# Simultaneous Equations: Question 20

Discussion in 'General Electronics Chat' started by HadMatter, Oct 27, 2009.

1. ### HadMatter Thread Starter New Member

Aug 16, 2009
9
4
Hi guys. First time caller here. Before getting into my question I just want to say how much I love both the philosophy and content of this site. A new human age of information and technology is dawning - in the way of how we learn and share - and you guys are leading the charge. New realms of understanding and possibilities are opening up before me, and for that I thank you. ... I could gush and kiss ass all day, but let's move on.

Ok, if it isn't clear from the thread topic, I'm having trouble with Question 20 of the Simultaneous Equations worksheet. My grasp on the algebraic concept of solving systems of equations is strong, but time and time again I come up with resistor values different from the answers given. I'm plugging in the same approach I used successfully in questions 15 - 17, so I'm thinking maybe my understanding of how the use of a rheostat affects voltage is lacking - by which I mean the fact that R2 is connected to the circuit both as a fixed resistor and by the variable wiper. It seems like the electricity would follow "path of least resistance," but is there some crazy double voltage / parallel voodoo at work here? I'd love to think that the given answers are wrong, but since I didn't find any other posts on this, it seems unlikely [and all the more personally deflating =( ] Here are the equations I'm using:

Given that R = Rtotal (Vr / Vtotal).....

R1 = (R1 + R2 + 10K) (30/100)
&
R2 = (R1 + R2 + 10K) (40/100)

I'd appreciate any insight you guys could offer. I'm just going in circles at this point.

2. ### HadMatter Thread Starter New Member

Aug 16, 2009
9
4
No wait, I take it back!!!

UNPOST!! UNPOST!!

Geez, no sooner than I press the 'post thread' button..... I figured it out. My problem was that while my equation for the fixed R1 was set up in a manner to represent the absence of any affect from R2, I was including R2 as part of total circuit resistance. It should have been:

R1 = (R1 + 10K) (30 / 100)

And, man, done properly it was a lot easier to work through - a lot fewer steps. Sorry for the false alarm, folks. Know that I didn't just post at the first sign of difficulty. Sadly, I've been puzzling over this for a few days now. I was just really focused on that weird 3-way R2 connection - trying to figure out how that might affect things differently. Maybe, in the end, I just needed to talk it out. Next time I'll try discussing it with an inanimate object or something first.

Moderators feel free to delete this thread -- hide my shame.

3. ### The Electrician AAC Fanatic!

Oct 9, 2007
2,666
473
There's no shame in such an experience. I can tell you from my own experience that beating your head against a wall for a while is the way the learning gets pounded into your head. We've all done it.

Those who give up at the first sign of difficulty won't learn what you've learned.

You won't forget what you've learned, will you?