Magic capacitors!
in [Design]
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From: John Larkin on 5 Aug 2010 11:13 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
From: Charlie E. on 5 Aug 2010 13:19 On Thu, 5 Aug 2010 02:40:12 +0100, "markp" <map.nospam(a)f2s.com> wrote: > >With all due respect, we don't, and shouldn't, say a capacitor 'stores >charge'. 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? > >Mark. > Well, theoretically, if the cap was say at ground potential on both plates before being added to the circuit, but the circuit is designed to run at some value above ground potential when running, it COULD get a few extra electrons added, giving a net increase of charge... ;-) Charlie
From: markp on 5 Aug 2010 13:41 "Charlie E." <edmondson(a)ieee.org> wrote in message news:6isl56tn6kicj23a8ej6g7hjaqk6tt3k9s(a)4ax.com... > On Thu, 5 Aug 2010 02:40:12 +0100, "markp" <map.nospam(a)f2s.com> wrote: > >> >>With all due respect, we don't, and shouldn't, say a capacitor 'stores >>charge'. 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? >> >>Mark. >> > > Well, theoretically, if the cap was say at ground potential on both > plates before being added to the circuit, but the circuit is designed > to run at some value above ground potential when running, it COULD get > a few extra electrons added, giving a net increase of charge... ;-) > > Charlie For there to be a net increase in charge one plate would have more electrical charge than the other (either positive or nagative). This would require more current going in than coming out (or vice versa). That doesn't happen in electrical circuits. I'm ignoring static electricity which is a different subject. Mark.
From: markp on 5 Aug 2010 13:43 "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! To be honest I'm staggered of how many people think like this, what they don't get is that a capacitor is *by definition* two electrodes separated by an insulator, it necessarily has two electrical charges, one positive and one negative if it is 'charged' by passing a current through it. Since the same current goes in and out of the capacitor, and there is an insulating barrier, electrons can't pass the barrier so they build up on one electrode, giving it a negative electrical charge (which by the way is where the voltage comes from. It's like pushing against an ever stronger spring, to get more electrons on that plate require more force, and that gets harder and harder and the plate becomes more and more negative). Since exactly the same number of electrons are coming out the other terminal, this means there must be an equal depletion of electrons on that electrode, yielding an equal and opposite electrical charge on that electrode. The sum of the amount of electrical charge therefore is a big fat zero. You can talk of a single capacitor electrode having a stored charge, and it does. But the definition of a capacitor is one that has two electrodes, the one has negative charge and the other equal, and opposite, charge. So a capacitor that is charged up this way cannot have a net storage of electrical charge. I'm ignoring static electricty, this situation is normal 'dynamic' electricty stuff. Mark.
From: Jim Thompson on 5 Aug 2010 14:29
On Thu, 05 Aug 2010 10:19:52 -0700, Charlie E. <edmondson(a)ieee.org> wrote: >On Thu, 5 Aug 2010 02:40:12 +0100, "markp" <map.nospam(a)f2s.com> wrote: > >> >>With all due respect, we don't, and shouldn't, say a capacitor 'stores >>charge'. 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? >> >>Mark. >> > >Well, theoretically, if the cap was say at ground potential on both >plates before being added to the circuit, but the circuit is designed >to run at some value above ground potential when running, it COULD get >a few extra electrons added, giving a net increase of charge... ;-) > >Charlie Larkin will further bloviate, but the _science_ says, "Within a closed Universe". ...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 | Phoenix, AZ, a city noted for many civic-oriented inventions such as automated garbage collection, has added a new tool to its Fire Department safety equipment... addressing problems with accidents involving so-called Smart Cars: A Hydraulically Assisted Spatula. |