# Capacitor confusion

Discussion in 'General Electronics Discussion' started by Tomstechnicalities, Mar 27, 2015.

1. ### Tomstechnicalities

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Mar 25, 2015
So I have been looking all over the place for information on capacitors. But still i'm confused. What are capacitors used for? If they store energy and then why isn't that we just use our own batteries? And how would I need to charge capacitor with AC voltage or DC voltage. Also what are farads? are they current, or how much energy the capacitor holds? I'm not really sure I'm just confused any help would be great thanks!!!

2. ### BobK

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1,688
Jan 5, 2010
Capacitors have many uses in electronics, including passing AC while blocking DC, timing circuits, filters, tuning circuits, suppressing voltage spikes, suppressing electrical noise, shifting phase to start an AC motor, limiting current in AC circuits, and the list goes on.

They do store energy, but not at the density of batteries. It would take a huge capacitor to store the same energy as an AA battery.

Farad is the unit of capacitance. It is equal to 1 Coulomb per volt. Which means it takes 1 Coulomb of charge to make 1V on a 1F capacitor. This is, by the way, a very large capacitance. Most capacitors are measured in microFarads or even picoFarads.

A C Farad capacitor holds E = 1/2 C V^2 when charged to voltage V.

Bob

Last edited: Mar 27, 2015
3. ### Tomstechnicalities

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Mar 25, 2015
Thanks!! this helps clear up quite alot of questions.

4. ### hevans1944Hop - AC8NS

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Jun 21, 2012
Capacitors are the electrostatic complement to electromagnetic inductors. Both capacitors and inductors store electrical energy, usually for only short periods of time because there are electrical losses associated with both devices. Capacitors store energy in the electrical field between two charged conductors; inductors store energy in the magnetic field surrounding a current-carrying conductor. Google is definitely your friend here! A good place to start is an introductory course in Physics with some calculus.

Or, if math and theory isn't exactly your thing right now, grab a goodly variety of capacitors from your local electronics emporium, a few resistors, a few LEDs, some dry-cell batteries and battery holders, a cheap multimeter and perhaps a solderless breadboard and begin experimenting. Some capacitors (electrolytics) are polarized, so you need to pay attention to that. Batteries and LEDs are polarized, too, and must be connected properly. Be prepared to "let the magic smoke out" of a few components until you get a "feel" for what your are doing. Be sure to memorize all the relations in that chart you are using as your avatar. Have fun! Learn by doing!

BTW, if you charge a largeish non-polarized capacitor (several microfarads) by temporarily connecting it across a battery, it will retain most of that charge for awhile when you disconnect the battery. If you then connect the capacitor across a largeish inductor (the primary of a power transformer weighing a few pounds will do), an amazing thing happens: the capacitor discharges into the inductance of the primary winding, which begins to build up a magnetic field, all the while the electrostatic field in the capacitor is diminishing. Then, as soon as the magnetic field in the inductor stops increasing, it begins to decrease and generates a voltage that charges the capacitor up in the opposite direction! When the magnetic field in the inductor ceases to decrease, the charge transferred back to the capacitor starts the cycle all over again, but now with the charge on the capacitor reversed. Electrical energy keeps sloshing back and forth between the capacitor and the inductor, slowly dissipating the energy in each cycle as heat in the resistance of the connecting wires and the resistance of the inductor winding. A clever person could design a circuit that would replenish the energy lost during each cycle, sort of like pushing on a playground swing at the right moment to sustain its motion. Voila! That clever person has constructed an LC oscillator! The period of oscillation will depend on the square root of the product of the value of capacitance and the value of inductance used. Larger capacitance and/or larger inductance means longer period of oscillation.

Making oscillators is just one thing you can do with capacitors. And, sometimes, you don't even need inductors to do that...

Tomstechnicalities and davenn like this.
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