# Help calculating impedance in RLC circuits please!

Discussion in 'Homework Help' started by cfinley, Jul 21, 2012.

1. ### cfinley Thread Starter New Member

Jul 21, 2012
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0
Dear all,

Having some issues with coursework. I've attached a circuit with some workings, (just ignore these), and I'm trying to calculate the impedance with little success. If anyone can get their head around the circuit I'd appreciate the help. I believe the equation I need to use is the following:

Z=√R2+(Xl - Xc)2

If this is the case and if anyone is able to show me how to substitute the data and calculate Z?

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2. ### panic mode Senior Member

Oct 10, 2011
1,328
305
the equation is correct (to find MAGNITUDE of impedance).

Impedances are
R=R (obviously)
Xl=jωL
Xc= 1/(jωC)

where
ω=2πf

and f is frequency of the AC source in Hertz.
when using your equation, only take magnitudes of each value.
for example

|Xl|=ωL
|Xc|= 1/(ωC)

also make sure to use PROPER units if you want to have ANY hope of ever getting this right.

for example:

k=kilo=x1000
M=mega=x1000000

m=milli =/1000
μ = micro = /1000000

R=2700000 Ω (=2.7 x 1000000 )
C= 0.000300 F (=300 / 1000000 )
L= 0.3 H (=300 / 1000 )

Last edited: Jul 21, 2012
3. ### cfinley Thread Starter New Member

Jul 21, 2012
5
0
Thanks so much for your help and a lot of that makes total sense,(especially the quantifying of units!) However I'm probably being a total numpty but even with your useful data i'm still struggling to find 'Z'-don't I need to calculate 'X' also in addition to the R,C and L you've been kind enough to help with!?

4. ### WBahn Moderator

Mar 31, 2012
18,079
4,917
He provided you with the relations needed to calculate Xl and Xc from L and C near the top of his response.

5. ### panic mode Senior Member

Oct 10, 2011
1,328
305
There is no 'X', it is 'Xl' for inductor and 'Xc' for capacitor (subscript indicates type of device and L=inductor, C=capacitor; i just used lowercase).

you just need frequency, everything else is there, just plug in values...
calculate ω
get R (given)
calculate |Xl|
calculate |Xc|
apply equation with root of squares...

that will give you magnitude (only) of the impedance Z.

Last edited: Jul 21, 2012
6. ### cfinley Thread Starter New Member

Jul 21, 2012
5
0
Ok I think I'm getting there...slowly! Only problem i'm not too sure about now is now is how do I go about calculating the frequency,(as this is not given in the question)? Also what does ω denote? Is this the frequency? Once I've nailed this frequency business I'm sure I'll be there!

Ps Sorry, I'm new to all this!

7. ### cfinley Thread Starter New Member

Jul 21, 2012
5
0

ω=2πf

so does not quantify frequency. However in order to calculate ω do I need to know the frequency of the circuit as this is not given in the question. Do I have all the data provided in the question in order to calculate this myself? In addition to this would you mind helping me out with the "π" also, ie:

ω = 2 x ? x f (frequency I do not have!)

Many thanks, (once again!)

Apr 5, 2008
15,796
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Last edited: Jul 22, 2012
9. ### WBahn Moderator

Mar 31, 2012
18,079
4,917
The 'π' is this font's (rather pathetic) rendering of pi (3.14159....).

What is it that you are being asked to do in this problem? If all that has been given is what's in that diagram and you are asked to find the impedance, then you will have to find it as a function of the unspecified frequency.

10. ### panic mode Senior Member

Oct 10, 2011
1,328
305
yes, pi=3.14159...
and yes, you need to know frequency...
f is frequency in Hz or 1/s
ω is angular frequency in rad/s

11. ### WBahn Moderator

Mar 31, 2012
18,079
4,917
f is in Hz or cycles/s or periods/s (the 'cycles' is part of the unit).

ω is angular frequency in rad/s or 1/s (the 'rad', while descriptive, is ultimately dimensionless).

12. ### ochoya New Member

Jul 27, 2012
7
0
think on the line of a standard frequency value like 50Hz american i think is 60Hz,i dont know if it make sense.new member