I’ve read answers on Quora about why capacitors are used, and they all pretty much said they store charge, or filter out specific frequencies. But what does that actually mean? What do you achieve by doing that? Could anyone give real-world examples? (For example what do they actually do in a light bulb circuit, why is their role important, what if you didn’t add them to the circuit, or what about in mobile phones?) I see answers like the two I listed above, but they don’t actually tell me what they mean.

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    $\begingroup$ Have you looked at any books or articles on electronics? $\endgroup$ – Raghu Parthasarathy Nov 25 '20 at 18:01
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    $\begingroup$ What some thing "actually does" often can be explained on more than one level and, often can be explained in more than one context or from more then one point of view. Also when we say that X does Y (e.g., capacitors filter out specific frequencies), sometimes what we're really saying is that X plays a crucial role in some larger thing that does Y. There is no reason for you to think that capacitors don't do all of the things you've heard about. $\endgroup$ – Solomon Slow Nov 25 '20 at 18:02

..they all pretty much said they store charge, filter out specific frequencies. But what does that actually mean? What do you achieve by doing that?

There are numerous practical uses of a capacitor's ability to store energy, filter out specific frequencies, and improve the efficiency in delivering power (power factor correction). The following are a few examples. I leave it to you to look up exactly how capacitors perform these functions, if you are interested.

1. Stored Energy-

Depending on the capacitance of the capacitor, the energy stored in a capacitor can be used in camera flashes and some audio equipment applications to name a few. In this regard the capacitor acts somewhat like a battery for relatively low power applications.

2. Frequency Filtration or Selection-

When a variable capacitor is used in parallel with an inductor (an LC circuit) it becomes part of a "tuning" circuit . When the resonant frequency of the LC circuit is tuned (by adjusting the capacitance) to match the incoming carrier signal from a radio station, that particular radio station can be heard. Capacitors are also used for filtering in UPS (Uninterruptible Power Supplies).

3. Power Factor Correction-

Power factor correction is the use of capacitors to reduce the reactive power component of an AC circuit (notably inductive reactance) thereby reducing the current and improving efficiency in delivering power. In the case of large motor loads, capacitors placed in parallel with the motor load(s) reduces the current needed to produce the same electrical power to the motor(s). Electrical utilities like this because it reduces the size of the supply conductors needed, and reduces $I^{2}R$ losses in the supply conductors.

Hope this helps.

  • $\begingroup$ @pasabaporaqui My error. I have it in the wrong grouping. Caps used for filtering in UPS not power. Corrected. Thanks. $\endgroup$ – Bob D Nov 25 '20 at 21:24
  • $\begingroup$ Their use in camera flashes is based on their ability to release stored charges in a shorter time than a battery does, I would add that detail there...adobe might wonder why not just use the battery instead. $\endgroup$ – Triatticus Nov 25 '20 at 22:46

Three useful things that capacitors do in everyday life:

  1. They can store electrical energy and release it quickly. Some devices require a high voltage to start them, then a much lowers voltage once they are running. An example is a fluorescent light. It would be very inefficient and expensive to build in a high voltage transformer that was only used for the first fraction of a second when the light was turned on. Instead the light has a capacitor (the round cylinder that needed replacing every few years in old fluorescent tubes) which stores electrical energy and then releases it in a short spike of high voltage.
  2. Working the other way round, a capacitor can also protect electrical equipment from voltage spikes from the mains. Put a capacitor close to where your equipment is connected to the mains and it will absorb the energy in a short voltage spike and smooth out the voltage.
  3. A capacitor can be used to build a circuit that has a specific resonant frequency - it has a low resistance to an input in which the voltage oscillates with a particular frequency, and a high resistance to inputs with any other frequency. This can be used to pick out one particular frequency from signal containing multiple frequencies. This allows your mobile phone to pick out the signal from a specific cell tower, and allows your laptop to distinguish your WiFi router from your neighbour’s.
  • $\begingroup$ About the fluorescent starter: it is not the stater the one who produces the kick, but the ballast (see en.wikipedia.org/wiki/Glow_switch_starter). In addition to the capacitor in the starter, a second one is usually added to correct the lamp power factor. Thus, all lamp has two capacitors, but none with the purpose of produce a high voltage trigger. $\endgroup$ – pasaba por aqui Nov 25 '20 at 20:54

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