Charge Conservation - Hint of the Day
in [Design]
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From: John Larkin on 21 Jul 2010 10:00 On Wed, 21 Jul 2010 07:59:29 +0100, John Devereux <john(a)devereux.me.uk> wrote: >John Fields <jfields(a)austininstruments.com> writes: > >> On Tue, 20 Jul 2010 08:24:26 -0700, Jim Thompson >> <To-Email-Use-The-Envelope-Icon(a)On-My-Web-Site.com> wrote: >> >>>Charge Conservation - Hint of the Day: >>> >>>How many Coulombs can a 1mH inductor charged to 1A deliver? >> >> --- >> Well, let's see... >> >> Since the collapsing magnetic field around an ideal one henry inductor >> with one ampere of current in it will deliver about 6.24e18 electrons >> into any load when its current source is abruptly terminated, > >No, that's is the initial *rate* at which it will "deliver" charge, >i.e. 1 amp equals ~6.24e18 electrons *per second*. > >Unfortunately Jim is being purposefully vague with the critical points >and refuses to define his terms. Then, when someone uses a definition >different from his private one, he will turn around and "prove them >wrong". Two weeks later, by appointment only. > >For example, how are we supposed to interpret "deliver"? > >A certain amount of charge will flow *through* the load, completing a >circuit back to the other end of the inductor. But has it been >"delivered"? In any normal electronic engineering context we would say >so, but here? It all returned to the source, so maybe not. You can >sensibly *define* it either way. > >> a one millihenry coil, under the same conditions, should deliver about >> about three orders of magnitude less, about one millicoulomb. >> >> JF Of course if you connect a "charged" inductor to a resistor, literal charges - electrons - will enter one end of the resistor and exit the other. The resistor is not electrostatically charged by that flow, but some number of ampere-seconds does flow through the resistor, and ampere-seconds are coulombs. The resistor converts the charge flow to heat. If a current source is connected to a capacitor, we circuit designers say that charge is put *into* the capacitor, in the sense that some number of ampere-seconds - coulombs - flow through the cap. Unlike the resistor, those coulombs change the internal state of the cap in a way that allows us to disconnect the cap, wait a while, and then use the "charged" cap as a source to make current. It sure looks like we poked coulombs *into* (not just onto, in the electrostatic sense) the capacitor, and we can extract them later, and the numbers balance precisely. So we EEs talk as if charge is stored in capacitors and batteries. Physics purists may quibble, but it works. John
From: John Larkin on 21 Jul 2010 10:05 On Wed, 21 Jul 2010 07:51:08 -0500, "George Jefferson" <phreon111(a)gmail.com> wrote: > > >"Jim Thompson" <To-Email-Use-The-Envelope-Icon(a)On-My-Web-Site.com> wrote in >message news:jqfb469iggbg6dkeu84c8kgp3m9g0alr3m(a)4ax.com... >> Charge Conservation - Hint of the Day: >> >> How many Coulombs can a 1mH inductor charged to 1A deliver? >> >> > >Charged to 1A? hahaha, are you serious? Do you mean 1A flowing through it >after steady state has been reached? > > Steady-state doesn't matter but sure, why not? It's common to talk about charging an inductor, namely connecting a voltage to it and allowing it to integrate up a current. Those are just shorthand words that EEs use; so quibble all you want. What matters is that we get the numbers right. Go turn on your television or start your car. Do they work? John
From: John Larkin on 21 Jul 2010 10:09 On Tue, 20 Jul 2010 11:35:47 -0700, Tim Wescott <tim(a)seemywebsite.com> wrote: >On 07/20/2010 09:32 AM, Jim Thompson wrote: >> On Tue, 20 Jul 2010 08:53:22 -0700, Tim Wescott<tim(a)seemywebsite.com> >> wrote: >> >>> On 07/20/2010 08:24 AM, Jim Thompson wrote: >>>> Charge Conservation - Hint of the Day: >>>> >>>> How many Coulombs can a 1mH inductor charged to 1A deliver? >>> >>> That's insufficient information, and I rather expect that you know it. >> >> No. It's provided to cause young bucks to do some thinking. Looks >> like it didn't work with you :-( > >You did _not_ give enough information: if you don't know why you should >respectfully ask -- or do some thinking yourself. I purposely remained >mysterious because I didn't want to ruin your fun. His version of fun lately seems to have a lot to do with "young bucks", and I don't think he is fantasizing about adolescent male deer. John
From: George Herold on 21 Jul 2010 10:28 On Jul 20, 8:32 pm, Tim Wescott <t...(a)seemywebsite.com> wrote: > On 07/20/2010 09:32 AM, Jim Thompson wrote: > > > On Tue, 20 Jul 2010 08:53:22 -0700, Tim Wescott<t...(a)seemywebsite.com> > > wrote: > > >> On 07/20/2010 08:24 AM, Jim Thompson wrote: > >>> Charge Conservation - Hint of the Day: > > >>> How many Coulombs can a 1mH inductor charged to 1A deliver? > > >> That's insufficient information, and I rather expect that you know it. > > > No. It's provided to cause young bucks to do some thinking. Looks > > like it didn't work with you :-( > > > (Except that it did annoy Larkin, yet again... so a partial success > > :-) > > Since you didn't answer I have to assume that you couldn't. > > Either this is a trick question, and the answer is "however many excess > electrons it has sitting on it when I hand it to you", or the answer is > "that depends on the coil resistance". > > A 1mH superconducting inductor with 1A will deliver (or flow, if you > want to quibble about the common EE definition of "deliver") an infinite > charge to a dead short, assuming all conductors are also zero resistance. > > Otherwise a 1mH inductor that sees R ohms of total circuit resistance in > the inductor and the load (charge target?) will see it's current decay > as (1A)*e^-(R/L)*t; this will integrate to (1A) * (L/R). So for 1 ohm > total resistance that'd be 1mC, for a 10 ohm total resistance that'd be > 100uC, for a 0.1 ohm total resistance it'd be 10mC, etc. > > Answers involving loads that aren't purely resistive are more > complicated, but still obvious if you can understand the above. > > But to answer how much charge that 1mH inductor _can possibly_ deliver > when it has 1A flowing through it depends on the particular inductor's > winding resistance and possibly also on whether it's really a 1mH > inductor when it has 1A flowing through it. > > You may want to pop over to the closest ASU campus that presents EEE 202 > and see if you can audit the course. This problem is no great mystery > for someone who's gotten through sophomore electronics engineering. > > -- > > Tim Wescott > Wescott Design Serviceshttp://www.wescottdesign.com > > Do you need to implement control loops in software? > "Applied Control Theory for Embedded Systems" was written for you. > See details athttp://www.wescottdesign.com/actfes/actfes.html Hi Tim, I agree with your calculation. But not the interpretation. Sure you integrate current over time and you get charge. But this is not the charge delivered to a resistor, it is how much charge flowed through it. (Oh unless that's what is meant by delivered.) George H.
From: Jim Thompson on 21 Jul 2010 10:41
On Wed, 21 Jul 2010 07:09:09 -0700, John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >On Tue, 20 Jul 2010 11:35:47 -0700, Tim Wescott <tim(a)seemywebsite.com> >wrote: > >>On 07/20/2010 09:32 AM, Jim Thompson wrote: >>> On Tue, 20 Jul 2010 08:53:22 -0700, Tim Wescott<tim(a)seemywebsite.com> >>> wrote: >>> >>>> On 07/20/2010 08:24 AM, Jim Thompson wrote: >>>>> Charge Conservation - Hint of the Day: >>>>> >>>>> How many Coulombs can a 1mH inductor charged to 1A deliver? >>>> >>>> That's insufficient information, and I rather expect that you know it. >>> >>> No. It's provided to cause young bucks to do some thinking. Looks >>> like it didn't work with you :-( >> >>You did _not_ give enough information: if you don't know why you should >>respectfully ask -- or do some thinking yourself. I purposely remained >>mysterious because I didn't want to ruin your fun. > >His version of fun lately seems to have a lot to do with "young >bucks", and I don't think he is fantasizing about adolescent male >deer. > >John Cluck! Cluck! Cluck! ...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 | |