Magic capacitors!
in [Design]
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From: Morris Slutsky on 10 Aug 2010 10:36 On Jul 21, 8:54 am, "George Jefferson" <phreon...(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). > > If you want to make it easier assume C2 is initially discharged. > > Is the energy before and after the same? If not explain why and why it is > not a violation of the conservation of energy law. I'd say that this puzzle demonstrates that ideal noninductive, nonresistive capacitors do not and cannot exist in the real world. Had they inductance but no resistance the charge would oscillate infinitely back and forth. Were there resistance, the oscillation would damp and the energy would be dissipated as heat. Allowing neither inductance nor resistance, not only does the oscillation never damp, but it would not even have a finite frequency. The charge would simply oscillate infinitely fast - except that this violates quantum mechanics. So this perfect, ideal capacitor violates QM! If we wanted to have this noninductive, nonresistive pair of capacitors in a QM system, we'd find that the oscillation frequency was in fact finite. Very high, but finite. The delta-E of 'missing energy' would equal Planks constant * frequency. Large but finite. And the system would decay radiatively. Possibly, in this case of extremely nonphyiscal components, a single photon would be emitted carrying away all this energy.
From: JosephKK on 10 Aug 2010 21:33 On Tue, 03 Aug 2010 20:56:47 -0700, Jim Thompson <To-Email-Use-The-Envelope-Icon(a)On-My-Web-Site.com> wrote: >On Tue, 03 Aug 2010 20:49:23 -0700, >"JosephKK"<quiettechblue(a)yahoo.com> wrote: > >>On Tue, 03 Aug 2010 15:26:13 -0700, Don Lancaster <don(a)tinaja.com> >>wrote: >> >>>On 7/21/2010 12:30 PM, Jim Thompson wrote: >>>> On Wed, 21 Jul 2010 12:14:04 -0700, John Larkin >>>> <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >>>> >>>>> On Wed, 21 Jul 2010 12:17:41 -0500, "George Jefferson" >>>>> <phreon111(a)gmail.com> wrote: >>>>> >><snip> >>>> >>>> Let the hedging begin... >>>> >>>> In Message-ID:<3b893612tjjndo8o4v1evro050nonjgp41(a)4ax.com> >>>> >>>> You said: >>>> >>>> "Right. If you dump all the energy from one charged cap into another, >>>> discharged, cap of a different value, and do it efficiently, charge is >>>> not conserved." >>>> >>>> Note the NOT CONSERVED. >>>> >>>> Now you say, "...the C*V (charge) on the first cap obviously becomes a >>>> different C*V on the second one". >>>> >>>> Where did the charge come from/go to? >>>> >>>> John "The Bloviator" Larkin is totally incapable of admitting error. >>>> >>>> I truly suspect you're too ignorant to understand :-( >>>> >>>> ...Jim Thompson >>> >>> >>>If you have a resistor between the capacitors,half the energy is >>>dissipated. The math does NOT depend on the resistance value. >> >>Sorry Don. that does not stand up to the physics or the arithmetic. > >Sorry, JosephKK, But it does. Shame :-( > >>> >>>In the case of zero resistance, you get infinite current and a huge >>>spark that radiates the equivalent energy. >> >>Energy may be radiated, but not by the switch. Nor is there a >>requirement for a spark. >>> >>>However, you can charge the second capacitor more efficiently with an >>>inductor in a resonant circuit. >> >>Not part of the question. > >Do the math JosephKK. You are a disappointment :-( > >Newbies. If you think differently, go into sales ;-) > > ...Jim Thompson Had to grind it a few cases. Ding Dang it, take some piano wire and hang it. My Apologies Don. The resistor always eats half the energy.
From: Bill Beaty on 11 Aug 2010 03:00 On Aug 8, 10:14 pm, John Larkin <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: > For Pete's sake, we all took physics. We understand this stuff. But we > need to talk about these things quickly... you should hear some of the > sessions around here. The "we" I'm talking about is little kids and grandmothers. [Move this thread to SEB instead of SED? :) ] If we want to explain basic circuitry to the general public, we first need to go over the traditional explanations with a fine tooth comb. Get rid of contradictions, and suddenly E&M stops resembling a black art. > >Besides endless Newsgroup fights, really this stuff is only important > >when teaching basic physics/electronics to newbies and when writing > >electronics textbooks. And further, it's only important if we've > >decided to avoid filling students' heads with misconceptions like > >single-wire capacitors or "capacitors store charge." > > > Funny, I took physics and EE courses and never noticed a conflict. By "electronics texts" I meant the ones aimed at the general public. EE courses have little to do with teaching newbies. "Newbies," as in kids' first introduction to electricity in 4th or 5th grade, or visitors wandering through the physics section of the science museum. What would capacitor operation look like, if we could see the current and voltage? Or "newbies," as in HS grads who never took physics, who forgot algebra after the exam, but now they want to grasp the operation of circuitry while mostly avoiding any encounters with Ohm's law. These concepts are appropriate for books like "Evil Genius Guide To Basic Electricity," or "There Are No Electrons." As you say, for EE undergrads, this Capacitors-store-charge stuff is trivial and almost beneath consideration except perhaps as a trick question on an exam. Unlike with EEs, for a total beginner it's a bit of work and a huge "Aha" to realize that capacitors *don't* store any charge, any more than inductors store charge. (Hmm, the idea of coil/capacitor duality becomes a bit clearer once you realize that both components can accumulate field energy, while neither one stores any charge.) > The EE profs always treated caps as storing charge. AoE resolves the situation in one sentence. Nobody notices traditional language which completely violates fundamental physics. I suspect it didn't stand out since we didn't have an intuitive grasp of the difference between coulombs and joules. The math works fine, so we didn't really need it. Now if undergrad textbooks had used explanations which violate energy conservation, we'd hear no end of ridicule from the physics crowd.
From: John Larkin on 11 Aug 2010 10:37 On Wed, 11 Aug 2010 00:00:10 -0700 (PDT), Bill Beaty <billb(a)eskimo.com> wrote: >On Aug 8, 10:14�pm, John Larkin ><jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: > >> For Pete's sake, we all took physics. We understand this stuff. But we >> need to talk about these things quickly... you should hear some of the >> sessions around here. > >The "we" I'm talking about is little kids and grandmothers. [Move >this thread to SEB instead of SED? :) ] If we want to explain >basic circuitry to the general public, we first need to go over the >traditional explanations with a fine tooth comb. Get rid of >contradictions, and suddenly E&M stops resembling a black art. Explain electronics to the general public? You've got to be kidding. The GP drives cars but hardly any understand Newton's laws, much less the rotational versions of same. When our building was being wired up, I showed one of the electricians a sketch of a triangle with a center-tap on one leg, the basic "stinger" diagram. He said "we don't do that theory stuff." But he sure could bend conduit beautifully; he was an artist. Teach them COE and some simple mechanics first. My wife wants me to put a windmill on our roof. John
From: Spehro Pefhany on 11 Aug 2010 10:53
On Wed, 11 Aug 2010 07:37:42 -0700, John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >On Wed, 11 Aug 2010 00:00:10 -0700 (PDT), Bill Beaty ><billb(a)eskimo.com> wrote: > >>On Aug 8, 10:14�pm, John Larkin >><jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: >> >>> For Pete's sake, we all took physics. We understand this stuff. But we >>> need to talk about these things quickly... you should hear some of the >>> sessions around here. >> >>The "we" I'm talking about is little kids and grandmothers. [Move >>this thread to SEB instead of SED? :) ] If we want to explain >>basic circuitry to the general public, we first need to go over the >>traditional explanations with a fine tooth comb. Get rid of >>contradictions, and suddenly E&M stops resembling a black art. > >Explain electronics to the general public? You've got to be kidding. >The GP drives cars but hardly any understand Newton's laws, much less >the rotational versions of same. > >When our building was being wired up, I showed one of the electricians >a sketch of a triangle with a center-tap on one leg, the basic >"stinger" diagram. He said "we don't do that theory stuff." But he >sure could bend conduit beautifully; he was an artist. > >Teach them COE and some simple mechanics first. My wife wants me to >put a windmill on our roof. > >John Exellent idea. Be sure to put a nice induction motor on there so that it can be spinning like mad even when it's dead calm. You'll be the envy of your neighbors. |