Magic capacitors!
in [Design]
|
From: VWWall on 6 Aug 2010 14:13 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.
From: markp on 6 Aug 2010 15:16 "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. Mark.
From: Grant on 6 Aug 2010 18:24 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 ;) 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. > >Mark. > > >
From: markp on 6 Aug 2010 19:14 "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.
From: Bill Beaty on 6 Aug 2010 21:38
On Jul 23, 10:04 am, Dirk Bruere at NeoPax <dirk.bru...(a)gmail.com> wrote: > On 21/07/2010 18:59, George Jefferson wrote: > > And do we also assume it does not oscillate nor radiate? With zero inductance it oscillates at infinite frequency. The single stored photon probably buds off into a daughter universe. |