Charge Conservation - Hint of the Day
in [Design]
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From: Jim Thompson on 21 Jul 2010 11:14 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 It is not me that's the BU pimp ;-) ...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 |
From: John Larkin on 21 Jul 2010 12:52 On Wed, 21 Jul 2010 07:28:10 -0700 (PDT), George Herold <gherold(a)teachspin.com> wrote: >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. That's a common way of stating it. When we put parts on boards, their electrostatic potential is not often of concern. So we say that we put charge into a capacitor or a battery, literally we say "charge a battery" or "charge a capacitor" as opposed to "run charge through a capacitor", and we measure how much in ampere-seconds, namely coulombs. Since both a cap and a battery save the ampere-seconds and can return/deliver them later, it's reasonable to think that they stored charge. The numbers work. Engineering is about what works. John
From: Jim Thompson on 21 Jul 2010 13:15 On Wed, 21 Jul 2010 09:52:25 -0700, John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: [snip] > >The numbers work. Engineering is about what works. > >John Engineering is about what works _all_the_time_, under _all_environmental_conditions_. Schlocky is for Larkin's. ...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 |
From: Tim Wescott on 21 Jul 2010 13:27 On 07/21/2010 07:28 AM, George Herold wrote: > 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.) See my preface. The notion of "delivered charge" crops up pretty consistently in electrical engineering, and it almost always means "charge delivered to (some lead of) some component (while ignoring charge coming out of one or more other leads)". If you're willing to do more math, and to posit a switch that magically opens when current drops to zero, without itself having any voltage drop, then you can get exactly the same results (i.e. -- the total delivered charge is limited by your desire or the total resistance in the circuit) by having the inductor charging up a capacitor (which will have no more net charge when the process is done than when it started). Or replace the switch by a real-world diode, and find that the total charge that will flow through the inductor is limited by the diode's characteristics as well as the rest of the circuit. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com Do you need to implement control loops in software? "Applied Control Theory for Embedded Systems" was written for you. See details at http://www.wescottdesign.com/actfes/actfes.html
From: Jim Thompson on 21 Jul 2010 14:03
On Wed, 21 Jul 2010 10:15:51 -0700, Jim Thompson <To-Email-Use-The-Envelope-Icon(a)On-My-Web-Site.com> wrote: >On Wed, 21 Jul 2010 09:52:25 -0700, John Larkin ><jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: > >[snip] >> >>The numbers work. Engineering is about what works. >> >>John > >Engineering is about what works _all_the_time_, under >_all_environmental_conditions_. > >Schlocky is for Larkin's. > > ...Jim Thompson For example, from _Consumer_Reports_, "Used Cars to Avoid": Audi; In the Best Used Cars List: Infiniti Q45 ...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 | |