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Higher Potential with reference point

  1. May 30, 2017 #1
    • Thread moved from the technical forums, so no Homework Template is shown
    Reference point was chosen to be at Va. I'm trying to understand the logic behind it.

    1. It is clear that Vc = - Vs because if Vc was the reference point at Va the voltage potential between Va and the reference point Vc is Vs. Therefore, Va = Vs, Va - Vc = Vs if Vc is reference. However, when Vc is the reference point, don't we have Vc -Va = -Vs? Va is reference so 0 volts which mean Vc = -Vs.

    2. At the Ohm law at IR1. If Va is the reference point which mean it is zero voltage. All the nodes are using it as a reference to ground and to my understand the voltage at the node is based on the higher potential - the lower potential. If that's the case, how can the higher potential be the reference point when it was given a value of zero with respect to all other nodes??
     

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  3. May 31, 2017 #2

    BvU

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    Post in homework. Use the template. What's the problem statement ?

    Va - Vc = Vs and Vc -Va = -Vs are exactly the same, identical and equivalent statement(s).

    "the voltage at the node is based on" No: the voltage at the node is the potential.
     
  4. May 31, 2017 #3
    Its not homework but I'm trying to refresh on my circuit analysis so I googled examples. As i was following the steps, I couldn't see the node reference is being the higher potential. I thought reference point is zero volts potential.
     
  5. May 31, 2017 #4

    jim hardy

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    That example is an awful thing to do to a beginner.

    Simplify your thinking. And use the simple rule for writing KVL equations. When you're just starting out and try to do them in your head you make mistakes.

    It matters not a whit which point you choose for reference, Kirchoff will give you the same answer.

    Put your pencil down on your reference point. Think of it as home plate.
    Now imagine yourself very small and walking around the circuit inside the wires
    Write down each voltage you traverse with the first polarity sign you encounter.
    When you get back to home plate write " = 0" . That's KVL for the path you chose.

    I think what's confusing you is the fact that voltage is a potential DIFFERENCE BETWEEN TWO POINTS .
    That's why voltmeters have two wires, the meter reports the difference between them.

    Here's how to approach that problem from the very basics.
    nodechoice1.jpg

    Here's the shortcut he threw at you before you were ready for it.
    nodechoice2.jpg
    And he's very sloppy about his labeling which i think further confused you. That's the trouble with internet sources.

    Can you label the current through R2 and write KVL around loop home base >>vb>>I2R2>>Vs >>home base ?

    I dont like his lack of rigor. He should have labeled his currents I1 and I2. Speaking aloud the names he chose makes you think it's product of I and R which gives voltage. Sloppy, sloppy work that confuses beginners .

    Good luck in your studies.

    old jim
     
    Last edited: May 31, 2017
  6. Jun 4, 2017 #5
     
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