Voltage Polarities

Discussion in 'Homework Help' started by SSkeeto, Oct 10, 2005.

  1. SSkeeto

    Thread Starter New Member

    Oct 10, 2005
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    Is there an easy way to know what the voltage polarity across a resistor must be? For example, in the circuit that I drew here: http://www.folderol.net/circuit.jpg

    How do I know where the plus and minus go on either side of the resistor R? I get different values of R depending on what the polarity is and the current directions that I choose...

    Also, can all of the currents that join at the top node be going away from that node? Or does at least one current have to be going toward the node?

    Thanks.
     
  2. Snow Asassin

    New Member

    Oct 10, 2005
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    The selection of polarity is arbitrary, it doewsn't matter.
    In response to your question about nodal analysis, no it also does not matter, if you choose either the sumer of the current entering the node=0, or sum of currents leaving=0, all that means is that you will have some currents which are negative (meaning they go in the opposite direction)
    Good Luck
     
  3. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    It only matters if you wish to know what VM1 or VM2 will read in the below circuit.

    After all, the circuit is an exercise in Kirchoff's laws.
     
  4. n9xv

    Senior Member

    Jan 18, 2005
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    In terms of current flow (electron flow - negative to positive), the current entering a resistance will always leave that point negative with respect to the current exiting that resistance. In the above examplem the unknown R is directly connected to the voltage source. That point in the circuit is obviously going to be positive.

    In the above attachment VM1 = VM2 = 120-Volts.
     
  5. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    True. Both meters in the example read 120, but VM2 reads negative 120.

    Obviously the experience of a person can overcome that easily, but, those coming here with some questions are inexperienced and polarity around the circuit would be important to their learning.
     
  6. n9xv

    Senior Member

    Jan 18, 2005
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    But,

    VM1 & VM2 are physically the same connection. How can one read positive and one read negative. In other words, you dont need both meters to indicate the same voltage. It's a parallel circuit at that point in the circuit. Current is not changing direction at that point in the circuit.
     
  7. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    You can get the voltage to read differently by where you position the leads. VM1 has the positive lead on top while VM2 has the positive lead on the bottom.

    I purposely choose the same potential to illustrate that point. I'm from the old school of going from negative to positive when I connect a DVM. That alone doesnt make my way the correct and proper way, it only has some semblence of order.

    Anyway, those are my thoughts on the comments from the arbitrary polarity issue.
     
  8. n9xv

    Senior Member

    Jan 18, 2005
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    I guess I did'nt realize that you were intentially reversing the meter leads on VM2. I was just trying to understand how a common voltage was changing polarity. I assumed it to be a DC voltage anyway.
     
  9. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    Yes, it was a trick question.

    Our original poster hasn't returned so I don't know if they marked the polarities on their circuit congruent with the original posting, from the more negative to the more positive potential.
     
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