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Why do some capacitors have polarity?

(Asked once already, but for some reason was not visible.) That is sort of a blanket question. I'm wondering: 1) why only some, and not all or none? 2) what (in their physical structure) causes them to have polarity? 3) why do we make ones that have polarity? Is there something prohibitive, like cost, or safety, making it so in the production? (okay, not safety, as may attest anyone who has seen a reversed capacitor explode.)

whizz1234 years ago
Capacitors are measured in the unit called the Farad. Condensers are also capacitors. It refers to the amount of charge a capacitor can store.
That unit, the Farad, is very very big and poorly chosen. Most values you come across are microfarads. The microfarad is one millionth of a Farad! So you can see how big the Farad is. The pico farad is also use in radio work and other applications. This is one millionth of a microfarad! So you can see even more so the size of a Farad.
To get large value you need to use electrolytic capacitor. These are polarized meaning you have to connect positive voltages to the positive plate/electrode and neg to negative. Otherwise the fine coating that was formed when they were made will be destroyed. They are 'formed' when made. A 10,000 microfarad would be a very big capacitor even though that is only 0.01 Farad!
Small capacitors like 0.01 microfarad are common in radio sets etc. This is only 0.0001 Farad.
Most capacitors can be non polarized up to about 1 microfarad. So can be connected either way around.
There are very many different materials used to make capacitors including paper, plastics, air and tantalum.
The voltage they work at varies immensely depending on contruction and materials made from.
The old vane type tuning 'condenser' used in early radio sets were air gapped. The beauty of air is that should there be a voltage spark over it is repaired all by itself. The 'stuff between the plates of a capacitor is known as the dialectric and this too changes the capacitance depending on what it is. eg mica. Some caps are close tolerance meaning they are close to what the published value is, mica. Or maybe ceramic where the tolerance is poor but may work at a high capacitance - not Farads though!
With the Leyden Jar the capacity will be low but the voltage working will be high - possible thousands of volts providing it is well constrcted with almost no leaky resistance anywhere.
Not my subject and to damn old to remember school but I believe the Leden is non polarized so if you put in a charge you will get out that charge in the same way it went in. In easier to follow words the positive in terminal will be the positive out one.
The last point is that a capacitor will charge up to a value of the input voltage. Small capacity ones will be almost immediately charged whereas ones with a large capacity, large/very large, ones will take a little time. This is when fed with DC. Altogether different with AC/alternating current as that will 'pass through' a capacitor. The bigger capacity it has the easier AC will pass.
DJ
Only electrolytic capacitors need to be polarised. Its a matter of their internal construction, where the dielectric film is created BY the polarisation, and can be broken down by reverse polarisation. The only way to make BIG value capacitors is generally by using electrolytics, so we need them. It is not a problem in practice.
V-Man737 (author)  steveastrouk5 years ago
so...
all (and only) electrolytic capacitors have polarity?
Polarization creates the dielectric film? how?
By "big values" you mean Farads (as opposed to voltage rating)?
If I decided to extrapolate this into a Leyden jar, how would I go about making one with polarity? How would I make one side positive or negative?
So full of questions, like a new child...
There are some nonpolarised electrolytics, used in some audio work. They have low breakdown voltages, and low cap values. No, 10-47,000uF is big in electrolytics. Farad rated caps are made in another technology. Leyden jars don't have a lot of capacity, but they do have high breakdown voltages.