Negative Currents and direction???

Discussion in 'Homework Help' started by hunterage2000, May 13, 2010.

  1. hunterage2000

    Thread Starter Active Member

    May 2, 2010
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    When I think Im making progress I fall upon another problem I dont understand. I really would like to know why the currents through R1 and R2 are negatige and R3 is positive. Without the ground the numbers add up and I dont really understand what ground is for! Can someone shed some light on the subject please
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
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    Look at voltage drops across the individual resistors to see the direction of the currents.
     
  3. hunterage2000

    Thread Starter Active Member

    May 2, 2010
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    Not sure what you mean Ive got V1 = 24 V2 = 4 V3 = 4 Where does the direction come from because I thought it flowed like it does in the pic here.
     
  4. beenthere

    Retired Moderator

    Apr 20, 2004
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    It probably does. You have the polarity of the voltage source as a guide for that. Measuring current is a relative process, but it's hard to see how one resistor could have a current backwards to the others.
     
  5. Ghar

    Active Member

    Mar 8, 2010
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    The directions you drew are correct.

    A negative sign just means the calculation assumed a different direction. In this case the numbers are not consistent, because like I said your drawing is correct.

    How did you get those numbers exactly?

    The numbers are valid if you defined the currents in a specific way.
     
  6. sgardner025

    Active Member

    Nov 5, 2009
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    Yeah you have to pay attention to how the ammeter is connected in the program.
     
  7. someonesdad

    Senior Member

    Jul 7, 2009
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    Sum the currents into the top node. Is there a problem? (Think conservation of charge.)
     
  8. hunterage2000

    Thread Starter Active Member

    May 2, 2010
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    Yeah Ghar, I used LT Spice to draw the circuit and those are the values it gave. But on paper all calculations were positive?? Thats why I was wondering why 2 of the values are neg and 1 pos.
     
  9. Ghar

    Active Member

    Mar 8, 2010
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    I think LT Spice lets you pick which direction you choose as positive, there's a little arrow that shows up when you go to click on the element.
     
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