# frequencies below resonance

Discussion in 'Homework Help' started by Steven5757, Jul 20, 2012.

1. ### Steven5757 Thread Starter New Member

Jul 2, 2012
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I have a question regarding: at all frequencies below resonance, a series RLC circuit has a net_______ reactance. Any assistance would be helpful.
Thank you Steven

Nov 4, 2008
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Capacitive

3. ### WBahn Moderator

Mar 31, 2012
23,201
6,993
Instead of just memorizing the answer, you want to understand the concepts well enough to reason out the answer. There are several ways to do this.

You could just brute force it and right down the equation for the equivalent reactance of a series RLC circuit and then see what happens as you lower the frequency below the resonant frequency.

But without doing any math you can get at the answer intuitively if you have a basic understanding of how inductors behave at the frequency extremes, namely that a capacitor looks like an open at DC and a short at very high (i.e., infinite) frequencies while an inductor is the opposite and looks like a short at DC and an open at very high frequencies. So, as you come down in frequency from the resonant frequency the inductor is looking more and more like a short. Well, it if were a short, then it would be the same as removing it from the circuit and just having a series RC circuit, so it must look like a net capacitive reactance.

4. ### BillB3857 AAC Fanatic!

Feb 28, 2009
2,461
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One of the most valuable lessons I learned when in tech school was to not try to remember details but have a full understanding of the principle of operation. With that understanding you can use "reasoning power" to get a grip on almost anything. Schools today don't seem to be teaching that concept. Sad day, indeed.\

edit: Another tidbit I learned along the way was the law of extremes. For the OP, take the frequency to 0 to determine the most reactive component.

Last edited: Jul 20, 2012
5. ### Papabravo Expert

Feb 24, 2006
11,749
2,483
Answer is obvious from a graph of Xc and Xl with respect to frequency.
So what happens above resonance?
That's for extra credit!!

6. ### WBahn Moderator

Mar 31, 2012
23,201
6,993
To the OP: What may not be evident from all the responses, is that the answer to this question can be seen by looking at it from many different perspectives. You should take the time to consider all of the ones mentioned because, in doing so, you add tools to your toolbox. Then, in the future, when faced with more subtle problem in which it may be difficult or impossible to approach it from some of these directions, you can consider all of those tools and as long as just one of them works, the problem is solvable.