# LC Waveform question

Discussion in 'Homework Help' started by Corey Esson, Feb 28, 2015.

1. ### Corey Esson Thread Starter Member

Feb 8, 2015
30
1
Hi all, I am having some trouble with a question here.

I have a waveform that Voltage is leading current by 45 degrees. The question goes as such:

If the value of the reactive component (I think this means the inductor) was increased, how would it change the waveform?

A. The phase shift between the current and the voltage would reverse.
B. The phase shift between the current and the voltage would remain the same. (I know this is wrong)
C. The phase shift between the current and the voltage would increase.
D. The phase shift between the current and the voltage would decrease.

Correct me if I'm wrong here but, this would not be a resonant circuit because the phases are not equal, so I'm pretty sure that A is not the right answer because then the voltage would be lagging the current and this is a big shift?
I think, and am pretty sure that it is C because with all the pounding of the text book and the internet it would make sense that the the phase shift would increase to around 90 degrees out of phase?

This is my reasoning and hope and am confident that this is right.
Thank you very much in advance for any and all answers as well as any extra tutelage.

Esson

2. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
783
Is this a parallel or series circuit?
Also if it's an LC circuit, either element can be regarded as having reactance. Which of the two is then changing?

3. ### Corey Esson Thread Starter Member

Feb 8, 2015
30
1
Hi t_n_k, it doesn't say whether its a parallel or series circuit. What I typed is verbatim on my lesson. The whole lesson deals with both though. I assume if it doesn't say, then it could be either? It also could be a RLC circuit also?
I also just assume that the inductor is what is changing?
Thank you!

4. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
783
If the total circuit impedance produces a 45 degree lag in current wrt voltage then one must expect that there is some resistance involved. For a series RLC case then the inductive reactance must be dominating the circuit response. So you would be correct in choosing option C if the inductive reactance increased.
If the capacitive reactance increased then you would have a different choice.

5. ### Corey Esson Thread Starter Member

Feb 8, 2015
30
1
Thank you very much t_n_k! Glad I at least thought I was right and someone confirmed it. Again, thank you!

Corey Esson

6. ### WBahn Moderator

Mar 31, 2012
18,079
4,917
The question doesn't seem to indicate that this is even an LC circuit, that is only indicated by the thread title -- why do you think that it is? It seems like it could be just an RC or RL circuit with the expectation that you determine which it is based on the leading/lagging relationship given. That it is not an LC circuit is hinted pretty strongly by the problem stating that "the" reactive component, implying a single reactive component whereas an LC circuit has at least two reactive components.

7. ### Corey Esson Thread Starter Member

Feb 8, 2015
30
1
WBahn, thank you very much for chimming in.
All I know, is that the lesson is on resonant circuits so I just assume that it was an RLC or LC circuit? The good thing is that I submitted the lesson online and C was the correct answer.
I surely thank and appreciate yours and t_n_k responses and tutiledge.
Corey