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>> Hello there! Read this statement:
I've read it.
>>A capacitor is a reactance that changes with time.
This is a statement!.
>>How true or false is this??
This a question asking if the word 'this' is true or false!.
It can used as an 'adjective' or a 'pronoun', depending upon the context, its neither true or false.
>> The statement is as is: DON'T add even a word.
I did not add a word to your statement!.
A capacitor generally has a tolerance of ± 20%.
As temperatures increase, (powersupply for example) and decrease the value (µF) will shift. hence it's reactance.
Also the value will drift over time as the insulation value between the plates deterioates. This is most noticeable in electrolytics.
1 / (2*pi*ƒ*Xc) = µF ( if i'm correct.)
If µF value changes, reactance Xc changes as well.
The statement is FALSE. A capacitor has a reactance. Reactance is a function of frequency and capacitence, not time. Capacitence is considered constant over "short" time scales such as seconds, minutes, and hours. Over time periods such as years or decades the capacitence of a capacitor may change which will of course change the reactance according to the formula already mentioned.
It's a picky point but if Xc is in ohms and frequency is in Hertz then the capacitence will be in farads, not micro Farads.
My thoughts are exactly like Papabravo. I'm in a disagreement with an instructor!!
He put this statement in a test and it was a true or false question! I put false of course and I was marked wrong..........!!
His reasoning:
Xc= 1/2pi*f*c
and since f= 1/T then Xc changes with time!!
I told him this is wrong because BIG T is defined to be the period it takes the signal to complete one cycle. It is not small t we see in the AC graph.
I know what he means but he didn't know how to ask the question. You need to be really precise with such questions. He should say changes with frequency ......................or at least periodic time(although I don't like it this way either). You change the frequency in an AC source. You don't say I will change big T.
His statement clearly implies that Xc changes as time passes, and this is of course is incorrect.
You'll find that many instructors and teachers don't have much of a clue, and that many exam questions are misleading or just plain incorrect - unfortunately if your answer doesn't match what is in the answers used to mark the exam (almost certainly marked by someone with no electronics knowledge at all), then you get the question wrong.
There's an old saying, which I've never had cause to disbelieve "those who can, do, and those who can't, teach"
You'll find that many instructors and teachers don't have much of a clue, and that many exam questions are misleading or just plain incorrect - unfortunately if your answer doesn't match what is in the answers used to mark the exam (almost certainly marked by someone with no electronics knowledge at all), then you get the question wrong.
There's an old saying, which I've never had cause to disbelieve "those who can, do, and those who can't, teach"
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