question about my calculations

[dak246]

Im studying up on basic electronics theory because im still kind of new to all of this but now im stuck. I cant seem to get to the right answers for this practice problem:

Given that E=10V rms, f=15kHz, R=8.2k, L=40mH, and C=0.004uF;

Find the values of IR, IL, IC, IX, IS, ZT and the phase angle; the second letters of each should be read as subscripts.

I am getting the following;
IR=1.2mA, IL=2.7mA, IC=3.8mA, IX=1.1mA, IS=1.7mA and ZT=6046 Ohms

In getting those values i had to also get XL=3769.9 Ohms
XC=2652.6 Ohms and XT=8950.2 Ohms

Supposedly the answers should be as follows;
IR=1.22mA, IL=3.62mA, IC=2.76mA, IX=0.855mA, IS=1.49mA and ZT=6710 Ohms and the phase angle should be +35 degs.

Also since according to my calculations XL > XC, shouldnt the phase angle be negative because the source current will be leading the source voltage?

Thanks for any help.

 

[Roff]

Can you tell us how these components are interconnected, i.e., provide a schematic?

 

[dak246]

Its a simple circuit, consisting of one inductor, one capacitor and one resistor, each parallel to each other and connected across an alternating voltage source.

 

[andy257]

Hi there, ive give it a go but its been a while since ive done this.

**broken link removed**

I get:

XL = 3769.91 ohms
XC = 2652.58 ohms

IR = 1.22mA
IL = 2.65mA
IC = 3.77mA

Is = 1.65mA

Z = 6060 ohms

(sorry i am not familiar with XT?) is that total reactance? i would need to know the resistance of the coil to go any further.

The phase angle i get is

ø = TAN -1 (IL - IC) / IR = -42.55 deg


I am no professor in this field but i have give it my best shot with the information you have provided. Perhaps one of the mods can double check this. A word of warning, teachers etc can make mistakes also, your expected results could be wrong. I know this sounds a little stupid but it has happened to me before. You spend alot of time trying to find their answer but it is impossible because it isnt the correct answer in the first place.

**broken link removed**

p.s the answers have been derived using calculations to 2.d.p

hope it helps

andy

 

[dak246]

The resistance of the coil is assumed to be zero.

You are getting exactly what im getting. This makes me think that maybe the solution the book provided is wrong, especially since im almost positive the phase angle should be negative. The only thing is that im actually a finance major and i just do this as a hobby, so im kind of reluctant to think that im right and the solution given is wrong. Thank you for taking the time to help me.

 

[zevon8]

Never assume the text book is correct at all times. I remember my basic electricity text that had examples for Ohm's Law involving a toaster that would have made stale bread, but never have got warm enough to singe hair. Also a transmission theory text where someone dropped a decimal place and it would have taken a nuclear reactor generating station to power one of the RF amplifiers they gave as an example.