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Finding potential at a specific point in the circuit Help

  1. Apr 14, 2009 #1
    Finding potential at a specific point in the circuit... Help!!

    1. The problem statement, all variables and given/known data

    ............................. ground
    ...................................|
    ----R2-------------------
    |.......................|..........|
    |.......................|..........|
    |.......................|..........|
    R1....................R3........R4
    |.......................|...........|
    |.......................---------
    |.......................|
    Emf1..................Emf2
    |.......................|
    |.......................|
    V2-----R5-------V1

    dots are for spacing that is all


    The resistances are all 2 ohms and emf1 = 5V and emf2 =12 V
    what are the potentials at points V1 and V2



    2. Relevant equations

    currents are noted by what resistor they flow through
    I1 = I2 = I5

    I used the following equations
    junction rule
    I1 = I4 + I3
    loop rule
    Emf1 - I1R1-I1R2-I3R3-emf2-1R5 = 0

    Emf1 - I1R1-I1R2-I4R4-emf2-1R5 = 0


    3. The attempt at a solution

    Using the above I found I1 = 1amp, I3 = 0.5 amp and I4 = 0.5 amp

    Now where i am stuck is when trying to find the potential at a point. This is the first time i seen such a problem related to a circuit. Before I seen difference between two points or something like that or voltage at a resistor or something but V1 and V2 are on the wires. I tried taking two seperate pathes between V1 and V2 but that did not pan out.

    I just looking for some hints on how to approach

    thanks in advance
     
    Last edited: Apr 14, 2009
  2. jcsd
  3. Apr 14, 2009 #2

    LowlyPion

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    Homework Helper

    Re: Finding potential at a specific point in the circuit... Help!!

    I think you are making it more complicated than it needs to be.

    Since R3 || R4 that means you can replace it with the equivalent R for the 2.

    I can't see which direction your Voltage sources are pointing, but you seem to indicate that they are opposing each other through R5, so that suggests that difference of the Voltages 7V is the effective voltage delivered through all the resistors. So I = 7v/Req of all the resistors.

    Armed with the current I, then simply figure your V1 and V2 from the relative step up or step down from the ground reference.
     
  4. Apr 14, 2009 #3
    Re: Finding potential at a specific point in the circuit... Help!!

    [Solved]

    Thank you so much Lowly, now that you mention it. The solution is easy. -11V for V1 and - 9V for V2.

    I probably did make it over complicated with loops, but is there any circumstance where one cannot reduce all the resitances to a single Req.
     
  5. Apr 14, 2009 #4

    LowlyPion

    User Avatar
    Homework Helper

    Re: Finding potential at a specific point in the circuit... Help!!

    It depends really on what you are trying to do. If all the resistances do resolve into an equivalent then of course. But if you have a mesh of nodes with multiple active sources, then maybe you would want to just simplify the ones that make your life easier.

    Of course a computer programmed generalized solver would just analyze every node and loop and take the mess of equations and solve them faster than you can say Bob's your uncle. Basically I guess between the two extremes let common sense be your guide.
     
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