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When is this ever true?

  1. Jul 23, 2013 #1
    I've seen this simple circuit design many times when the text is illustrating different points. Sometimes, again for illustrative purposes, the text may say that I3 is negative. When would this ever happen? If I3 were negative, the node between components 3 & 4 would have to be lower than ground, and, further, the red extraordinary node would have to be lower yet. It seems intuitive to me to conclude that the voltage at the node in red will never be at a lower potential than ground in this design. If I3 is negative, it would seem obvious to me that I2 must also be negative (with polarity defined as is it) - but when could this be the case?

    Perhaps I3 could be negative if different node is selected as the reference, but as it stands I cannot see a way that this claim would ever be true.

    ciHq6ix.jpg
     
  2. jcsd
  3. Jul 23, 2013 #2

    psparky

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    IN my mind, I3 can never be negative in this circuit.

    Lets say the battery is 20 volts and 10 volts drop across that first resistor. YOu are then left with 10 volts at the red node. No matter how you slice it, current is going to travel in the shown direction just because that voltage will always be higher than the negative side of the battery.

    You could say that I3 is negative if the arrow pointed the other way, but that's it.
     
  4. Jul 23, 2013 #3
    Eactly. Although, I did not intend to post this picture implying the elements are resistors. I had forgotten that european resistor symbols are rectangles.
     
  5. Jul 23, 2013 #4

    psparky

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    Doesnt matter, I just pictured them as loads.

    You might be able to stop the current with a capacitor or diode, but the current cant reverse direction in I3.
     
  6. Jul 23, 2013 #5
    Unless they're using electron flow for current.
     
  7. Jul 23, 2013 #6
    If you are doing a node analysis for the red dot - it is ( may be ) simpler to have all of the currents flowing into - or out of the red dot - EG I1+I2+I3 = 0A ...... so you may draw the arrows how you like.

    My point being - that the issue of positive or negative is a matter of perspective.
     
  8. Jul 23, 2013 #7

    Redbelly98

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    Is there any restriction on what they could be? If element 4 is a voltage source, a sufficiently charged capacitor, or a current source, it is possible for i3 to be negative.
     
  9. Jul 23, 2013 #8

    jim hardy

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    My guess would be it is simply to demonstrate that, even if you initially assume I3 in wrong direction, Kirchoff's principles will cause the arithmetic solution give you the correct answer - a negative number for I3.

    My high school electronics teacher pounded this point into us boys so we'd develop rigor (and faith) in our math .

    old jim
     
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