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Finding the voltage in a resistance

  1. Feb 27, 2014 #1
    1. The problem statement, all variables and given/known data
    Given the circuit in the image (with a current source of 2A and several resistors), what's the voltage in R3?
    rz_circuit.jpg
    http://postimg.org/image/yl6czh9u7/

    The answer options are:
    1.1V
    0.77V
    50V
    0.55V

    2. Relevant equations
    Ohm's law.
    V=RI

    3. The attempt at a solution
    I tried to simplify the circuit. The circuit equivalent resistor I found was
    (((((R6 || R7) + R5) || R8) + R4) || R3) + R1 + R2
    From my calculation the equivalent resistor is 32,5Ω
    And the voltage of the circuit is 65V
    If the voltage in R1 is 10V (2A x 5Ω) and in R2 is 30V (2A x 15Ω) in R3 it should be something like 25V... but that is none of the options!
    Thanks for any help.
     
    Last edited: Feb 27, 2014
  2. jcsd
  3. Feb 27, 2014 #2

    mfb

    User Avatar
    2016 Award

    Staff: Mentor

    I agree with your analysis and I got the same result.

    Here is a nice shortcut once you have the expression for the equivalent resistor: you have R3 and an equivalent resistance of 25 Ohm in parallel, so the 2 A will be split in two equal parts - R3 will get 1 A and therefore a voltage of 25 V.
     
  4. Feb 27, 2014 #3
    I tried to solve this problem hundreds of times using different approaches and the result was always 25Ω.
    Maybe the professor mistook when he chose the answer options...
     
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