# Impedance

Discussion in 'Homework Help' started by thepipersson1980, Aug 23, 2014.

Aug 23, 2014
8
0
Hi I'm just trying to work out where i have gone wrong

R = 3.1 m Ohms
L = 280 mH
C = 150 uF
F = 120KHz
W = 2piF
Xl = WL
Xc = 1/WC

Z = Sqrt R^2+(Xl-Xc)^2

Now my answer according to my scientific calculator is 211115 ohms but I have been told the answer 211008 is more accurate.... please can you give me your thoughts

2. ### ericgibbs AAC Fanatic!

Jan 29, 2010
3,222
554
hi,
I make it.
Series Res&Ind&Cap:
R=.0031 XL=211115 XC=0.009
Complex: .0031 -j211115.0
Polar: 211115 Ohm 90.00 Deg

Ask the person who said its not accurate, why.?
E

Aug 23, 2014
8
0
LOL I would do, however it is my tutor and I don't want to appear cheeky as I only have one question left and i want him to pass me.... when I'm done though I will be asking that question!!

4. ### gneill New Member

Feb 7, 2014
9
5
Your tutor is likely using 3.14 as the value for pi; only two decimal places (or three significant figures) for the constant. But as a general principle you should, whenever possible, use more figures for known constants than are provided in your given values. This keeps the uncertainty in your calculated results dependent upon the uncertainties in your given (or measured values) rather than introducing more uncertainties with the constants.

So really, you did the appropriate thing using more figures for pi, so your result would be "more accurate" if the given values were absolutely accurate (no uncertainty in the values). But you cannot assume that the given values are perfect. That's why the concept of significant figures is taught.

How many significant figures for your result would be appropriate given the values for the components that you were given? Use several more digits for pi than are in your given values but express your results in scientific notation to the appropriate number of significant figures to match the data that you were given. This should satisfy everyone.

I suspect that your tutor was trying to remind you of this concept in a round about way.