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Homework Help: [Circuits] Calculating voltage and current in a circuit

  1. Jan 20, 2014 #1
    1. The problem statement, all variables and given/known data


    2. Relevant equations

    3. The attempt at a solution


    How does the author do the below?

    $$v_{ i }=2li=42V$$

    What equation is he/she using?
  2. jcsd
  3. Jan 20, 2014 #2


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    Staff: Mentor

    That should be 21i. 21 Ω is the parallel combination of the 70 Ω and 30 Ω resistors. The author is calculating the voltage drop across that pair.
  4. Feb 10, 2014 #3
    They seemed to have applied KCL at the middle node: $$i_1=i_2+I_0$$

    But what about the other side of the same node? It looks like they only looked at the left-side of that node. Is that acceptable?
  5. Feb 10, 2014 #4


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    Staff: Mentor

    The current Io takes care of whatever is happening on the other side; the node was partitioned in order to "get at" the current Io . If you want, think of the horizontal wire as a zero ohm resistor separating two nodes :smile:
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