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This is a bad question unless "misconceptions in learning" is a topic that people here would like. The topic of common misconceptions is pretty common here, and often appears in questions or answers. Feel free to close or delete, or whatever those with moderator privileges feel like, I already had the question resolved for myself. If so, maybe you could create the tag "learning-errors" and add it to this question, I can't because I lack 300 reputation.

I have learned about high-pass and low-pass RC filters. Initially, it seemed like loss of amplitude was inherent to AC passing a capacitor, in the same way that it is with phase shift. But this isn't the case, right? Amplitude loss is instead caused by combination of a resistor and capacitor. Is there no inherent loss of amplitude for AC passing through a capacitor in itself (as in a circuit with just a capacitor)? If anyone needs a schematic of the circuit just described, a circuit with just a capacitor, here it is:

enter image description here

(5 or so people actually requested that one be added because it was apparently impossible to understand otherwise. )

The context that the "inherent amplitude loss" meme was presented, was alongside an inherent effect that is actually true, the 90° phase shift inherent to capacitors when using AC. In AC, "the current leads the voltage by 90°", under some ideal conditions. I managed to pick up that the amplitude loss in RC filters and such, was also inherent to capacitor, which is false. To be clear on it, I asked here. As stated, this is a dumb question, because it is not about something that is right, but about something that is wrong. I pointed that out in the beginning of the question so that no one got taken by surprise on that.

enter image description here

My suggestion to this community, if this question is to be redeemed, is to add the tag "learning-errors", tag it with that. But I don't know if electricalengineering stack exchange wants to focus on that topic, I just know that it is often brought up in different questions and answers. Or leave it closed, who cares.

To give some credit to my claim that this might be a common "learning-error", I just saw the quote "reactive circuit elements always introduce phase shift in addition to amplitude effect" (source), and it sure seems to suggest capacitors, as a reactive element, would have an inherent amplitude effect, which they do not in a "pure capacitance" circuit if I understand right. So it is possible although maybe hard to see that it might be reasonable that beginners might interpret that in the way I did. Semantically, "always introduce amplitude effects" can easily be interpreted to mean what it actually means, and I asked because I wanted to be clear that this claim was false, which it is. There is no inherent amplitude loss for AC across a capacitor.

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    \$\begingroup\$ Please draw a schematic and show the signals you want to compare. \$\endgroup\$
    – Eugene Sh.
    Nov 10 at 19:13
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    \$\begingroup\$ If I understand what you're asking, you're looking for what a capacitor does in isolation... but if all you have is a capacitor, hanging off the end of a signal generator (or even connected across a signal generator), you aren't going to have any amplitude change. However, a resistor would have the same properties in that configuration.... \$\endgroup\$
    – W5VO
    Nov 10 at 19:18
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    \$\begingroup\$ What do you mean by "a circuit with just a capacitor"? What do you mean by "AC passing a capacitor"? What do you mean by "combination of resistor and capacitor"? You really need to add some schematics, because your language is very vague. \$\endgroup\$ Nov 10 at 19:34
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    \$\begingroup\$ What do you mean by "amplitude loss"? There is a voltage across the capacitor. There is also current through it. But "amplitude loss" is an insufficiently clear term. \$\endgroup\$ Nov 10 at 19:35
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    \$\begingroup\$ Sorry, the schematic is not understood anyway. Please add one. \$\endgroup\$
    – winny
    Nov 10 at 19:38
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A schematic is better than words - no matter how simple it is.

schematic

simulate this circuit – Schematic created using CircuitLab

Figure 1. As far as V1 is concerned the loads presented by C1-R1 and R2-C2 are identical.

The circuit of R2-C2 should make it intuitively obvious that there will be a phase shift due to the charging of C2 via R2 and that it will lag V1.

schematic

simulate this circuit

*Figure 2. If the circuit is not loaded (or a very high impedance load is connected such as a voltmeter) then no significant current will flow and the output voltage will be in phase with the source, there will be no amplitude drop and the output voltage will be the same as V1.

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  • \$\begingroup\$ What I take with me is that there is no amplitude loss with a capacitor in itself. Some other thing I thought about, the " phase shift due to the charging of C2 via R2", is it the same as the 90° phase shift from current flowing mostly when capacitor charges and discharges, and not at the voltage peaks of the AC current? Or, are there two different components to the phase shift that is usually talked about? I thought about that for days now, can't entirely say what is what. \$\endgroup\$
    – Ohm
    Nov 10 at 20:26
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Components which have two terminals, and which are linear have an impedance. The impedance is the (phasor) ratio between the current through the component and the voltage drop across the component.

Capacitors, like resistors and inductors, have impedances, and those impedances depend upon frequencies.

Networks with three or more terminals can have properties that are more complex than impedances. For example, the network enclosed in the box below has three terminals, and it makes sense to speak of the "amplitude loss" between the input voltage (relative to ground) and the output voltage (relative to ground).

schematic

simulate this circuit – Schematic created using CircuitLab

What you may mean by amplitude loss in the case of a two-terminal component, I do not know. Amplitude loss implies that there are two amplitudes being related. What are these two amplitudes? That question needs to be answered before one can meaningfully answer the question of whether or not capacitors have "amplitude loss".

You could mean that an (ideal) capacitor does not convert electrical power into heat. If that is what you mean, then that is true. A resistor converts electrical power into heat, but an ideal capacitor does not. (Nor does an ideal inductor). For this reason, the impedance of these components is called reactive rather than resistive.

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  • \$\begingroup\$ I get that capacitors impede current and that is is frequency dependent, lower impedance at higher frequencies. And that the circuit you show is a low-pass RC filter, because there is less pressure drop over the resistor when the impedance of C1 is high at low frequencies. It was just that I had capacitors explained to me (based on my own limited understanding) in a way that made it seem like they had an inherent amplitude loss effect. Learning wrong is usually a bad idea, so, I just asked to clarify that it was wrong. \$\endgroup\$
    – Ohm
    Nov 10 at 20:22
  • \$\begingroup\$ Again, what would be the meaning of "amplitude loss" for a two terminal component? Amplitude loss implies that there are two amplitudes being related. What are these two amplitudes? \$\endgroup\$ Nov 10 at 20:24
  • \$\begingroup\$ I'm saying that the concept of "inherent amplitude loss" probably makes no sense because it is probably false. I either misunderstood, or had it explained, wrongly, and when I looked more at the details I noticed that. "Misconceptions in learning" questions might be better fit elsewhere, although that is a pretty common topic on this forum too, I see it mentioned in lots of questions or answers. \$\endgroup\$
    – Ohm
    Nov 10 at 20:31
  • \$\begingroup\$ (I think how I phrased the question it is clear it is a "misconception in learning" question, but I get if that is not the type of questions that should be asked here. ) \$\endgroup\$
    – Ohm
    Nov 10 at 20:32
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    \$\begingroup\$ In this case, a schematic would have helped clarify, whether you believe so or not. \$\endgroup\$ Nov 10 at 21:11
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It depends how you want to think of it.

If you have a high pass RC filter, and feed it high frequency sine wave, impedance of capacitor is near zero so there is no voltage loss over capacitor since it is so small compared to resistor.

If you feed the same filter with low frequency sine wave, impedance of capacitor is not zero so it is a voltage divider with the resistor so amplitude is smaller as there will be voltage drop over the capacitor.

Another way to look at is in time domain. High frequency sine wave is so fast changing that capacitor does not have much time to build up charge and thus accumulate voltage over it. Low frequency sine wave changes so slowly that capacitor has time to charge and accumulate voltage over it.

On the other hand, if you put an ideal AC source over an ideal capacitance, the voltage of the capacitance is always at same voltage and phase with the source, it's just that neither exist in the real world.

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  • \$\begingroup\$ Yes I understand that, the hi pass / low pass RC filters. It took me a few days to get them, but once I got it it is really easy. But what I wanted to clear up is that I got the impression that some amplitude loss was caused by the capacitor itself, specifically. This seems to be false, but I entirely get how capacitors combined with resistors can produce amplitude loss on output signal. \$\endgroup\$
    – Ohm
    Nov 10 at 19:55

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