misunderstanding unfortunately, and leads to >>>>>>>>>>>>>> conclusions >>>>>>>>>>>>>> that >>>>>>>>>>>>>> electrical charge is not conserved. In fact, in a closed >>>>>>>>>>>>>> system >>>>>>>>>>>>>> where >>>>>>>>>>>>>> no >>>>>>>>>>>>>> electrical charge can get in or out, within that system >>>>>>>>>>>>>> electrical >>>>>>>>>>>>>> charge >>>>>>>>>>>>>> *is* conserved, it's actually a fundamental law of physics >>>>>>>>>>>>>> (along >>>>>>>>>>>>>> with >>>>>>>>>>>>>> conservation of energy and momentum, again for closed >>>>>>>>>>>>>> systems). >>>>>>>>>>>>>> >>>>>>>>>>>>>> The same current flows in and out of a capacitor when it is >>>>>>>>>>>>>> being >>>>>>>>>>>>>> 'charged' >>>>>>>>>>>>>> (I assume you are not going to deny that). Note I said the >>>>>>>>>>>>>> same >>>>>>>>>>>>>> current, >>>>>>>>>>>>>> but >>>>>>>>>>>>>> they are not made of the same electrons because those can't >>>>>>>>>>>>>> cross >>>>>>>>>>>>>> the >>>>>>>>>>>>>> plate >>>>>>>>>>>>>> barrier. The same amount of electrical charge that goes in >>>>>>>>>>>>>> comes >>>>>>>>>>>>>> right >>>>>>>>>>>>>> out >>>>>>>>>>>>>> again. How can the capacitor possibly end up with a net charge >>>>>>>>>>>>>> in >>>>>>>>>>>>>> it? >>>>>>>>>>>>>> If >>>>>>>>>>>>>> it >>>>>>>>>>>>>> can, where has the electrical charge come from? Have electrons >>>>>>>>>>>>>> just >>>>>>>>>>>>>> been >>>>>>>>>>>>>> conjured up out of nowhere? >>>>>>>>>>>>> It's a different convention. Words. But the units work and the >>>>>>>>>>>>> numbers >>>>>>>>>>>>> work, so we use it. Call our kind of charge "charge separation" >>>>>>>>>>>>> or >>>>>>>>>>>>> "plate charge differential" if it makes you happier. >>>>>>>>>>>>> >>>>>>>>>>>>> Do you design electronics? Do mosfet data sheets refer to >>>>>>>>>>>>> stored >>>>>>>>>>>>> gate >>>>>>>>>>>>> energy, or stored gate charge? >>>>>>>>>>>>> >>>>>>>>>>>>> John >>>>>>>>>>>> Yes, I'm actually an electronics design consultant. >>>>>>>>>>>> Take a look at this: >>>>>>>>>>>> www.irf.com/technical-info/appnotes/mosfet.pdf >>>>>>>>>>>> >>>>>>>>>>>> Note the equivalent circuits, which show capacitance between the >>>>>>>>>>>> gate, >>>>>>>>>>>> source and drain. They talk about 'gate charge' as being a >>>>>>>>>>>> conveient way >>>>>>>>>>>> of >>>>>>>>>>>> relating the capacitance charging and discharging (energy, not >>>>>>>>>>>> electrical >>>>>>>>>>>> charge) with current, and hence time. Again, confusion can >>>>>>>>>>>> arrise >>>>>>>>>>>> because >>>>>>>>>>>> they use the word 'charge' in two contexts. >>>>>>>>>>> They use "charge", in many places, the same way most electronics >>>>>>>>>>> engineers use the word, namely C*V. >>>>>>>>>>> >>>>>>>>>>>> The fact is that since current flows in and out of these >>>>>>>>>>>> capacitors >>>>>>>>>>>> in >>>>>>>>>>>> equal >>>>>>>>>>>> amounts the net stored electrical charge on each one is zero. >>>>>>>>>>>> However >>>>>>>>>>>> the >>>>>>>>>>>> Q=CV equation relates to the magnitude of charge that each of >>>>>>>>>>>> the >>>>>>>>>>>> plates >>>>>>>>>>>> of >>>>>>>>>>>> these capacitances carries, but for each capacitor there is >>>>>>>>>>>> another >>>>>>>>>>>> plate >>>>>>>>>>>> with equal and opposite charge. >>>>>>>>>>> Exactly. We say a cap is "charged" if C*V <> 0. In fact, C*V is >>>>>>>>>>> the >>>>>>>>>>> exact charge. We say that a cap integrates charge into voltage, >>>>>>>>>>> and >>>>>>>>>>> that it can return that same charge as we drain it down to zero >>>>>>>>>>> volts. >>>>>>>>>>> So it's handy to think that a cap can store that charge for us, >>>>>>>>>>> which >>>>>>>>>>> it actually does. >>>>>>>>>>> >>>>>>>>>>>> Here is a good derivation of the elecrostatic forces between the >>>>>>>>>>>> plates >>>>>>>>>>>> of >>>>>>>>>>>> a >>>>>>>>>>>> parallel plate capacitor. Note that the electrical charge on >>>>>>>>>>>> each >>>>>>>>>>>> plate >>>>>>>>>>>> has >>>>>>>>>>>> the same magnitude Q, but one is positive and the other >>>>>>>>>>>> negative. >>>>>>>>>>>> If you >>>>>>>>>>>> think this is not correct maybe you should contact the >>>>>>>>>>>> University >>>>>>>>>>>> of >>>>>>>>>>>> Pennsylvania and tell them :) >>>>>>>>>>>> http://dept.physics.upenn.edu/~uglabs/lab_manual/electric_forces.pdf >>>>>>>>>>> The plates need not gave the same absolute Q, because the overall >>>>>>>>>>> cap >>>>>>>>>>> can have a net charge, what we electronic guys would call an >>>>>>>>>>> electrostatic charge. That ususlly doesn't matter to us, so we >>>>>>>>>>> use >>>>>>>>>>> the >>>>>>>>>>> conviently short word "charge" to mean C*V, where V is the sort >>>>>>>>>>> of >>>>>>>>>>> potential difference we measure with a 2-terminal voltmeter. When >>>>>>>>>>> we >>>>>>>>>>> rarely refer to physics-type net charge, we say "electrostatic >>>>>>>>>>> charge." >>>>>>>>>>> >>>>>>>>>>> To express the concept of "charge on a capacitor" any way other >>>>>>>>>>> than >>>>>>>>>>> the way we use it would be grammatically and numerically very >>>>>>>>>>> messy. >>>>>>>>>>> >>>>>>>>>>> But when we use the term this way, we have to be careful to >>>>>>>>>>> remember >>>>>>>>>>> what it means to us, and we can't blindly say stuff like "charge >>>>>>>>>>> is >>>>>>>>>>> conserved" without thinking carefully. It's safe to say "energy >>>>>>>>>>> is >>>>>>>>>>> conserved." >>>>>>>>>>> >>>>>>>>>>> John >>>>>>>>>> I'm giving up. Your concept of charge is obviously not Coulombs! >>>>>>>>> Q=C*V is coulombs. The units work. If a cap stores 5 coulombs, I >>>>>>>>> can >>>>>>>>> load it at 2.5 amps for 2 seconds, and then the charge is gone. It >>>>>>>>> works. >>>>>>>>> >>>>>>>>> John >>>>>>>> Actually the capacitor stores + 5 Coulombs on one plate, and -5 >>>>>>>> Coulombs on >>>>>>>> the other plate. When you 'load it at 2.5 Amps for 2.5 seconds' the >>>>>>>> negatively charged electrons move from the negatively charged plate, >>>>>>>> through >>>>>>>> the external circuit, and into the positively charged plate. At the >>>>>>>> end >>>>>>>> there are zero Coulombs stored on both plates. This is why the maths >>>>>>>> works, >>>>>>>> at no point does the capacitor as a whole ever have net stored >>>>>>>> charge, >>>>>>>> the >>>>>>>> sum of the charges of both plates is always zero. In your system of >>>>>>>> stored >>>>>>>> charge you cannot explain why, if the same current is flowing in and >>>>>>>> out of >>>>>>>> the capacitor, it doesn't violate the Law of Conservation of Charge. >>>>>>>> >>>>>>>> >>>>>>> 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 >>>>>>> >>>>>> >>>>>> 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. >>>>>> >>>>> >>>>> From: >>>>> http://www.google.com/url?sa=t&source=web&cd=94&ved=0CB4QFjADOFo&url=http%3A%2F%2Furegina.ca%2F~benslamk%2Fteach%2Fphys201%2Flect201-6.pdf&ei=RkxcTP32IIH4swOZgZFB&usg=AFQjCNHh0gfLiX04UuCcC1Jjwdja7IvM6g >>>>> >>>>> "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." >>>>> >>>>> -- >>>>> Virg Wall, P.E. >>>> >>>>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. >>> >>> Simply because the strict interpretation you argued for is useless for >>> practical electronics design ;) >> >> 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! >> >>> >>> 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. >>> >> >> 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": >> http://en.wikipedia.org/wiki/Capacitance) >> >> Mark. > >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 >+/-q on the plates, related to voltage by the q=CV equation. > >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. 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
From: markp on 7 Aug 2010 11:43 "John Larkin" <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote in message news:s2tq56p0ugnumreagnvkc57sv0su58e95o(a)4ax.com... > On Sat, 7 Aug 2010 11:52:23 +0100, "markp" <map.nospam(a)f2s.com> wrote: > >> >>"markp" <map.nospam(a)f2s.com> wrote in message >>news:8c3jbcFr8rU1(a)mid.individual.net... >>> >>> "Grant" <omg(a)grrr.id.au> wrote in message >>> news:ko2p56p8k3pdimdkkdnp627vi2q6eqfdm1(a)4ax.com... >>>> On Fri, 6 Aug 2010 20:16:38 +0100, "markp" <map.nospam(a)f2s.com> wrote: >>>> >>>>> >>>>>"VWWall" <vwall(a)large.invalid> wrote in message >>>>>news:_M-dnTT-O9YTzcHRnZ2dnUVZ_vOdnZ2d(a)earthlink.com... >>>>>> markp wrote: >>>>>>> "John Larkin" <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote in >>>>>>> message news:258o56l0k2k1id6bm27oevicacm6ifd5ft(a)4ax.com... >>>>>>>> On Fri, 6 Aug 2010 15:05:38 +0100, "markp" <map.nospam(a)f2s.com> >>>>>>>> wrote: >>>>>>>> >>>>>>>>> "John Larkin" <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote >>>>>>>>> in >>>>>>>>> message >>>>>>>>> news:u4sm56lncllfueedvo2ahi4ihr8j00gu3p(a)4ax.com... >>>>>>>>>> On Thu, 5 Aug 2010 18:43:28 +0100, "markp" <map.nospam(a)f2s.com> >>>>>>>>>> wrote: >>>>>>>>>> >>>>>>>>>>> "John Larkin" <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote >>>>>>>>>>> in >>>>>>>>>>> message >>>>>>>>>>> news:7tjl5615o4lklftqq34fncd86soc75forh(a)4ax.com... >>>>>>>>>>>> On Thu, 5 Aug 2010 13:20:12 +0100, "markp" <map.nospam(a)f2s.com> >>>>>>>>>>>> wrote: >>>>>>>>>>>> >>>>>>>>>>>>> "John Larkin" <jjlarkin(a)highNOTlandTHIStechnologyPART.com> >>>>>>>>>>>>> wrote >>>>>>>>>>>>> in >>>>>>>>>>>>> message >>>>>>>>>>>>> news:889k5654o0h9qfgs3cej7gfe99ahsg42am(a)4ax.com... >>>>>>>>>>>>>> On Thu, 5 Aug 2010 02:40:12 +0100, "markp" >>>>>>>>>>>>>> <map.nospam(a)f2s.com> >>>>>>>>>>>>>> wrote: >>>>>>>>>>>>>> >>>>>>>>>>>>>>> "John Larkin" <jjlarkin(a)highNOTlandTHIStechnologyPART.com> >>>>>>>>>>>>>>> wrote >>>>>>>>>>>>>>> in >>>>>>>>>>>>>>> message >>>>>>>>>>>>>>> news:ci3k56d0kga1776gghosaq09q2e0i2ahhq(a)4ax.com... >>>>>>>>>>>>>>>> On Wed, 4 Aug 2010 16:55:16 +0100, "markp" >>>>>>>>>>>>>>>> <map.nospam(a)f2s.com> >>>>>>>>>>>>>>>> wrote: >>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>> "John Larkin" <jjlarkin(a)highNOTlandTHIStechnologyPART.com> >>>>>>>>>>>>>>>>> wrote in >>>>>>>>>>>>>>>>> message >>>>>>>>>>>>>>>>> news:2vge46h4sragrk4jdn6sasde6hg2r52nos(a)4ax.com... >>>>>>>>>>>>>>>>>> On Wed, 21 Jul 2010 12:17:41 -0500, "George Jefferson" >>>>>>>>>>>>>>>>>> <phreon111(a)gmail.com> wrote: >>>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>>>> "John Larkin" >>>>>>>>>>>>>>>>>>> <jjlarkin(a)highNOTlandTHIStechnologyPART.com> >>>>>>>>>>>>>>>>>>> wrote >>>>>>>>>>>>>>>>>>> in >>>>>>>>>>>>>>>>>>> message >>>>>>>>>>>>>>>>>>> news:dj7e465sga7fe3nq7hfl3f0uk601pvrem8(a)4ax.com... >>>>>>>>>>>>>>>>>>>> On Wed, 21 Jul 2010 11:19:31 -0500, "George Jefferson" >>>>>>>>>>>>>>>>>>>> <phreon111(a)gmail.com> wrote: >>>>>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>>>>>> "John Larkin" >>>>>>>>>>>>>>>>>>>>> <jjlarkin(a)highNOTlandTHIStechnologyPART.com> >>>>>>>>>>>>>>>>>>>>> wrote >>>>>>>>>>>>>>>>>>>>> in >>>>>>>>>>>>>>>>>>>>> message >>>>>>>>>>>>>>>>>>>>> news:s43e46la1p1vt11527eg3ptl9ulm44dfrj(a)4ax.com... >>>>>>>>>>>>>>>>>>>>>> On Wed, 21 Jul 2010 07:54:03 -0500, "George >>>>>>>>>>>>>>>>>>>>>> Jefferson" >>>>>>>>>>>>>>>>>>>>>> <phreon111(a)gmail.com> wrote: >>>>>>>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>>>>>>>> Suppose you have two capacitors connected as >>>>>>>>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>>>>>>>> --*-- >>>>>>>>>>>>>>>>>>>>>>> | | >>>>>>>>>>>>>>>>>>>>>>> C1 C2 >>>>>>>>>>>>>>>>>>>>>>> | | >>>>>>>>>>>>>>>>>>>>>>> ----- >>>>>>>>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>>>>>>>> where * is a switch. >>>>>>>>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>>>>>>>> What is the total energy before and after the switch >>>>>>>>>>>>>>>>>>>>>>> is >>>>>>>>>>>>>>>>>>>>>>> closed(in >>>>>>>>>>>>>>>>>>>>>>> general). >>>>>>>>>>>>>>>>>>>>>> Energy is conserved, so it's the same, if you account >>>>>>>>>>>>>>>>>>>>>> for >>>>>>>>>>>>>>>>>>>>>> all >>>>>>>>>>>>>>>>>>>>>> the >>>>>>>>>>>>>>>>>>>>>> manifestations of energy. >>>>>>>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>>>>>> You didn't answer the question. I assume this because >>>>>>>>>>>>>>>>>>>>> you >>>>>>>>>>>>>>>>>>>>> don't >>>>>>>>>>>>>>>>>>>>> know. >>>>>>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>>>>> State the question unambiguously and I will. >>>>>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>>>>> As I said, the puzzle is both ancient and trivial, so >>>>>>>>>>>>>>>>>>>> probably >>>>>>>>>>>>>>>>>>>> JT >>>>>>>>>>>>>>>>>>>> invented it. There are web sites and even academic >>>>>>>>>>>>>>>>>>>> papers >>>>>>>>>>>>>>>>>>>> devoted >>>>>>>>>>>>>>>>>>>> to >>>>>>>>>>>>>>>>>>>> it. Given all that, how could I not understand it? >>>>>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>>>> Um you don't get it. Your ignorance in basic electronics >>>>>>>>>>>>>>>>>>> amazes >>>>>>>>>>>>>>>>>>> me. >>>>>>>>>>>>>>>>>> That's funny. But people can choose to be amazed in all >>>>>>>>>>>>>>>>>> sorts >>>>>>>>>>>>>>>>>> of >>>>>>>>>>>>>>>>>> ways. >>>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>>> Michael >>>>>>>>>>>>>>>>>>> got it(although he didn't explain where the energy went >>>>>>>>>>>>>>>>>>> but I >>>>>>>>>>>>>>>>>>> think >>>>>>>>>>>>>>>>>>> gets >>>>>>>>>>>>>>>>>>> it). >>>>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>>>> Assume the second cap is initially "uncharged" and has >>>>>>>>>>>>>>>>>>> the >>>>>>>>>>>>>>>>>>> same >>>>>>>>>>>>>>>>>>> capacitance >>>>>>>>>>>>>>>>>>> as the first. >>>>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>>>> Then the initial energy is >>>>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>>>> Wi = 1/2*C*V^2 >>>>>>>>>>>>>>>>>>> Wf = 2*1/2*C*(V/2)^2 = 1/4*C*V^2 = 1/2*Wi >>>>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>>>> Hence the final energy of the system 1/2 what we started >>>>>>>>>>>>>>>>>>> with. >>>>>>>>>>>>>>>>>> Miraculous calculation. Yours and about 300 web sites >>>>>>>>>>>>>>>>>> that >>>>>>>>>>>>>>>>>> admire >>>>>>>>>>>>>>>>>> this >>>>>>>>>>>>>>>>>> puzzle. >>>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>>> You didn't wxplain where the energy went - see those 300 >>>>>>>>>>>>>>>>>> web >>>>>>>>>>>>>>>>>> sites - >>>>>>>>>>>>>>>>>> but you are assuming losses. Another solution is that no >>>>>>>>>>>>>>>>>> energy is >>>>>>>>>>>>>>>>>> lost, and it rings forever, in which case the final state >>>>>>>>>>>>>>>>>> that >>>>>>>>>>>>>>>>>> you >>>>>>>>>>>>>>>>>> cite never happens. The exact waveforms are actually >>>>>>>>>>>>>>>>>> interesting. >>>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>>>> I'd really like to hear your explanation but I know >>>>>>>>>>>>>>>>>>> thats >>>>>>>>>>>>>>>>>>> impossible(as >>>>>>>>>>>>>>>>>>> you'll steal someone elses). After all your the one that >>>>>>>>>>>>>>>>>>> believes >>>>>>>>>>>>>>>>>>> charge >>>>>>>>>>>>>>>>>>> isn't conserved... heres your change to *prove* it. >>>>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>>> Check my previous posts. I noted the exact waveform >>>>>>>>>>>>>>>>>> across >>>>>>>>>>>>>>>>>> a >>>>>>>>>>>>>>>>>> resistive >>>>>>>>>>>>>>>>>> switch, for any values of C1 and C2, and an independent >>>>>>>>>>>>>>>>>> way >>>>>>>>>>>>>>>>>> to >>>>>>>>>>>>>>>>>> compute >>>>>>>>>>>>>>>>>> the energy lost in that switch. >>>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>>> Given an inductor, one can move all the energy from one >>>>>>>>>>>>>>>>>> charged >>>>>>>>>>>>>>>>>> cap >>>>>>>>>>>>>>>>>> to >>>>>>>>>>>>>>>>>> another, uncharged one. If the C values are unequal, the >>>>>>>>>>>>>>>>>> C*V >>>>>>>>>>>>>>>>>> (charge) >>>>>>>>>>>>>>>>>> on the first cap obviously becomes a different C*V on the >>>>>>>>>>>>>>>>>> second >>>>>>>>>>>>>>>>>> one. >>>>>>>>>>>>>>>>>> I noted that here some weeks ago, too. >>>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>>> This is all EE101 stuff. >>>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>>> John >>>>>>>>>>>>>>>>> Yes, Q=CV equation is somewhat misleading in this context. >>>>>>>>>>>>>>>>> A >>>>>>>>>>>>>>>>> capacitor >>>>>>>>>>>>>>>>> doesn't store electrical charge, it stores energy. This is >>>>>>>>>>>>>>>>> a >>>>>>>>>>>>>>>>> very >>>>>>>>>>>>>>>>> common >>>>>>>>>>>>>>>>> misconception, when we say 'charge a capacitor' we don't >>>>>>>>>>>>>>>>> mean >>>>>>>>>>>>>>>>> put >>>>>>>>>>>>>>>>> electrical >>>>>>>>>>>>>>>>> charge into it, we mean put energy into it. The plates are >>>>>>>>>>>>>>>>> equal and >>>>>>>>>>>>>>>>> opposite in electrical charge due to an abundance of >>>>>>>>>>>>>>>>> electrons >>>>>>>>>>>>>>>>> on >>>>>>>>>>>>>>>>> one >>>>>>>>>>>>>>>>> plate >>>>>>>>>>>>>>>>> and an equal and opposite charge on the other. The total >>>>>>>>>>>>>>>>> stored >>>>>>>>>>>>>>>>> electrical >>>>>>>>>>>>>>>>> charge in a capacitor is zero, and the Q=CV equation >>>>>>>>>>>>>>>>> relates >>>>>>>>>>>>>>>>> to >>>>>>>>>>>>>>>>> how >>>>>>>>>>>>>>>>> much >>>>>>>>>>>>>>>>> charge flowed *in and out* of the capacitor (in fact since >>>>>>>>>>>>>>>>> electrons >>>>>>>>>>>>>>>>> can't >>>>>>>>>>>>>>>>> cross the barrier between the plates, it actually >>>>>>>>>>>>>>>>> describes >>>>>>>>>>>>>>>>> the >>>>>>>>>>>>>>>>> *modulus* >>>>>>>>>>>>>>>>> of >>>>>>>>>>>>>>>>> the abundance of charge on each plate, one abundance is >>>>>>>>>>>>>>>>> positive and >>>>>>>>>>>>>>>>> the >>>>>>>>>>>>>>>>> other is negative). >>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>> Mark. >>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>> That's not what they taught us in college, and that's not >>>>>>>>>>>>>>>> the >>>>>>>>>>>>>>>> way we >>>>>>>>>>>>>>>> do engineering. 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. >>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>> John >>>>>>>>>>>>>>>> >>>>>>>>>>>>>>> With all due respect, we don't, and shouldn't, say a >>>>>>>>>>>>>>> capacitor >>>>>>>>>>>>>>> 'stores >>>>>>>>>>>>>>> charge'. >>>>>>>>>>>>>> What do you mean by "we"? Electronic design engineers do this >>>>>>>>>>>>>> all >>>>>>>>>>>>>> the >>>>>>>>>>>>>> time, with reference to capacitors and batteries. You pump X >>>>>>>>>>>>>> coulombs >>>>>>>>>>>>>> into a cap; it becomes stored, coincidentally as C*V. You can >>>>>>>>>>>>>> extract >>>>>>>>>>>>>> those coulombs later, and the accounting is correct. The math >>>>>>>>>>>>>> works. >>>>>>>>>>>>>> The gear works. >>>>>>>>>>>>>> >>>>>>>>>>>>>> The misconception comes from the use of the word charge when >>>>>>>>>>>>>>> talking about putting energy into a capacitor, and more >>>>>>>>>>>>>>> explictly >>>>>>>>>>>>>>> the >>>>>>>>>>>>>>> significant lack of clarification given on this when being >>>>>>>>>>>>>>> taught. >>>>>>>>>>>>>>> This >>>>>>>>>>>>>>> is >>>>>>>>>>>>>>> compounded by a confusion of the q=C*V equation which >>>>>>>>>>>>>>> actually >>>>>>>>>>>>>>> relates >>>>>>>>>>>>>>> to >>>>>>>>>>>>>>> the charge on the plates, but one of the plates is of the >>>>>>>>>>>>>>> same >>>>>>>>>>>>>>> value >>>>>>>>>>>>>>> but >>>>>>>>>>>>>>> opposite in polarity, so the sum of those is zero. This is >>>>>>>>>>>>>>> an >>>>>>>>>>>>>>> extremely >>>>>>>>>>>>>>> popular misunderstanding unfortunately, and leads to >>>>>>>>>>>>>>> conclusions >>>>>>>>>>>>>>> that >>>>>>>>>>>>>>> electrical charge is not conserved. In fact, in a closed >>>>>>>>>>>>>>> system >>>>>>>>>>>>>>> where >>>>>>>>>>>>>>> no >>>>>>>>>>>>>>> electrical charge can get in or out, within that system >>>>>>>>>>>>>>> electrical >>>>>>>>>>>>>>> charge >>>>>>>>>>>>>>> *is* conserved, it's actually a fundamental law of physics >>>>>>>>>>>>>>> (along >>>>>>>>>>>>>>> with >>>>>>>>>>>>>>> conservation of energy and momentum, again for closed >>>>>>>>>>>>>>> systems). >>>>>>>>>>>>>>> >>>>>>>>>>>>>>> The same current flows in and out of a capacitor when it is >>>>>>>>>>>>>>> being >>>>>>>>>>>>>>> 'charged' >>>>>>>>>>>>>>> (I assume you are not going to deny that). Note I said the >>>>>>>>>>>>>>> same >>>>>>>>>>>>>>> current, >>>>>>>>>>>>>>> but >>>>>>>>>>>>>>> they are not made of the same electrons because those can't >>>>>>>>>>>>>>> cross >>>>>>>>>>>>>>> the >>>>>>>>>>>>>>> plate >>>>>>>>>>>>>>> barrier. The same amount of electrical charge that goes in >>>>>>>>>>>>>>> comes >>>>>>>>>>>>>>> right >>>>>>>>>>>>>>> out >>>>>>>>>>>>>>> again. How can the capacitor possibly end up with a net >>>>>>>>>>>>>>> charge >>>>>>>>>>>>>>> in >>>>>>>>>>>>>>> it? >>>>>>>>>>>>>>> If >>>>>>>>>>>>>>> it >>>>>>>>>>>>>>> can, where has the electrical charge come from? Have >>>>>>>>>>>>>>> electrons >>>>>>>>>>>>>>> just >>>>>>>>>>>>>>> been >>>>>>>>>>>>>>> conjured up out of nowhere? >>>>>>>>>>>>>> It's a different convention. Words. But the units work and >>>>>>>>>>>>>> the >>>>>>>>>>>>>> numbers >>>>>>>>>>>>>> work, so we use it. Call our kind of charge "charge >>>>>>>>>>>>>> separation" >>>>>>>>>>>>>> or >>>>>>>>>>>>>> "plate charge differential" if it makes you happier. >>>>>>>>>>>>>> >>>>>>>>>>>>>> Do you design electronics? Do mosfet data sheets refer to >>>>>>>>>>>>>> stored >>>>>>>>>>>>>> gate >>>>>>>>>>>>>> energy, or stored gate charge? >>>>>>>>>>>>>> >>>>>>>>>>>>>> John >>>>>>>>>>>>> Yes, I'm actually an electronics design consultant. >>>>>>>>>>>>> Take a look at this: >>>>>>>>>>>>> www.irf.com/technical-info/appnotes/mosfet.pdf >>>>>>>>>>>>> >>>>>>>>>>>>> Note the equivalent circuits, which show capacitance between >>>>>>>>>>>>> the >>>>>>>>>>>>> gate, >>>>>>>>>>>>> source and drain. They talk about 'gate charge' as being a >>>>>>>>>>>>> conveient way >>>>>>>>>>>>> of >>>>>>>>>>>>> relating the capacitance charging and discharging (energy, not >>>>>>>>>>>>> electrical >>>>>>>>>>>>> charge) with current, and hence time. Again, confusion can >>>>>>>>>>>>> arrise >>>>>>>>>>>>> because >>>>>>>>>>>>> they use the word 'charge' in two contexts. >>>>>>>>>>>> They use "charge", in many places, the same way most >>>>>>>>>>>> electronics >>>>>>>>>>>> engineers use the word, namely C*V. >>>>>>>>>>>> >>>>>>>>>>>>> The fact is that since current flows in and out of these >>>>>>>>>>>>> capacitors >>>>>>>>>>>>> in >>>>>>>>>>>>> equal >>>>>>>>>>>>> amounts the net stored electrical charge on each one is zero. >>>>>>>>>>>>> However >>>>>>>>>>>>> the >>>>>>>>>>>>> Q=CV equation relates to the magnitude of charge that each of >>>>>>>>>>>>> the >>>>>>>>>>>>> plates >>>>>>>>>>>>> of >>>>>>>>>>>>> these capacitances carries, but for each capacitor there is >>>>>>>>>>>>> another >>>>>>>>>>>>> plate >>>>>>>>>>>>> with equal and opposite charge. >>>>>>>>>>>> Exactly. We say a cap is "charged" if C*V <> 0. In fact, C*V is >>>>>>>>>>>> the >>>>>>>>>>>> exact charge. We say that a cap integrates charge into voltage, >>>>>>>>>>>> and >>>>>>>>>>>> that it can return that same charge as we drain it down to zero >>>>>>>>>>>> volts. >>>>>>>>>>>> So it's handy to think that a cap can store that charge for us, >>>>>>>>>>>> which >>>>>>>>>>>> it actually does. >>>>>>>>>>>> >>>>>>>>>>>>> Here is a good derivation of the elecrostatic forces between >>>>>>>>>>>>> the >>>>>>>>>>>>> plates >>>>>>>>>>>>> of >>>>>>>>>>>>> a >>>>>>>>>>>>> parallel plate capacitor. Note that the electrical charge on >>>>>>>>>>>>> each >>>>>>>>>>>>> plate >>>>>>>>>>>>> has >>>>>>>>>>>>> the same magnitude Q, but one is positive and the other >>>>>>>>>>>>> negative. >>>>>>>>>>>>> If you >>>>>>>>>>>>> think this is not correct maybe you should contact the >>>>>>>>>>>>> University >>>>>>>>>>>>> of >>>>>>>>>>>>> Pennsylvania and tell them :) >>>>>>>>>>>>> http://dept.physics.upenn.edu/~uglabs/lab_manual/electric_forces.pdf >>>>>>>>>>>> The plates need not gave the same absolute Q, because the >>>>>>>>>>>> overall >>>>>>>>>>>> cap >>>>>>>>>>>> can have a net charge, what we electronic guys would call an >>>>>>>>>>>> electrostatic charge. That ususlly doesn't matter to us, so we >>>>>>>>>>>> use >>>>>>>>>>>> the >>>>>>>>>>>> conviently short word "charge" to mean C*V, where V is the sort >>>>>>>>>>>> of >>>>>>>>>>>> potential difference we measure with a 2-terminal voltmeter. >>>>>>>>>>>> When >>>>>>>>>>>> we >>>>>>>>>>>> rarely refer to physics-type net charge, we say "electrostatic >>>>>>>>>>>> charge." >>>>>>>>>>>> >>>>>>>>>>>> To express the concept of "charge on a capacitor" any way other >>>>>>>>>>>> than >>>>>>>>>>>> the way we use it would be grammatically and numerically very >>>>>>>>>>>> messy. >>>>>>>>>>>> >>>>>>>>>>>> But when we use the term this way, we have to be careful to >>>>>>>>>>>> remember >>>>>>>>>>>> what it means to us, and we can't blindly say stuff like >>>>>>>>>>>> "charge >>>>>>>>>>>> is >>>>>>>>>>>> conserved" without thinking carefully. It's safe to say "energy >>>>>>>>>>>> is >>>>>>>>>>>> conserved." >>>>>>>>>>>> >>>>>>>>>>>> John >>>>>>>>>>> I'm giving up. Your concept of charge is obviously not Coulombs! >>>>>>>>>> Q=C*V is coulombs. The units work. If a cap stores 5 coulombs, I >>>>>>>>>> can >>>>>>>>>> load it at 2.5 amps for 2 seconds, and then the charge is gone. >>>>>>>>>> It >>>>>>>>>> works. >>>>>>>>>> >>>>>>>>>> John >>>>>>>>> Actually the capacitor stores + 5 Coulombs on one plate, and -5 >>>>>>>>> Coulombs on >>>>>>>>> the other plate. When you 'load it at 2.5 Amps for 2.5 seconds' >>>>>>>>> the >>>>>>>>> negatively charged electrons move from the negatively charged >>>>>>>>> plate, >>>>>>>>> through >>>>>>>>> the external circuit, and into the positively charged plate. At >>>>>>>>> the >>>>>>>>> end >>>>>>>>> there are zero Coulombs stored on both plates. This is why the >>>>>>>>> maths >>>>>>>>> works, >>>>>>>>> at no point does the capacitor as a whole ever have net stored >>>>>>>>> charge, >>>>>>>>> the >>>>>>>>> sum of the charges of both plates is always zero. In your system >>>>>>>>> of >>>>>>>>> stored >>>>>>>>> charge you cannot explain why, if the same current is flowing in >>>>>>>>> and >>>>>>>>> out of >>>>>>>>> the capacitor, it doesn't violate the Law of Conservation of >>>>>>>>> Charge. >>>>>>>>> >>>>>>>>> >>>>>>>> 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 >>>>>>>> >>>>>>> >>>>>>> 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. >>>>>>> >>>>>> >>>>>> From: >>>>>> http://www.google.com/url?sa=t&source=web&cd=94&ved=0CB4QFjADOFo&url=http%3A%2F%2Furegina.ca%2F~benslamk%2Fteach%2Fphys201%2Flect201-6.pdf&ei=RkxcTP32IIH4swOZgZFB&usg=AFQjCNHh0gfLiX04UuCcC1Jjwdja7IvM6g >>>>>> >>>>>> "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." >>>>>> >>>>>> -- >>>>>> Virg Wall, P.E. >>>>> >>>>>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. >>>> >>>> Simply because the strict interpretation you argued for is useless for >>>> practical electronics design ;) >>> >>> 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! >>> >>>> >>>> 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. >>>> >>> >>> 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": >>> http://en.wikipedia.org/wiki/Capacitance) >>> >>> Mark. >> >>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 >>+/-q on the plates, related to voltage by the q=CV equation. >> >>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. > > 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. 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). > > 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.) 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. > > John >
From: Jim Thompson on 7 Aug 2010 12:51 On Sat, 07 Aug 2010 08:17:36 -0700, John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: [snip] > >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. ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ > [snip] 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 -- | James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | Spice is like a sports car... Performance only as good as the person behind the wheel.
From: Phil Hobbs on 7 Aug 2010 15:30
On 8/7/2010 11:17 AM, John Larkin wrote: > On Sat, 7 Aug 2010 11:52:23 +0100, "markp"<map.nospam(a)f2s.com> wrote: > <sniiiiiiip> >> >> "markp"<map.nospam(a)f2s.com> wrote in message >> news:8c3jbcFr8rU1(a)mid.individual.net... >>> >> 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 >> +/-q on the plates, related to voltage by the q=CV equation. >> >> 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. > > 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. 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 > than a physicist might get legal-picky about. (Unless that physicist > designs circuits, too.) 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 -- Dr Philip C D Hobbs Principal ElectroOptical Innovations 55 Orchard Rd Briarcliff Manor NY 10510 845-480-2058 email: hobbs at electrooptical dot net http://electrooptical.net |