Let's Take A Vote...
in [Design]
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From: John Larkin on 24 Jul 2010 11:48 On Sat, 24 Jul 2010 09:24:16 +0100, John Devereux <john(a)devereux.me.uk> wrote: >Jim Thompson <To-Email-Use-The-Envelope-Icon(a)On-My-Web-Site.com> writes: > >> Let's Take A Vote... >> >> While I write this up, hopefully sometime this weekend, let me ask for >> votes... >> >> How many think, as Larkin opines, "charge is not conserved" ?? >> >> How many think charge IS conserved ?? >> >> Just curious what I'm up against here. > >It depends on the context and your definitions, as already pointed out >and which you still refuse to provide. Are you using a definition which >says capacitors store charge or not? Is the quantity Q=CV to be regarded >as "charge" or not? Is charge "delivered" or does it "flow through"? > >*Without* any context, I would have said "charge is conserved". You >don't need to spend three weeks proving this, just point to Kirchoff. > >But the context of the original thread was all abouit switched >capacitors and whether the "capacitors charge" was always conserved when >transfered to another. We routinely refer to the quantity Q=CV as the >capacitors "charge", it is this quantity which is not conserved, I.e., >you can sum them before and after the switching operation and it is >different. Not sophistry, just a normal use of terms in a circuit >description. > >And it is obvious that this was the intended usage, since otherwise the >"charge of the capacitor" is always zero! > >Basically, our routine use of the word is ambiguous, you can easily >"prove someone wrong" by assuming the opposite usage to that intended, Exactly. That's the difference between the "charge is conserved" physics perspective, and the way EEs work. The physics argument can be made that a small 1F supercap, with 1 volt across its terminals, that is floating 10 volts above ground, has a charge of around 10 pC, namely 10 volts times its capacitance to the universe. EEs would prefer to think of that cap as having a charge of 1C, namely 1F*1V. Technically I suppose we should say something like "the capacitor sustains an internal charge separation of 1C" but we just say it's charged to 1C, or "holds a charge" of 1C. We can pump that sort of charge into and out of caps and do the accounting correctly. The way that electronic designers (and the authors of mosfet data sheets) talk is the latter; we say "charge" instead of "charge saparation" because it's easier. We say "electrostatic charge" in the rare cases when the first convention is intended. Spice, of course, won't allow electrostatic charge to be considered. Without a path to ground, it will throw up its figurative hands and declare a singular matrix. The EE sort of charge is obviously conserved only in some special cases. All I ever said was that such cases exist (with examples) and that circuit designers should be careful about generally depending on "conservation of charge" when they do math. If they do math. If all they ever do is drive Spice and fiddle, no problem. John
From: John Larkin on 24 Jul 2010 11:51 On Sat, 24 Jul 2010 09:45:53 +0100, Martin Brown <|||newspam|||@nezumi.demon.co.uk> wrote: >On 23/07/2010 23:52, Jim Thompson wrote: >> Let's Take A Vote... >> >> While I write this up, hopefully sometime this weekend, let me ask for >> votes... >> >> How many think, as Larkin opines, "charge is not conserved" ?? > >Hopefully not too many. But it is difficult to predict the behaviour of >electronics engineers - about half of them think Einstein was wrong :( >> >> How many think charge IS conserved ?? > >Just about every physicist on the planet since Ben Franklin. > >It was the inconsistency of Ampere's Law with conservation of charge >that led Maxwell to formulate his famous equations and show that >oscillating fields of electromagnetic radiation travel at a constant >speed c in a vacuum. >> >> Just curious what I'm up against here. >> >> ...Jim Thompson > >A idealised physics version of your original capacitor problem but >without the switch can be stated as the following problem: > >Two identical metal spheres with capacitance C are used. >Initially one is uncharged and the other with a charge Q > >They are brought together from infinity until they touch. > >Describe what happens and how the charge is distributed after they are >in electrical contact. You can add an infinite ground plane under the >experiment if it makes you feel better about the circuit analogue. > Where can I buy 0.33 uF surface-mount metal spheres? Are they expensive? I'd need ROHS, of course, on reels. [1] John [1] extra credit: how big would they be?
From: John Fields on 24 Jul 2010 12:49 On Sat, 24 Jul 2010 08:30:43 -0700, John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >On Sat, 24 Jul 2010 09:04:41 -0500, John Fields ><jfields(a)austininstruments.com> wrote: > >>On Sat, 24 Jul 2010 08:15:03 -0500, John Fields >><jfields(a)austininstruments.com> wrote: >>On F>> >>>On Fri, 23 Jul 2010 17:38:45 -0700, John Larkin >>><jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >>>>Charge is measured in coulombs. Force is measured in newtons. So how >>>>is charge "a measure of force"? >>> >>>--- >>>news:2apl46hr8s01os8dv1aipdm19bcf64nec4(a)4ax.com >> >>--- >>Oh, and the first sentence of the cited Wikipedia article reads: >> >>"Electric charge is a physical property of matter which causes it >>to experience a force when near other electrically charged matter." >> >> >>JF > >If you experience a pig, does that make you a pig? --- That's just a silly diversionary tactic; measuring a force doesn't make you the force. --- >Look at the SI units if you want to determine if things are the same. --- That's just another silly diversionary tactic. Take a look at the leaves of a gold-leaf electroscope which has been charged; that force remains the same regardless of the system used to describe it. --- >Is a pig a Field? --- More silly diversion... A red herring this time. http://www.youtube.com/watch?v=wNRH7_Kd5Yc JF
From: Jim Thompson on 24 Jul 2010 12:52 On Sat, 24 Jul 2010 08:30:43 -0700, John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >On Sat, 24 Jul 2010 09:04:41 -0500, John Fields ><jfields(a)austininstruments.com> wrote: > >>On Sat, 24 Jul 2010 08:15:03 -0500, John Fields >><jfields(a)austininstruments.com> wrote: >> >>>On Fri, 23 Jul 2010 17:38:45 -0700, John Larkin >>><jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >>> >>>>On Fri, 23 Jul 2010 19:30:25 -0500, John Fields >>>><jfields(a)austininstruments.com> wrote: >>>> >>>>>On Fri, 23 Jul 2010 15:52:06 -0700, Jim Thompson >>>>><To-Email-Use-The-Envelope-Icon(a)On-My-Web-Site.com> wrote: >>>>> >>>>>>Let's Take A Vote... >>>>>> >>>>>>While I write this up, hopefully sometime this weekend, let me ask for >>>>>>votes... >>>>>> >>>>>>How many think, as Larkin opines, "charge is not conserved" ?? >>>>>> >>>>>>How many think charge IS conserved ?? >>>>>> >>>>>>Just curious what I'm up against here. >>>>> >>>>>--- >>>>>Cordially, Jim, >>>>> >>>>>All you're up against is Larkin's sophistry, and whether any of us >>>>>votes as to whether charge is conserved or not is immaterial, since >>>>>nature rules. >>>>> >>>>>Post what you've got and let the chips fall where they may, there's >>>>>always Wikipedia which supports your position: >>>>> >>>>>http://en.wikipedia.org/wiki/Electric_charge#Conservation_of_electric_charge >>>>> >>>>> >>>>>On the other hand, if Larkin's right and energy is conserved but >>>>>charge isn't, then since charge is a measure of force, unbalanced lost >>>>>charge might be able to be used for propulsion. >>>> >>>>Charge is measured in coulombs. Force is measured in newtons. So how >>>>is charge "a measure of force"? >>> >>>--- >>>news:2apl46hr8s01os8dv1aipdm19bcf64nec4(a)4ax.com >> >>--- >>Oh, and the first sentence of the cited Wikipedia article reads: >> >>"Electric charge is a physical property of matter which causes it >>to experience a force when near other electrically charged matter." >> >> >>JF > >If you experience a pig, does that make you a pig? > >Look at the SI units if you want to determine if things are the same. > >Is a pig a Field? > >John No, But you are. ...Jim Thompson -- | James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | Spice is like a sports car... Only as good as the person behind the wheel.
From: John Larkin on 24 Jul 2010 13:13
On Sat, 24 Jul 2010 11:49:38 -0500, John Fields <jfields(a)austininstruments.com> wrote: >On Sat, 24 Jul 2010 08:30:43 -0700, John Larkin ><jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: > >>On Sat, 24 Jul 2010 09:04:41 -0500, John Fields >><jfields(a)austininstruments.com> wrote: >> >>>On Sat, 24 Jul 2010 08:15:03 -0500, John Fields >>><jfields(a)austininstruments.com> wrote: >>>On F>> >>>>On Fri, 23 Jul 2010 17:38:45 -0700, John Larkin >>>><jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: > > >>>>>Charge is measured in coulombs. Force is measured in newtons. So how >>>>>is charge "a measure of force"? >>>> >>>>--- >>>>news:2apl46hr8s01os8dv1aipdm19bcf64nec4(a)4ax.com >>> >>>--- >>>Oh, and the first sentence of the cited Wikipedia article reads: >>> >>>"Electric charge is a physical property of matter which causes it >>>to experience a force when near other electrically charged matter." >>> >>> >>>JF >> >>If you experience a pig, does that make you a pig? > >--- >That's just a silly diversionary tactic; measuring a force doesn't >make you the force. >--- > >>Look at the SI units if you want to determine if things are the same. > >--- >That's just another silly diversionary tactic. Were you ever taught dimensional analysis? Obviously not. Give it a try: http://en.wikipedia.org/wiki/Dimensional_analysis The basic concept is that you can test all sorts of relationships for plausibility by reducing their SI units. If the units don't agree, the things can't be equal. Newtons aren't coulombs, so charge can't be force. It's that simple. They taught us this our first freshman semister in EE school, in a course called "Engineering Design Analysis" which was taught in a small classroom by the Dean of Engineering, just to get us started on the right track. It's been a great friend ever since. John |