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Homework Help: Choose a voltage V <2.5 V

  1. Sep 9, 2012 #1
    1.Choose a voltage v < 2.5 V and values for the resistors R1, R2, R3, and R4 in
    the circuit of Fig. 3.90 so that i1 =1 A, i2 =1.2 A, i3 =8 A, and i4 = 3.1 A.

    2. Relevant equations

    KCL/KVL/OHms Law

    3. The attempt at a solution

    If Is = I1 + I2 + I3 + I4,

    Then, Is = [itex]\frac{v}{Req}[/itex]

    Req = [itex]\frac{1}{R1}[/itex] + [itex]\frac{1}{R2}[/itex] + [itex]\frac{1}{R3}[/itex] + [itex]\frac{1}{R4}[/itex].

    After this, I get stuck...

    Attached Files:

  2. jcsd
  3. Sep 9, 2012 #2
    You're over thinking the problem. For parallel branches, the voltage is the same to all branches of the node. For simplicity choose 1V and for R1 choose 1 ohm so that I1 = 1A. Now you have the voltage and current for each branch so just solve for resistance via ohm's law.
  4. Sep 9, 2012 #3
    So because we can choose any v that is less than 2.5V, you picked 1V correct?
  5. Sep 9, 2012 #4
    Taking V as 1A and using the given values for the currents, I solved for R1, R2, R3, and R4:

    R1 = 1V/1A = 1Ω

    R2 = 1V/1.2A = 0.83Ω

    R3 = 1V/8A = 0.125Ω

    R4 = 1V/3.1A = 0.323Ω

    Is this correct?
  6. Sep 10, 2012 #5
    Looks good. And if for some reason you need the voltage to be higher than you can just scale the resistance proportionately to maintain the same current.
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