Polarity with KVL

Discussion in 'Homework Help' started by yan500, Jul 17, 2011.

  1. yan500

    Thread Starter Member

    Jul 12, 2011
    48
    0
    Hi guys,

    Just a general question here: So how does one determine the polarity of resistors in a circuit when using KVL? Are there special rules?
     
  2. steveb

    Senior Member

    Jul 3, 2008
    2,433
    469
    Yes, there are special rules, although the rules depend on the current direction convention you use. The ebooks here use electron current as the convention for current direction, so consult this source for proper rules and definitions, if you prefer that method.

    If you use the positive current convention (which means the current direction arrow points in the direction of flow for positive charges, or opposite the direction of negative charges), then the arrow points from + to -, with the point of the arrow at the negative side.

    So, the basic answer to your question is that you determine polarity of resistors by first defining the current direction through each resistor with arrows. Once the arrow is defined, then the polarity is defined (with polarity depending on the current convention you are using). Note that you do not need to know the current direction beforehand. You can chose it arbitrarily, and if you happen to guess wrong, the equations will yield a negative sign for that current (and voltage). The negative sign simply means that the current is flowing opposite the direction of the arrow.
     
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  3. yan500

    Thread Starter Member

    Jul 12, 2011
    48
    0
    Thank you! That really clarified it for me.
     
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