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Basic Circuit Analysis

  1. Sep 26, 2014 #1
    Hi guys. Really elementary question here, I've been kicking myself for ages trying to figure this out, but I suppose everybody needs a little nudge sometimes.

    I have been staring at this circuit diagram:


    From this page:


    which is basically a rundown of Kirchoff's Voltage Law, and I have been wracking my brain for why voltage V3 is labeled (-) with respect to the power source and (+) with respect to ground.

    If I follow a current running from the positive terminal of the power source, then it splits up into the two branches to go through R2 or R3 and R4, respectively. The directionality of the voltage (to my intuition) should be the same for V3 as it is for V4. It is dropping voltage, not adding to it.

    Thanks for any help you can provide!
  2. jcsd
  3. Sep 27, 2014 #2
    I'm guessing it was done to teach a concept. I would never label it that way in real life.

    In this case, all it means is that V3 is probably going to be a negative number (and V4 will be positive).
  4. Sep 27, 2014 #3

    jim hardy

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    Second for post #2.
    When sliderules roamed the earth and i was being taught KVL , teacher made it a major point to write those little polarity + & - signs .
    He said emphatically: "If polarity is not intuitive by looking, just guess - if you guess wrong you'll get a negative number for that one. It only means you guessed wrong, your answer will be correct for the polarity you've shown. "
    That's why it is so important to be rigorous and neat in your algebra. No shortcuts or you'll start dropping signs.

    old jim
  5. Sep 27, 2014 #4
    Okay, so it's just for the sake of making a point. Thanks you guys!

    I just want to check that it's the same for KCL: pick your current directions, and if you're wrong, then you'll get a negative sign for that particular branch of current.
  6. Sep 27, 2014 #5
    Also, "when sliderules roamed the earth". lol.
  7. Sep 27, 2014 #6

    jim hardy

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    Yes, same for KCL.

    Meticulous attention to detail will pay off down the road. Form your good habits early.
  8. Sep 27, 2014 #7
    Thank you sirs, I will watch my signs with care.
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