Charge Conservation - Hint of the Day

That's insufficient information, and I rather expect that you know it.

--

Tim Wescott Wescott Design Services

Do you need to implement control loops in software? "Applied Control Theory for Embedded Systems" was written for you. See details at

He promised us, a couple of weeks ago, a "mathematical proof" of something or other charge related. Is this lame snarky "hint" the best he's been able to come up with?

John

Cluck! Cluck! Cluck! Cluck! ...Jim Thompson

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 :-) ...Jim Thompson

=A0 =A0 ...Jim Thompson ...

Anywhere between zero and infinity depending on the load - of course it can only deliver 1 Coulomb per second regardless of the load.

kevin

Hi Jim,

I think it depends on your definitions,

I think most would say it will start out "delivering" one Coulomb per second, I.e. that is what flows through an external circuit. The subsequent behaviour, and hence total number of Coulombs "delivered", depends on what it is actually connected to. The charge delivered would be unlimited for a superconducting shorted-out inductor, and would be infinitesimal if the inductor was open circuited.

But in the "all charge is conserved" picture all the charge that exits one terminal gets returned to the other. So Zero.

So which definition do you want to use?

And then of course there is the secret Jim Thompson Interpretation of Electrical Circuits... :)

Into what?

Zero 'total' charge is my answer, unless the inductor has a few stray charges on it's surface.

George H.

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.

At least you exhibit some thought on the problem. Thanks!

...Jim Thompson

Cluck! Cluck! Cluck! Cluck! ESAD!

I may be a snarky ol' git, but I'm GOOD ;-)

I'm a graphical/pictorial sort of guy...

Normally I'd do this sort of thing on a white board, before an audience at one of my seminars, only an outline in front of me, then "play" the audience.

So I've been struggling to create the pictorial on a series of sheets of paper to properly describe (and solve the issue).

It's funny, the only time I've had time to think this through has been while sitting in doctors' waiting rooms. This morning, at Barnet-Dulaney (wife's eyes, YAG lasers, cataracts), I fitted the pictorial together in my head.

Within the next 48 hours you, John "The Bloviator" Larkin (and ALL your supplicants) are going to be BURNED AT THE STAKE...

One-by-one every one of your STUPID STUPID STUPID statements will be refuted (message ID's supplied to prove you made the STUPID STUPID STUPID statements).

I'm working now on how to distribute the material so that it can't be disseminated, so that everyone can have a good laugh... BEHIND YOUR WORTHLESS BACK ;-)

RECANT! RECANT! THE END IS NEAR :-) ...Jim Thompson

Is this the way your seminars end?

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

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.

--- 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, a one millihenry coil, under the same conditions, should deliver about about three orders of magnitude less, about one millicoulomb.

JF

Sheeeesh, Tim! Did I hit a nerve? I can teach anything over there.

My first month in AZ I stood up in an ASU graduate school class and asked, "When do you plan to start teaching this course on a college level?"

Got tossed. 10 minutes later (after the Dean and I had a "discussion") I had full credit for the course.

I posed the Inductor Question as a mind teaser.

Looks like you need to quaff a few... don't be so uptight ;-)

This group has really gone to hell... like Democrats... "discussion" allowed only if you kiss up to "The Bloviator". ...Jim Thompson

Please, John Fields, Please don't shoot from the hip :-) ...Jim Thompson

This is more like the way his life will end. Paranoid ravings.

John

So sayeth the namby pamby jack-wagon of this group :-)

Will you sleep well tonight ?:-) ...Jim Thompson

Dumping that inductor into 1 ohm makes the exact same waveform across the resistor as if you discharged a 1 millifarad cap charged to 1 volt. Ditto 0.001 coulombs. Nice crosscheck.

John