Discussion in 'Feedback and Suggestions' started by Tesla00010010, Jan 27, 2009.

1. ### Tesla00010010 Thread Starter Member

Jun 20, 2008
21
0
Hello to everyone

i just wanted to comment about the article "Capacitors and Calculus" on the first volume of this textbooks.
The problem is that the author shows voltage and current changes in a capacitor in a linear way rather than logarithmically as it really is.I pressume that the author did it that way for didactical purposes by showing simple geometrical forms and the rate of changes of the magnitudes,but i think it can be very misleading to new students since he doesn't mention that what he wrote does not happens in real life.I would like to know what u think about this.

Thanks

2. ### mik3 Senior Member

Feb 4, 2008
4,846
69
Ic=C*dV/dt

If you increase the voltage V across the capacitor linearly with time then the term dV/dt will increase linearly and thus the current because C is constant.

3. ### Tesla00010010 Thread Starter Member

Jun 20, 2008
21
0
i understand what you mean,but is there a real way of increasing voltage linearly?the capacitor charges always exponentially or at least that is what i have seen so far not only in theory but measuring in a real circuit too

4. ### Tesla00010010 Thread Starter Member

Jun 20, 2008
21
0
I think you are right,im confusing the voltage in capacitor with a fixed voltage source value and in the article he mentions a variable voltage source. which would indeed give a linear current through the capacitor.Anyways,i think its worth to point out the difference because i got confused.Thanks

5. ### kubeek Expert

Sep 20, 2005
5,413
1,030
If you connect it to a triangle voltage source with near-zero source impedance, then it surely will increase voltage linearily.

6. ### studiot AAC Fanatic!

Nov 9, 2007
5,003
521
Tesla I think you have a valid point.

Looking at the page concerned the capacitor is question is supplied from a DC battery. Therefore it will indeed charge or discharge in an exponential fashion.

Two things to remember however.

Firstly
Capacitors are normally ised in AC circuits, not DC ones. i.e. they pass AC, not DC.
they will then be charged and discharged from a time varying voltage, which may be sinusoidal, or Kubeeks triangle or whatever. Then they current waveform will not be exponential, but will be the same shape as the driving AC voltage.

Secondly
We often approximate a small portion of a non linear curve by a straight line.

Perhaps a note to this effect might be usefully added to the text?

7. ### mik3 Senior Member

Feb 4, 2008
4,846
69
In the text the author explains that the pot is varied linearly and thus the DC voltage form the battery becomes variable voltage.

8. ### Ratch New Member

Mar 20, 2007
1,068
4
Tesla000100010,

The voltage across the capacitor will vary exponentially (not logrithmically) if the capacitor is charged (with energy) from a constant voltage source. The voltage will vary linearly if the capacitor is charged (with energy) from a constant current source, V=Q/C . So if the charge imbalance is increased at a constant rate, and it will be from a constant current source, then the voltage increase will be linear.

Ratch

9. ### jpanhalt Expert

Jan 18, 2008
6,805
1,399
The paragraph immediately before the graph in question begins,
It seems clear to me what is meant.

John

10. ### studiot AAC Fanatic!

Nov 9, 2007
5,003
521
What you say is strictly true,
but

how many engineers do you guys know who can stand and twiddle a pot at an exactly linear rate, repetitively or otherwise?

Seems to me to be a remarkably clumsy artifice to foist onto a beginner.

It is not what the experienced engineer finds clear, but what is most easily digestible by a beginner.

Especially after (s)he has been shown the exponential charge curve for a capacitor.

I vote for improvement.

Last edited: Jan 28, 2009
11. ### jpanhalt Expert

Jan 18, 2008
6,805
1,399
You don't mean to say that Maxwell's Demon doesn't really exist either, do you?

Although I don't think the description in AAC will quite reach the stature of Maxwell's thought experiment, it is one thing to criticize the accuracy of a statement, and quite another to disagree with the pedagogical approach. I found the author's description clear and not at all ambiguous as to whether it was intended as a real or thought experiment.

John

12. ### studiot AAC Fanatic!

Nov 9, 2007
5,003
521
I thought Maxwell's Demon was still on his Christmas hols.

13. ### Wendy Moderator

Mar 24, 2008
21,494
2,961
Maxwell's Demon is right outside my door, raining water that is freezing when it hits the ground. Call it the Dallas Texas equivalent of snow.