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MESH Current, Voltage Polarities

  1. Dec 4, 2009 #1
    Hello,

    I'm trying to figure out MESH Analysis but for the life of me I can't seem to get the Voltage polarities on the resistors right. I usually like to assign my loop currents clockwise so I can get in some sort of a routine(I know it makes no difference). Can someone please help me explain the voltage polarities across resistors?

    Here is a DC example,
    http://www.art-sci.udel.edu/ghw/phys245/05S/examples/images/Mesh-ex1.gif

    Can someone please explain to me a method for choosing voltage polarities in these loops and give the MESH equations? I think I am just missing something basic, so a detailed explanation would be nice.
    Thanks
     
  2. jcsd
  3. Dec 4, 2009 #2

    berkeman

    User Avatar

    Staff: Mentor

    Please show us what you think they are first, and then we can comment. Them's the PF Rules.

    Welcome to the PF!
     
  4. Dec 4, 2009 #3
    Alright,

    Here`s how I would approach this problem,
    Left Loop : -9v + I1(5k) +(I1-I2)(6k)=0 (Clockwise Current Direction)
    Right Loop: -9v - (I1-I2)(6K) + I2(12K) = 0 (Clockwise Current Direction)

    I`m not sure if this is right or not, it`s what I came up with. I think my problem is I am not 100% confident in my methods which would lead me to mistakes on tougher questions. Anyways, I`d appreciate your input.
    Thanks again.
     
  5. Dec 5, 2009 #4
    The thing I'm getting stuck on is always hearing "The polarities within each loop for each resistor are determined by the assumed direction of current."
    Can someone explain this concept for me please?
     
  6. Dec 5, 2009 #5
    The part where current flows into to a resistor is more positive than where it leaves.

    In mesh analysis, assume that current flows in a loop in some direction.
     
  7. Dec 5, 2009 #6
    Ahhhhh. Thank you. Thank you soooo much. I knew it was something basic I was missing.
     
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