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Homework Help: Electrical Question- Please help me

  1. Jul 30, 2010 #1
    1. The problem statement, all variables and given/known data

    A bridge initially has resistances equal to R1=100ohms, R2= 200ohms, R3= 250ohms, R4= 300ohms. Determine output voltage at this condition for an input voltage of 3V. What is the bridge output if R1 changes to 125ohms?

    2. Relevant equations

    Havent been given any. I am assuming something to do with Ohms Law?

    3. The attempt at a solution

    I used Ohms Law. But I dont know if I am on the right track.

    I= E/ R
    I= 3/ 850 = 0.00353 amps.

    Now I dont know what to do. Or if I was even supposed to use this Law.

    Any help would be much appreciated :) Thanks alot.
  2. jcsd
  3. Jul 30, 2010 #2
    I'm actually just learning about them myself so I could be wrong but I may be able to help. Firstly the equivalent resistance of parallel resistors isn't just the sum of resistances so that doesn't work because in a bridge you usually have two banks of resistors that are in series connected in parallel which is in turn connected to some sort of measuring device (galvanometer) at a point in between them.

    So across that measuring device, you will have a positive and negative terminal and you want the potential difference between them

    So, voltage across top of the circuit (current flows in direction of positive charge) = V = 3v

    So the voltage in the (+) terminal is = V x (R2/ R(eq) of R1 and R2 in serial)
    voltage in the (-) terminal is = V x (R4 / R(eq) of R3 and R4), just find the difference between them and I think that should be it.
  4. Aug 1, 2010 #3
    Thanks for your help!

    What does everything think of what I have done here?

    Eo= Ei(R1/R1+R2) – (R3/R3+R4)
    Eo= 3V (100/100+150) - )200/250+300)
    = -.164V
    Let us change R1 to 125 ohms:

    Eo= Ei(R1/R1+R2) – (R3/R3+R4)
    Eo= 3V (125/125+150) - )200/250+300)
    = 0V
  5. Aug 7, 2010 #4


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    Science Advisor

    The method looks OK except you seem to be using the wrong values for some of the components.
    R1=100ohms, R2= 200ohms, R3= 250ohms, R4= 300ohms in the first post.

    You are using 150 ohms for R2 and 200 ohms for R3.
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