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Homework Help: Two easy circuit-analysis problems

  1. Oct 15, 2007 #1
    First problem:

    1. The problem statement, all variables and given/known data

    I'm supposed to find the voltage across the 8k resistor in the following circuit:
    http://img79.imageshack.us/img79/4139/circuit1lk0.jpg [Broken]

    3. The attempt at a solution

    This just seems so mindboggling easy. My first thought is to use kirckhoffs current laws on all three nodes, giving me three equations with three unknowns.

    The problem is this always ends up giving me a 0 = 0 equation. I'm guessing this might be because the equation for the upper left node = 1, the down left node = 2 and the node on the right = 3, so you could say left node equations = right node equation. I don't know, at any rate I always get a 0 = 0 equation so there's no help there either.

    I've also tried using the node-voltage method, but this also turns out wrong every time. Admittedly this may be calculation errors on my side, but to some extent since I can't see the paths back to whatever node I chose as the reference node I've got a feeling it won't be correct anyway.

    The problem seems so easy, and I know for a fact it's supposed to be easy, but i'm somehow missing something essential here, any help would be greatly appreciated.

    Problem two:
    Nevermind this one, I figured it out =\

    1. The problem statement, all variables and given/known data
    I'm supposed to find the Thevénin equivalent voltage of the following circuit:
    http://img156.imageshack.us/img156/9977/circuit3ab6.jpg [Broken]

    Again this is a somewhat easy problem.
    I start by simplifying the voltage source to a power source:
    230v / 20 = 11.5 A in paralell with a 20 ohm resistance and a 30 ohm resistor, which together becomes a 12 ohm resistor.

    So now I've got this 11.5 A power source in paralell with a 12 ohm resistor in paralell with a 12 A power source.

    First question: Considering I can now splice these two power sources together, how come I need to subtract them from eachother when their current goes the same way? I know that's what I'm supposed to do, I'd just think it would be 12 + 11.5 instead of 12 - 11.5.

    Second question: Aren't I correct in my observation that since the 12A current is stronger than the 11.5 one, the current after I put these together as one current source will be 0.5 but pointed downwards on the left side?

    Anyway at the end I get a Tvh of -3.75, but it's not supposed to be negative. But I can't see how it's supposed to not become negative considering i'm almost positive that will be the direction of the current after adding the two sources up.

    This is probably because I can't figure out the exact mechanics on adding two current sources together. All I know is I'm supposed to use kirckhoff's current law to do it, but I've really got no idea how, I just take it for granted as a rule that I can.
    Last edited by a moderator: May 3, 2017
  2. jcsd
  3. Oct 15, 2007 #2
    Ok never mind problem two, I realize why now. 11.5 ampere goes down, 12 goes up, this means that the current across the 12 ohm resistor and the other 12 ohm resistor is a total of 0.5, this has to go upwards from the left current source because the 12 ampere current is headed upwards. Phew.

    Still stumped at the first question though. I'm gonna try the node voltage method one more time ;o.

    Edit: And don't ya know it, 5 minutes after I made this thread this one was right aswell. I get -12 V. I'd still like to ask though wether this is solvable in any easier way than using the node-voltage method? As I'm fairly sure it's supposed to be.
    Last edited: Oct 15, 2007
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