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I'm trying to better understand the concept of active and passive components, mainly with respect to capacitors and batteries.

I found that - according to Wikipedia - there are two definitions of passivity, one that focus on the inability of have power gain, and one that looks at the inability to generate power.

In this answer the answerer says that it depends on the linearity of the controlling function, and the subsequent ability to control current and/or voltage, so a diode is active because of its ability to control current (while it would be passive if we look at its ability to generate power).

With all this in mind, I was wondering about capacitors and batteries: capacitors are passive elements, while I have seen batteries categorized as active, as direct generators; but capacitors can be used to power a circuit, so I was trying to understand the difference between them: are batteries active? If so, can capacitors used as batteries be seen as active? If not, what's the difference?

Mauro
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    I wouldn't worry about this kind of classification too much since it seems to be for teaching purposes but doesn't actually matter in real life or even in theory. Because when working with a component in theory or practice you will work with it according to the properties and behaviour of component itself, not whether it's classified as active or passive so it doesn't matter. I have *never* used a component thinking "this is a passive component so it must be treated this way". Whether you call a circuit as a whole active or passive (i.e. requires an extra energy source) matters much more. – DKNguyen Mar 21 '22 at 15:10
  • In AC theory, components always have passive and reactive elements. Research [AC theory](http://www.ibiblio.org/kuphaldt/electricCircuits/AC/index.html) for more info. – rdtsc Mar 21 '22 at 16:07
  • @DKNguyen I started trying to understand it when I was asked about explain the difference and I was trying to explain it - it's kinda of a teaching issue: I intuitively understand about active and passive circuits, but when asked why a capacitor used as source is passive and a battery is active I wasn't really able to answer. – Mauro Mar 24 '22 at 12:24
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    @Mauro Well you could add reactive with the other two. I've never thought about it that way or seen anyone do that until rdtsc just now but it seems like it would help you. Much more prevalent in the biological sciences, I think this saying also applies here: Ultimately, things are what they are no matter how you choose to classify them. It's okay to have classifications that paint broadstrokes but aren't so well defined at the boundaries, as long as the classification is useful. – DKNguyen Mar 24 '22 at 14:07

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