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Voltcom9

Anyone know anything about RLC circuits?

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Anyone able to answer the following question? It relates to a series RLC circuit. For some odd reason all my math work seems to match aside from my inductance of the inductor, which is way out. Not sure if anyone on here would know but hell I'll ask anyways.


Here's the question:

A resistor has a voltage of 25V, an inductor has a voltage of 12V, and a capacitor has a voltage of 45v. If the total impedance is .5ohms find: ET, angle, leading or lagging, and the inductance of the inductor.

If you are familiar with RLC circuits let me know what answers you got. Thanks.




My math results showed:

ET as 41.4v
Angle (25/33)tan-1 = 37 degrees
It was Leading (as it was more capacitive)
Inductance was 384.4uF

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There certainly is a big problem if you found an inductance measured in uF.

Also, you must be leaving something out of your problem. Since you're not given the frequency, you must be only be able to deduce it at the very end. That "impedance" value you cite, is it just the magnitude |Z| or the actual value of Z, containing only a real part?

Edit: are you supposed to find an actual numerical value or just a binded relationship?

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Thanks for checking this out. The question never provided a frequency. It was asking what the inductance of the inductor would be. I am guessing the frequency would be 60Hz but it is not specified.
Yes the Z value is the total impedance of the circuit.
I am looking for the specified inductance. For some reason this question doesn't seem to jive with me.

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There are just the 3 components in this series circuit. The resistor, the inductor and then the capacitor in the AC circuit.

I got my "Z" impedance by using the square root of R^2+(XL-XC)^2. I must have punched the numbers in wrong.

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I'm not certain how you can calculate precise values for the impedance or capacitance without knowing the frequency -unless finding it is part of the problem.

Without it, at most you can find a relationship between L and C:

LC = V_L/V_C= 12/45
You may also find ET (magnitude) and using the phase angle, also find I_real= 50A and I_imm=-66 A. This gives you R=2 Ohm, |X_C|= 45/66 Ohm and |X_L|= 12/66 Ohm.

However without the frequency, as I said, you can't actually find the impedance value. With an indeterminate frequency (and without the circuit being in resonance), there are infinite triplets of L/C/ω that would satisfy the conditions.

If it's really 60 Hz though, then X_L= ωL => L= 31.8 mH and X_C=1/ωC => C= 3.5 mF

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