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Confused on how to find current! also why is this resistor labled +/- not -/+!

  1. Jan 16, 2006 #1
    Here is the diagram and work the answer booklet says:

    Okay first question, If you loook on the image and look at the Current Source Is, the arrow shows its going up and to the left, so that means its going to go through the 2.5 OHM resistor, but you can see the picture its labeled +^v^v - so that means its going to not have a voltage drop but an increase and isn't the rule it should always decrease with the flow of current? all the other resistors follow this rule in the diagram.

    Okay now to the real problem:
    I'm confused on how they found i2, which i'm assuming is the current flowing into Resistor with 2 OHMS, with voltage of v2.
    They say:
    v2 = v1.5/1.5 OHM - v14/15OHM = 6v/1.5 ohm - 14v/14 ohm = 3A

    Why are they only using v1.5 and v14 to find this? if u used KVL, wouldn't u sum up the middle loop and get:
    start at V2 going clock wise, -V2 - V4 + V14 = 0;
    so V2 = -V4 + V14
    but they arn't even using V4 instead they are using V1.5, but why? Thanks
  2. jcsd
  3. Jan 16, 2006 #2


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    Homework Helper

    1. The power supply (current source) is the symbol on the left + 20V - indicating that the current flows from the bottom to the top part of that circuit.

    2. The 4A that flows in that branch at the left of the circuit split ups at the branch point between the 1.5 ohm and 2 ohm resistor. The amount of current carrying on in the 2 ohm resistor is therefore 4A minus the current that branched off through the 14 ohm resistor.

    3. The diagram are using non standard symbols, so I'm not shure what the Is symbol represents, maybe an ammeter?
  4. Jan 16, 2006 #3


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    Staff: Mentor

    The sign for the voltages is arbitrary. If the +/- is shown one way, you will get a positive voltage for an answer, and if the +/- is flipped, you will get a negative voltage for the answer. The +/- just shows the way that the V(x) voltage was solved for.
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