Capacitors do store charge. Even though the net charge of the entire device is zero, there’s still charge accumulation on each of the plates.
Some more info (because I’m sitting here at work watching a screen to tell me something useful and it is taking forever):
Another use for capacitors is in AC load balancing (technically this is just a variation of the temporary store and release of energy). Charging up energy storage devices like inductors and capacitors takes energy, and even though that energy is released later in the AC sine wave cycle, the generator still has to supply the extra energy required to do it. We tend to break up power into two components, the “real” power, which is the power used by things like light bulbs and such, and the “imaginary” or “reactive” power, which is the stuff that goes into charging and discharging capacitors and inductors.
Capacitors and inductors kinda work opposite of each other. While capacitors are charging in an AC sine wave, inductors are discharging, and when inductors are charging, capacitors are discharging. So, what you can do is if your load is inductive, you can add capacitors to the circuit to balance it out. Likewise, if the load is capacitive, you can add inductors to balance it out.
What’s the benefit of balancing it all out? Well, if you’ve got it perfectly balanced, then the reactive power just ends up bouncing back and forth between the capacitors and inductors, which means that the generator only has to supply the real part of the power. This makes things much more efficient and lightens the load on the generator.
Residential power in general tends to be somewhat inductive, due mostly to things like motors in vacuum cleaners, hair dryers, washing machines, etc. So, to balance it all out, the power company adds capacitors to the line at the substation. Many appliances these days will have power factor correction capacitors installed in them as well.
If the load in your house isn’t balanced, don’t worry. The power company only charges you for the real power (measured in watts). They usually don’t charge you for the reactive power (measured in vars) unless your power factor is really far out of whack. Power factor is the measure of how inductive or capacitive your house is. If you want the ugly math version, it’s the cosine of the phase angle between the voltage and the current.
But how can you be so sure? Every month it seems like somebody is slipping in a few extra vars into my electric bill.
And some more info:
There are different types of capacitors, and they are often used differently in circuits.
For example, those round tin can style capacitors are usually electroltyic capacitors. They are made with one or two sheets of metal with a layer of goo in between, which is rolled up like a fruit roll-up and shoved into a can (some only have one sheet because they use the goo as the anode instead of using two plates). These work great for bulk filtering, but they are horrible at high frequencies.
Typical electrolytic capacitor:
http://www.solarbotics.com/assets/images/cp4700uf/cp4700uf_pl.jpg
On the other hand, you’ve got ceramic disk capacitors, which look like this:
http://www.guitarpartscanada.com/images/caps.jpg
Ceramic disk capacitors are great for higher frequencies, but you can’t use big values of them to do bulk filtering because ceramic disk capacitors get freaking huge for higher values of capacitance.
So, what you often see in circuits where they are trying to do something like keep a voltage source stable is they’ll use a big fat electrolytic in parallel with a ceramic disk.The electrolytic does most of the work, and the small ceramic disk filters off the high frequency stuff that the big electrolytic misses.
You’ve got things like tantalum capacitors which are small but have a greater capacitance for their size than ceramic disks, but they cost more than ceramic disks too. So there’s all kinds of different materials and different tradeoffs.
Manufacturing cost sometimes factors into it. If you are building a small circuit board with all surface mount parts, it costs a lot more to stick a through-hole electrolytic capacitor on there. If you use too small of parts you can also cause problems, like having the part shift around and make poor contact when it goes through the soldering machine. Sometimes you end up using a part that may not be the ideal part for how you envisioned the circuit working, but the part you want would drive up the manufacturing cost too much so you use something cheaper and almost as good instead.
If you’ve got the old fashioned spinny wheel type of meter, they don’t measure vars at all. They only measure watts. The evil power company can’t charge you for what they can’t measure.
If you’ve got the new super duper whiz bang fancy shmancy digital meters, they usually can measure vars, and often they will report your power factor back to the power company. Unless your power factor is really far out of whack (like you are running a couple of 10,000 hp motors in your basement for some reason) the power company doesn’t really do anything with this information (most residential service specifies that they only charge you for the watts). If your power factor is really far out, one of two things will happen. Either (1) an extra charge may show up on your utility bill or (2) nothing will happen. In either case, some dude or dudette from the power company may come out and investigate.
Now if you are a business or a manufacturing plant, those guys operate under different rules, and they get whacked big time financially if they don’t keep their power factor in line.
A question for you capacitor experts: I have a less-than-two-years-old Samsung HD TV with a blown capacitor (and a one-year warranty) of the following specs:
2200 microfarads, 10 V (I’ve got room on the board to upgrade up to 35 V, I think), 105 degrees C, with the leads coming out of one end.
None of the area Radio Shacks carry capacitors with these specs exactly. Two of the stores have one of these:
2200 microfarads, 35 V, but rated only to 85 degrees C.
My question is, would it be okay to go with one of the locally-available caps, or should I mail order or special order a more exact replacement? Is the difference between 85 and 105 C that significant?
Thanks in advance for your replies.
going lower in voltage or temperature rating is not good.
The temperature rating is just for meeting the rated lifetime. The 85C one will work, although it may not last as long (but probably long enough).
I know, but I’m trying to suss out how serious the difference is…
Sounds good enough for me, then. I was afraid of a catastrophic failure or insufficiency that would jeopardize the TV as a whole, permanently… but if I have to replace the cap. every few years, I can live with that.
What’s weird to me is how little of a selection in capacitors one finds at Radio Shack these days; it seems like their present offerings in electrical and electronic components is just a token smattering compared to what they used to carry (say, twenty years ago). 
Yes, Electronics as a hobby is nearly dead.
I end up mail ordering almost all my parts from DigiKey or Mouser.
For that matter, it’s a lot less than they used to offer ten years ago, when I used to rely on them for my high school electronics projects.* That’s because “The Shack” (as they now wish to be known) are rebranding themselves as a consumer electronics store, not a hobbyist shop. That’s a mistake, in my view - they can’t compare with the “big box” stores on standard consumer electronics. But it is what it is.
*In fact, the store manager was an electrical engineer. Good guy - helped me out more than a few times, when I’d bitten off more than I could chew on a project.
Ironically, there are very complex components available today dirt cheap that make it possible for a hobbyist to build more fantastic things these days than they could ever dream of 20 years ago.
I have a large electroltyic capacitor from High School days.
Do they still work decades later?
Should I start out charging it with a low voltage, discharge it and work upward to not get a fault in the insulator?
It is out in the shop so I can’t tell you the specs, it is the size of a thermos, and was alot of fun back in the day.
Charge it up, put your gum wrapper foil bridge across the terminals, …light show.
I would charge it up to full voltage through a very large-value resistor, say 1MΩ.
If you really want to be careful, you could start at say 1/4 voltage, and measure the leakage current until it stabilizes, and then work your way to full voltage. That said, most old caps are surprisingly stable, and it probably would be fine just applying full voltage to it.
BTW, this is known as “re-forming.”
A lot depends on what they used for the electrolyte. Some of them age better than others.
I would be a little leery about just applying a full voltage to it, especially if it is decades old. beowulff’s link about re-forming is good. Just be aware that whatever you do there is a chance that the cap could explode (much more likely to occur if you just apply a full voltage than if you slowly work your way up), so plan accordingly. Aim the top of the can at somebody you don’t like. 
There are still a few Radio Shacks around that carry what they did 10 years ago, maybe 15. I use their web site to check which stores have transistors and classic ICs like a 555, and some of them have a pretty good set of basic components, sockets, etc… They seem to be stores run by the same management for a long time. I think most of the outlets only want to carry high profit items like cell phones, and don’t have anyone in the store who knows what components are anyway. Ask for a capacitor at those places, and they’ll point to a corner in the back and say ‘If we have it, it would be back there’.
As for being a hobbyist store, they couldn’t pay the rent with that. Even at the inflated prices for parts, the percentage of people who would shop at a hobbyist store and the amount of money they spend never paid the bills for Radio Shack. CB radios, phones, scanners, TRS-80s and other popular electonic specialties have always been their bread and butter. Their old model of low cost alternatives isn’t working for now, so they carry the regular consumer brands. The next electronic fad might change that again.
This. I’ve called seemingly half the RSes in NJ, and some of the clerks clearly didn’t know the first thing about capacitors – not even what they look like. As for selection, they’re better at the stores in strip centers as opposed to shopping malls, but not very good at either, unfortunately.
I visited Radio Shack some 5 or 6 years ago and asked for a breadboard.
I think you can see where this is going…
Be sure that the cap isn’t a special type. I’ve found that many of the caps that fail in these kinds of devices are low esr electrolytics. Even though there are several good supply stores in my area, I have to order those online.
Look very carefully at the label and Google any codes on it.
I used to work in sales for a capacitor manufacturer.
It sounds like you’ve got an aluminum electrolitic cap there, based on the values you mention.
Going up in voltage won’t be a problem, and it will to a small extent offset downgrading the temperature rating (in that a part rated for 35v won’t run as hot as one rated for 10v). More than space on the board, you need to be concerned with lead spacing.
But I’d still be leery of downgrading the temp on that. A cap that is hot run will at the least drastically shorten the life of the capacitor. At worst, the cap may fail and vent, causing damage to the board it is on.
I’d second the suggestion to check with Digikey, Mouser or Newark.
Digikey has a search engine http://search.digikey.com/scripts/DkSearch/dksus.dll?Cat=131081
Enter your requirements and see what they have in stock.