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Homework Help: Voltage at Vab, balancing, Increasing

  1. Sep 8, 2009 #1
    1. The problem statement, all variables and given/known data
    R1=350 Ohms
    R2=120 Ohms
    R3=124 Ohms
    R4=350 Ohms
    A is between R1 and R4, while B is between R2 and R3. SORRY FOR NOT HAVING IT IN THE PICTURE
    For the following circuit what is Vab? Now balance the circuit so Vab=0. After balancing what are the values of a shunt resistor which would increase Vab by 2.75% ( a shunt resistor is a resistor which is placed in parallel to another resistor in order to change its value.)

    2. Relevant equations
    Va=Vs R4(R1+R4)
    Vb=Vs R3/(R2+R3)

    3. The attempt at a solution

    So I basically have three parts to this problem. Find the value of Vab, then balance it, then increase it.

    for the the first part to find the value of Vab

    I just plug and chug for the above equations and got Va=2.5 V Vb=4.31 Vab=4.31-2.5=1.8 V that wasn't the hard part the hard part is that we didn't go over excatly how to do this but I found an example in the book but it just doesn't make since how to do the next two parts here is my work....if someone could do to things, tell me if I am right, and why! Because in physics its not really getting the right number it is why.


    Rx=R1*R3/R2=(350*124)/120=362 Ohms

    Rx=R1*R3/R4=124 Ohms

    RX=R2*R4/R3=339 Ohms

    is that all I have to do? I realized that R3=Rx would be the same as R2 so i didn't think i had to change it but then I dont know what to do after that if there is another step.

    THIRD PART increase Vab by 2.75%

    I have no idea it has work but it doesn't make sense where it comes from can someone explain to me how to do this part

  2. jcsd
  3. Sep 8, 2009 #2
    any help would be awesome!
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