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Homework Help: Circuit with Four Resistors and Two Switches

  1. Mar 2, 2016 #1
    1. The problem statement, all variables and given/known data
    Four resistors connected to a battery are shown in the figure (at bottom). The switch S2 is closed and switch S2 is left open. What is the magnitude of the potential difference Vcd

    2. Relevant equations
    Rp= Σ(1/Ri)-1

    Rs = ∑Ri

    V = IR
    3. The attempt at a solution
    Seeing as c and d aren't directly connected since S2 is open, I attempted to find the total resistance using the two resistance equations above and from that find the total current. After that, I divided the total current by two since the total resistance on both sides are the same, and multiplied that by the total of the top resistance. Next I added the current multiplied by the bottom left resistor and subtracted the current multiplied by the bottom right resistor, but I don't think this is correct.
    Last edited: Mar 2, 2016
  2. jcsd
  3. Mar 2, 2016 #2


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    Staff: Mentor

    Hi Jake Russell, Welcome to Physics Forums!

    The mage that you tried to attach isn't viewable. Can you upload it again?
  4. Mar 2, 2016 #3
    I ended up figuring out this question (I wasn't starting at a and not c), but I have another one. How would I find the current through the second switch when it is closed?
  5. Mar 2, 2016 #4


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    Staff: Mentor

    With switch S2 closed can you find the potential at node cd? (Since S2 joins c and d together, they become a single node).

    Knowing the potentials at a,b, and c, can you write a KCL equation at location c?
  6. Mar 2, 2016 #5


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    Science Advisor
    Homework Helper
    Gold Member

    Work out the voltage drop across each resistor (easy) then calculate the current in each resistor and mark it on the diagram. Apply KCL at either node c or d.

    Edit: cross posted with gneill.
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