Capacitive impedance

Discussion in 'General Electronics Chat' started by fosjoh41, Oct 7, 2013.

1. fosjoh41 Thread Starter New Member

Oct 7, 2013
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I am currently studying an electronics course and would very much appreciate guidance on the following question. Calculate capacitive impedance in the following circuit. Capacitor 200uf resistor 2ohm and 50mhz. The circuit is square. Capacitor is at the top, resistor on the right and the hertz reading at the bottom.

2. WBahn Moderator

Mar 31, 2012
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The very term "capacitive impedance" is a misnomer. The problem is therefore ambiguous because it isn't clear if it is asking for the impedance of the circuit or the capacitive reactance of the circuit.

Unfortunately, different authors, particularly of non-engineering texts but certainly not exclusive to non-engineering texts, are not very careful about the terms they use. So you need to look at the terms as the author this problem comes from and use those (and let us know what they are so that we can help you in a manner consistent with the problem).

And are you really sure that it is 50 millihertz?

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3. anhnha Active Member

Apr 19, 2012
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I think 50mhz is 50 megahertz not 50 millihertz.

4. WBahn Moderator

Mar 31, 2012
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Not necessarily. With a 200μF cap, the reactance at 50mHz is far more realistic than the reactance at 50MHz for anything resembling a real capacitor. I don't have anything like a good feel for this stuff, but I think that the self-resonance frequency of a cap this size is almost certainly going to be well below 1MHz and possibly below 10kHz. Above that, it actually looks more like an inductor.

But a specification of 50mHz is so unusual that, unless it is specifically that low for a reason, there is a good chance that the author was just being sloppy.

5. fosjoh41 Thread Starter New Member

Oct 7, 2013
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The question reads as follows: demonstrate how to calculate capacitive impedance using the following circuit. Capacitor is 200uf, resistor is 2M ohm and 50Hz. Does that make sense?

6. WBahn Moderator

Mar 31, 2012
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At the risk of pointing out the obvious, there is a HUGE difference between what you originally asked and what the actual problem asks. Can I assume that you do know the difference between "2ohm" versus "2M ohm" and "50mhz" versus "50Hz"? These aren't just "Oh, so what?" kinds of oversights.

Now, if you would, please look in your text and find what the text says regarding what "capacitive impedance" means.

7. studiot AAC Fanatic!

Nov 9, 2007
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There is yet one further piece of information missing.

fosjoh41 can you spot what it is?

8. THE_RB AAC Fanatic!

Feb 11, 2008
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Capacitive impedance = capacitive reactance (Xc). Just another (older?) way of saying it.

I can't see any information missing? He has all the information needed to calc the capacitive reactance.

9. #12 Expert

Nov 30, 2010
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Xc = 1/(2PiFC)

However, this will only work if you get your units correct. Megahertz? Millihertz? 2Megohms? 2 ohms? I gave you the formula. It's up to you to use it.

10. DerStrom8 Well-Known Member

Feb 20, 2011
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If he (or she) is looking for the capacitive reactance (which is the current, correct way of saying it), s/he should use the formula for capacitive reactance. If s/he is looking for the impedance, you have the real part, but still needs to find the capacitive reactance.

To the OP:

Just remember that Z = R + jX, where Z is the total impedance, R is the real part (resistance) and X is the imaginary part (the reactance). "j" is an imaginary number, you may also see it as an "i". Simply use the formula for Xc (capacitive reactance) and you have both answers.

Studiot, I think all the necessary information is there. Unless you're referring to the fact that s/he is missing a previous attempt

11. DerStrom8 Well-Known Member

Feb 20, 2011
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Quick note on that, C has to be in Farads, you can't just use the μF value you have now. You'll have to convert the units.

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12. WBahn Moderator

Mar 31, 2012
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Sure you can use μF. The prefix is part of the value just as the base unit is. What you can't do -- and which is what so many people do -- is ignore the units and crank just the numbers and then throw some units onto the result as an afterthought.

So as not to ruin the OP from being able to do their own cranking, let's consider the reactance of an 5mH inductor at 2MHz.

XL = ωL = 2πfL = 2π(2MHz)(5mH) = 6.26kΩ

You simply treat the units prefix as exactly what it is -- a factor that multiplies the numeric part of the value.

13. studiot AAC Fanatic!

Nov 9, 2007
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Well since some of our experts have missed it, I can't expect homework folks to see it.

No mention of whether the connection is series or parallel has been made.
Yet the OP must have studied this in DC circuitry, before moving on to AC.

14. WBahn Moderator

Mar 31, 2012
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It's series. That's the only reasonable interpretation I can make from the original post when he described the circuit as: "The circuit is square. Capacitor is at the top, resistor on the right and the hertz reading at the bottom."

15. studiot AAC Fanatic!

Nov 9, 2007
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I think it as important for the OP that he can distinguish and correctly report the difference between series and parallel as the difference between 50Mhz, 50mHz and 50Hz.

That was why I asked if he could spot it.

16. WBahn Moderator

Mar 31, 2012
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Okay. I understand your point. I don't know if the OP would be likely to get the hint since he would have been looking for information that was "missing" and not "present, but not well expressed". But now I think your point to him is pretty clear. I hope.

That's assuming, of course, that the OP is even still around. Lot of crickets over in that neck of the woods.

17. fosjoh41 Thread Starter New Member

Oct 7, 2013
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It's a series circuit and the value is Hz. The question is looking for capacitive impedance.

18. DerStrom8 Well-Known Member

Feb 20, 2011
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Do you mean capacitive reactance? Impedance is the whole thing (the 'real' resistance part and the 'imaginary' reactance part). The reactance is imaginary only, and is due to inductance or capacitance in the circuit.

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19. #12 Expert

Nov 30, 2010
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Thanks DS8. I have to look that up half the time because it just doesn't want to stick in my head.

20. DerStrom8 Well-Known Member

Feb 20, 2011
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Happy to help! It so happens I was just tutoring a student on this exact topic at the university where I work. Timing couldn't have been better

Matt