Capacitors, AC and DC confusion

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Spen
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Capacitors, AC and DC confusion

Postby Spen » Tue Jan 18, 2011 8:59 pm UTC

Hi, in physics we've recently been taught about capacitors and in all of the experiments we did the power supplies were set to DC. When I met up with my friend who's doing electronic engineering at uni he was telling me that capacitors were AC only which rather confused me (particularly as we were told that electrons just pile up on one plate until the capacitor is shorted at which point they all start flowing again). The physics teacher was also unsure as to why the equation for energy stored in a capacitor is 0.5*C*v^2, the coefficient of 0.5 seems to make a lot more sense with the idea of them being AC but the way that capacitors were explained is completely incompatible with my understanding of AC (the physics teacher said that she was unsure about them being AC components, oh and also I know you can get AC capacitors as I ended up removing one from an old knackered microwave oven hence the major confusion). Where am I going wrong here?
Thanks in advance
Spen

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Re: Capacitors, AC and DC confusion

Postby gorcee » Tue Jan 18, 2011 9:24 pm UTC

This article describes the coefficient of 1/2 in a clear and concise manner: http://en.wikipedia.org/wiki/Capacitance

In general, whenever you see a [imath]\frac{1}{2}ax^2[/imath] type of term, a good place to start thinking is that it's the result of integration of some linear relationship.

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Re: Capacitors, AC and DC confusion

Postby Sir_Elderberry » Tue Jan 18, 2011 9:34 pm UTC

Capacitors can be called "AC only" because they only "pass" AC current--that is, a current that changes quickly stops the capacitor from ever building up much opposing voltage, so it will pass through the capacitor, while one with a lower frequency will be more affected. This means you can use resistors and capacitors to build a "high-pass" filter, which allows signals of a certain frequency past while filtering out signals of a lower frequency. (It's not really a hard cut-off, but there's a "corner frequency" where things turn either way, basically.)

Depending on what your friend meant, he could also refer to the fact that a capacitor in a DC circuit quickly begins to approximate an open circuit, so it's a transient state. There's a word I'm looking for that means something like "steady state" or "equilibrium" but different, but basically in an AC circuit you can have a capacitor and it has reactance, periodic effects on the current, etc, whereas in a DC circuit it only affects things briefly until it charges up.
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Re: Capacitors, AC and DC confusion

Postby Spen » Tue Jan 18, 2011 9:46 pm UTC

Thanks to both of you!
Gorcee, you have just inspired an utter face palm moment, I hadn't made the connection from materials of area bounded by the curve, the line x=0 and the positive x-axis being equal to energy and that then being applicable to this, that and my calculus having been horribly weak at the point we were learning all of this! Thanks
Sir Elderberry, He's doing electronic engineering so he'll be using them as means to tune circuits, smooth out power fluctuations etc which would be why he was using them with AC whereas all of the experiments we have done with them were charging and discharing experiments (and discharging to make copper sheets look like swiss cheese!). That makes a lot more sense now and has cleared up the confusion so thanks!

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Re: Capacitors, AC and DC confusion

Postby Tass » Tue Jan 18, 2011 10:25 pm UTC

gorcee wrote:This article describes the coefficient of 1/2 in a clear and concise manner: http://en.wikipedia.org/wiki/Capacitance

In general, whenever you see a [imath]\frac{1}{2}ax^2[/imath] type of term, a good place to start thinking is that it's the result of integration of some linear relationship.


In other words: The area of a triangle.

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Re: Capacitors, AC and DC confusion

Postby wbeaty » Wed Jan 19, 2011 6:36 am UTC

Capacitors: feed them a constant current, and the voltage between their terminals will rise constantly. Or feed them a smoothly rising voltage, and they'll draw a constant current.

Of course with some, you can feed them twenty amps, and the voltage changes reeeeeeally slow:

3,000 Farad Ultracapacitor (2.7V max)
http://www.goldmine-elec-products.com/products.asp?dept=1028

V/T = I/C = 20A/3000F = .00667 volts/second

(Yes, if you short this one with a thin #26 wire, it drips to the floor as incandescent yellow frags. Haven't dared to try a thick wire)
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Re: Capacitors, AC and DC confusion

Postby Tass » Wed Jan 19, 2011 8:08 am UTC

wbeaty wrote: 3,000 Farad Ultracapacitor (2.7V max)
http://www.goldmine-elec-products.com/products.asp?dept=1028

V/T = I/C = 20A/3000F = .00667 volts/second


YAOUCH!

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Re: Capacitors, AC and DC confusion

Postby Link » Wed Jan 19, 2011 8:31 am UTC

Capacitors aren't AC only; that's bollix. Capacitors, and inductors for that matter, can just as easily be used in DC circuits as well. Generally, in DC, capacitors are more often than not used as timing devices (in combination with resistors), as energy storage, or as filters to keep high-frequency noise away from strictly-DC parts of the circuit. Inductors aren't as commonly used - but still usable - in DC; you can find them in switched-mode power supplies as energy storage and sometimes as a filter. In AC, capacitors and inductors act somewhat like a frequency-dependent resistor, although unlike a regular resistor, they introduce a phase shift between the voltage and the current.

The difference between AC and DC capacitors is that capacitors marketed as AC are optimised for use in AC circuits and those marketed as DC are optimised for DC usage. The underlying principles are the same, but the design and manufacturing process can be different. Any real capacitor may exhibit behaviours far from ideal due to parasitic properties (equivalent parallel and series resistances and inductances), but, for example, if you have a capacitor marketed as being "for AC", it will usually perform well in AC circuits -- but, as always, when in doubt, check the datasheet.

I also think it's rather strange that your physics teacher doesn't know where the factor of ½ comes from. That really seems like a thing a physics teacher should know. Does she likewise not know where the factor ½ in Ek=½mv² comes from? (If she doesn't: bad teacher. Bad! *Picks up squirt bottle*.)

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Re: Capacitors, AC and DC confusion

Postby eSOANEM » Wed Jan 19, 2011 5:56 pm UTC

Link wrote:The difference between AC and DC capacitors is that capacitors marketed as AC are optimised for use in AC circuits and those marketed as DC are optimised for DC usage. The underlying principles are the same, but the design and manufacturing process can be different. Any real capacitor may exhibit behaviours far from ideal due to parasitic properties (equivalent parallel and series resistances and inductances), but, for example, if you have a capacitor marketed as being "for AC", it will usually perform well in AC circuits -- but, as always, when in doubt, check the datasheet.


It's also important to note that DC capacitors (particularly those with larger capacitances) are likely to be electrolytic and so won't take kindly to being put in an AC circuit (or the wrong way round in a DC circuit either for that matter).
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Re: Capacitors, AC and DC confusion

Postby Spen » Wed Jan 19, 2011 9:35 pm UTC

Link wrote:
I also think it's rather strange that your physics teacher doesn't know where the factor of ½ comes from. That really seems like a thing a physics teacher should know. Does she likewise not know where the factor ½ in Ek=½mv² comes from? (If she doesn't: bad teacher. Bad! *Picks up squirt bottle*.)

We learnt the KE equation at GCSE when I was really not interested in physics (or maths for that matter) and since most of the rest of the class felt the same way the teacher didn't derive the equation (not forgetting that none of us had come across integration by this point).
The source of all of this confusion was my mate telling me that they were AC only and when I asked my physics teacher she seemed a bit unsure of it partly because she doesn't seem as strong on the electronics side of things as she is for the other bits of physics. She's also pretty good on helping with applying the physics to practical situations (Design and Technology projects where I'm involved often end up applying physics and maths and despite covering all of the theory the AQA specification doesn't seem to translate very well into applying it to a long lasting project, this is mainly because it's intended to lead into a physics or engineering degree rather than applying it to other subjects as you learn it).
eSOANEM, The experiment of putting an electrolytic capacitor into a DC circuit backwards was part of what confused me about my friends statement on the subject of them being AC only.

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Re: Capacitors, AC and DC confusion

Postby jjfortherear » Thu Jan 20, 2011 12:45 am UTC

holy moly it's wbeaty! off topic, but I was trying to find that discussion about an electrostatic magnet which was mentioned on this site (pointing to your site), but couldn't find it. where it be?
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Re: Capacitors, AC and DC confusion

Postby Link » Thu Jan 20, 2011 9:55 am UTC

eSOANEM wrote:
Link wrote:The difference between AC and DC capacitors is that capacitors marketed as AC are optimised for use in AC circuits and those marketed as DC are optimised for DC usage. The underlying principles are the same, but the design and manufacturing process can be different. Any real capacitor may exhibit behaviours far from ideal due to parasitic properties (equivalent parallel and series resistances and inductances), but, for example, if you have a capacitor marketed as being "for AC", it will usually perform well in AC circuits -- but, as always, when in doubt, check the datasheet.


It's also important to note that DC capacitors (particularly those with larger capacitances) are likely to be electrolytic and so won't take kindly to being put in an AC circuit (or the wrong way round in a DC circuit either for that matter).

True. A reverse-biased polar capacitor can give some nasty results (*fizzle-fizzle* *BANG* "Aah my face!") - although there are ways to avoid that, like using two capacitors of the (exact) same value in reverse-series with their negative ends tied together. Two capacitors combined like that will have half the value of either capacitor individually, but can withstand AC (although this is a tricky configuration and ideally shouldn't be used for anything commercial). Bipolar electrolytic capacitors are also available; they are essentially such a reverse-series combination put into a single package. Still, when it comes to ideal capacitors (bipolar with no parasitics), they can just as easily be used in AC as in DC.


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