Talk:Farad

In the picofarad article, there is the following statement : "the picofarad is the smallest measurable unit of electrical capacitance". I tagged it as dubious. Please confirm. --Edcolins 19:42, August 21, 2005 (UTC)

It's rubbish. I removed it. --Heron 20:25, 21 August 2005 (UTC)Reply

Thanks! --Edcolins 20:46, August 21, 2005 (UTC)

Not so far off for human sized systems. I believe it is the smallest you can buy as a capacitor. But inside ICs, the capacitances might be much smaller. Gah4 (talk) 21:07, 1 February 2022 (UTC)Reply

In the article, it says that commercial capacitors are in the range "from 100 fF to 5 kF". I think the "kF" is a mistake; the link given as reference is a component catalog where you can't even search for something beyond 1 mF. Besides, the article indicates that complete Earth can't even reach 1 F. --Comae (Talk) —Preceding undated comment added 19:16, 13 June 2010 (UTC).Reply

I think the word "accept" in the second paragraph should link to the old rock band Accept so I changed it accordingly. —Preceding unsigned comment added by 194.18.184.15 (talk) 11:57, 23 May 2011 (UTC)Reply

It's a copy paste of this page [1]

Is there a named unit for capacitance measured in elementary charges per volt?

Currently, the definition section says "A farad is the charge in coulombs which a capacitor will accept for the potential across it to change 1 volt." This is wrong for several reasons, the simplest being that a farad is not a unit of charge. The definition should simply state that "1 farad = 1 coulomb/volt" and perhaps mention that capacitance is measured in farads.71.224.118.215 (talk) 03:36, 13 December 2012 (UTC)Reply

The definition is accurate and precise. Think about it for a second. It is unfortunate it was messed up with the current faulty definition. You have to read the full sentence, and not stop half way. It says That a farad is the charge per volt. The new definition is not really a definition at all. "A farad is the value of capacitance..." is just poor writing. Also, in the new definition, "One farad is the value of capacitance that produces a potential difference of one volt when it has been charged by one coulomb.", the word "it" is used apparently referring back to the word farad. Again poor writing skills.

Please revert it back and think up a better formulation before changing the definition. — Preceding unsigned comment added by 213.153.22.143 (talk) 01:45, 11 April 2014 (UTC)Reply

As the article says, statF is rarely used, as the normal unit for CGS capacitance is cm. (Equivalent to statF, though.) Gah4 (talk) 21:12, 1 February 2022 (UTC)Reply

At the beginning, it says that it is the ability to store an electrical charge. This is most often wrong. It is commonly the ability to store a charge differential. If you ground one side, then relative to that ground, you can store charge. In the usual use, the net charge on a capacitor is zero, with a positive charge on one side, and negative charge on the other. But not always. One can give a capacitance to, for example, the whole earth. Gah4 (talk) 10:02, 26 August 2022 (UTC)Reply

I should have documented my reasoning at greater length when I originally removed (almost) all the equivalences, but here it goes. The equivalences are cited to the SI brochure, but of all the equivalences listed, only three actually appear: ${\displaystyle {\text{kg}}^{-1}{\text{m}}^{-2}{\text{s}}^{4}{\text{A}}^{2}}$  and C/V (page 138), and ${\displaystyle {\frac {A\cdot s}{V}}}$  (page 166). I don't think an equivalence must be cite-able, but it should be explainable in terms of a relationship between physical quantities; for example, in tesla I edit the page to explain the tesla as a measure of flux through a surface of defined area, rather than just presenting the equivalence ${\displaystyle {\text{T}}={\frac {\text{Wb}}{{\text{m}}^{2}}}}$  without comment. It is unlikely that similar explanations can be written for the farad article. Anything with squared coulombs or squared volts seemed hopeless to me, and the Hz/seconds and ohm/siemens substitutions look like substitutions just for the sake of substitutions. When those are eliminated, it leaves only the three equivalences from the SI brochure. ${\displaystyle {\frac {A\cdot s}{V}}}$  from the brochure is not in the current text of this article, but it is what clarity it would provide on top of C/V. Karl Dickman ^talk 03:21, 3 January 2023 (UTC)Reply

I think it is useful to express a farad as coulomb squared per joule, or joule per volt squared. It gives you an idea of what the relationship between a capacitor's capacitance, energy and charge / voltage is. Having these equivalences, one can easily rearrange the formulas to see that, for example, the energy stored in a capacitor is proportional to its capacitance times voltage squared. Sure, not all these equivalences have an apparent meaning (such as ${\displaystyle {\dfrac {{\text{C}}^{2}}{{\text{N}}{\cdot }{\text{m}}}}}$ ), but I find especially ${\displaystyle {\dfrac {\text{J}}{{\text{V}}^{2}}}}$  and ${\displaystyle {\dfrac {{\text{C}}^{2}}{\text{J}}}}$  quite useful. 90.183.96.145 (talk) 22:15, 7 January 2023 (UTC)Reply

Dear Mr. Dickman,

I already have corrected the otherwise useful equation, but NebY reverted it back:

E=(C.U^2)/2

so

2J/V^2

https://www.answers.com/physics/How_many_joules_are_in_a_farad

So please correct the incorrect unit.

Thanks. :-)

JK Xerostomus (talk) 19:52, 26 July 2023 (UTC)Reply

That is a very confused response at answers.com; I won't even try to explain it. Instead, note the straightforward answer below it. Alternatively, work through the equalities at Farad for yourself from left to right, confirm that

$${\displaystyle {\text{F}}={\dfrac {{\text{A}}{\cdot }{\text{s}}}{\text{V}}}={\dfrac {{\text{W}}{\cdot }{\text{s}}}{{\text{V}}^{2}}}}$$ 

and then consider that J = W⋅s . NebY (talk) 20:00, 26 July 2023 (UTC)Reply

Dear Mr. NebY,

I wrote you already on my talk page. The equation E=(C.U^2)/2 is for capacitors which have charge on *two* plates. So it is as if divided by two. So that is why this seems to be contradictory with your, as I think now, correct definition of the unit of energy of a single charge or of charge flowing through a wire.

There is a big discussion on it:

https://physics.stackexchange.com/questions/744497/confusion-involving-the-energy-stored-on-a-capacitor?rq=1

https://physics.stackexchange.com/questions/52661/how-can-you-calculate-or-convert-the-rm-wh-of-a-capacitor-whose-energy-is-g

So sorry for the confusion, I did not know that the equation is only for capacitors and is not a general equation. :-)

Sincerely

jk Xerostomus (talk) 06:01, 27 July 2023 (UTC)Reply