# More semantics - capacitors this time

Discussion in 'Off-Topic' started by Ratch, Oct 27, 2008.

1. ### Ratch Thread Starter New Member

Mar 20, 2007
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StephenDJ,

There used to be a formula I knew of for RC circuits where you could plug in the starting voltage or current, the ending voltage or current, and the amount of time in between, and project exactly what the voltage or current will be after exactly x amount time has passed. I know it rises/drops to 63% within the first time constant. But where's the formula for the other times? Also would like to have the vise versa: i.e. plug in the voltage or current and find the time.

I doubt that one formula could cover current/voltage and energize/de-energize. It is easy to figure out by differential equations or Laplace transforms by setting initial conditions.

mik3,

Does the above formula cover energizing or de-energizing? What defines a fully energized capacitor? Its dielectric breakdown, maybe?

Ratch

2. ### Ron H AAC Fanatic!

Apr 14, 2005
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One formula covers both charging and discharging:

V=Vf+(Vi-Vf)*e^(-t/(R*C))
Where
V=instantaneous voltage
Vi=initial voltage
Vf=final voltage

3. ### Ratch Thread Starter New Member

Mar 20, 2007
1,068
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Ron H,

After I saw your post, I worked it out for myself, and I agree with what you posted. My result is V = Va +(Vi-Va)*exp(-t/RC) where Vi is the initial voltage and Va is the applied step DC voltage. The instantaneous voltage will utlimately reach the applied step voltage whether the capacitor is being more energized or less energized. The subtraction of Vi and Va automatically takes care of the sign.

By the way, does this discussion appear to be "off topic" to you?

Ratch

4. ### Ron H AAC Fanatic!

Apr 14, 2005
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I'm baffled as to why it was moved to Off-Topic. I doubled posted, here and in the original thread. I wonder if mine will be moved also.

5. ### thingmaker3 Retired Moderator

May 16, 2005
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The following have nothing to do with the original thread:
and:
The first first statement is a personal opinion, not a formula or explanation. The second statement is a semantics discussion. If either can be argued to fit one of the other boards, or to fit into the original thread, feel free to present case for such. If one of the moderators agree with said argument, they will no doubt make the appropriate adjustment. If not, then not.

6. ### Ratch Thread Starter New Member

Mar 20, 2007
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thingmaker3,

OK, I will take you up on that. The first statement was a personal opinion, but an informed opinion. And it was identified as a personal opinion. I am sure I can find may personal opinions in these threads, some of which are not identified as opinions, which did not get moved. Ron H showed that one formula did exist for the instantaneous voltage of a capacitor after adding or removing energy from it. I verified his equation as correct, which proved my original opinion wrong. However, right or wrong, it still was "on topic".

The second statement you refer to is an alternate and more correct way of expressing what happens to a capacitor. After all, it takes energy to imbalance the charge of a capacitor. The energy changes, not the net charge. Any objection to that concept is a objection to the messenger, not the message. And again, whether agreement or disagreement is present, it is still "on topic". Whether the moderators restore this posting is not important. They have to justify to themselves and perhaps to the others in this forum whether they were correct.

Ratch

7. ### Ron H AAC Fanatic!

Apr 14, 2005
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I agree.
Ratch, I think your question "What defines a fully energized capacitor?" was a little strange. I have difficulty believing you didn't actually know what mik3 meant by "V=the steady state voltage across the capacitor when fully charged". I suspect you were using it as an excuse to get on your charge vs energy soapbox. If Thingmaker3 had the same opinion, it may explain the move.

8. ### Ratch Thread Starter New Member

Mar 20, 2007
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Ron H,

OK, let me explain it better. Mik stated "V=the steady state voltage across the capacitor when fully charged." So what is "fully charged". Surely not when the capacitor reaches the step voltage. If the step voltage is 10 volts, and the dielectric breakdown is 100 volts, then the capacitor has not been "fully charged" by the step voltage, has it? But regardless of the ambiguous description, it is still "on topic".

Ratch

9. ### Ron H AAC Fanatic!

Apr 14, 2005
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Well, Thingmaker3 gave this thread a good title. You are talking semantics.
Of course it's when the capacitor reaches the step voltage! IMHO, almost anyone reading this thread (except you?) would understand this, in light of the topic at hand. What could dielectric breakdown voltage possibly have to do with the voltage vs time RC charge/discharge curve?

10. ### Ratch Thread Starter New Member

Mar 20, 2007
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Ron H,

As I stated before, the dielectric breakdown determines the "fully charged" state of the capacitor. So why was "fully charged" mentioned in mik3's description of his formula in post #2 of this thread? You should be asking mik3 that question. It is a matter of definition, not semantics.

Ratch

11. ### beenthere Retired Moderator

Apr 20, 2004
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If you charge a capacitor to the point where the dielectric breaks down, you then have a fully destroyed capacitor.

12. ### Ratch Thread Starter New Member

Mar 20, 2007
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beenthere,

That's good. I like it. I guess I should have said rated working voltage.

Ratch

13. ### thingmaker3 Retired Moderator

May 16, 2005
5,072
6
It is possible to energize a capacitor beyond the rated working voltage without destroying it. Just how far will vary from situation to situation.

If you read a few practical electronics texts, you'll find that "fully charged" means exactly what Ron H says it does.

Language is an evolving thing, you see. It is not static, and it flatly refuses to conform to formal definitions in dusty old tomes. (A perusal of very old dictionaries will confirm this.) I know this can be frustrating - I find it annoying myself at time - but I suggest you learn to live with it.

One example of the evolution of language: use of the word "charge" to mean "energize" in this context.

14. ### Ratch Thread Starter New Member

Mar 20, 2007
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thingmaker3,

Yes, I am aware of that.

I would think that they do not even define "fully charged". They might mention it, but not define it.

You mean like the astronauts "walking" in space when they used to go EVA at the beginning of the space age? And do they walk away if they lose their supporting grip? Does that make sense? Is that the way you would describe it? Sometimes you have to stand up and fight against bad jargon. Otherwise descriptive words will eventually lose their meaning.

Ratch

15. ### thingmaker3 Retired Moderator

May 16, 2005
5,072
6
That is indeed another good example.
I think that one would be called "the long walk home."
Yes. Although not to you, perhaps.
And sometimes you have to realize you're just being overly critical.
Congratulations, Ratch! You've just stumbled across another of those logical fallacies you seem to enjoy indulging in! We call this one "Slipery Slope." Cultural ephasia will certainly not follow from linguistic evolution. We might admittedly end up with "peace-keeper missles" or a "permanant press" setting on our irons, but communication will by and large continue with effecacy.

Believe me, I empathise with your position even if I disagree with the merit of your quest. I myself continually fight a loosing battle against vocalisation of the L in "palm," or "calm," or "golf." I throw fits when I hear someone pronounce "coupon" with a wye sound after the hard c. My stomach sours when I hear an R in the second syllable of "sherbet." And I want to scream when the politicians say "nukular."

I've given up trying to get people to say "Fora" instead of "Forums." I can't stop evolution. Niether can you, my good fellow. Try if you must, but don't expect any followers.

But I do not, can not, and will not condone such battles when they distract from the honest quest for knowledge. The Feedback Forum is the appropriate place to challenge the language of the AAC text, not the Homework Forum. The Off Topic Forum is the appropriate place to debate semantics, not the Electronics Forums. (Yes, I call the Fora "Forums" now myself. Unlike some folk who post here, I know when to admit defeat and move on.)

16. ### Ron H AAC Fanatic!

Apr 14, 2005
7,050
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Super Moderators, I'm really glad you picked this up. I was quickly morphing from supporting Ratch to going into attack mode, which really is bad form, especially on these fora . His signature is really well chosen.

17. ### Ratch Thread Starter New Member

Mar 20, 2007
1,068
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thingmaker3,

Sorry to drag this up after all this time, but my preparations for my vacation back then did not allow me to devote the time your response back then that I should have done.

I think you will be happy to know that Webster's Unabridged Dictionary lists both "fora" and "forums" as the plural of "forum".

Ratch

18. ### thingmaker3 Retired Moderator

May 16, 2005
5,072
6
I do hope your vacation was enjoyable! I'm taking a three day weekend myself in just a few days.