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Electricity Circuit Analysis

  1. Jan 18, 2012 #1
    1. The problem statement, all variables and given/known data

    [attachment]

    2. Relevant equations

    I=V/R V=R*I R=V/I Rseries = R1 + R2 + R3 ...
    1/Rparallel = 1/R1 + 1/R2 + 1/R3 ...
    Kirchhoff Law


    3. The attempt at a solution
    Rsource = Vsource/Isource
    Rsource = 50/0.25
    Rsource = 200Ω

    I don't know what to do next!
     

    Attached Files:

  2. jcsd
  3. Jan 18, 2012 #2
    Anyone... Please help! I have tried so many times to figure this out but I can't! A hint my teacher gave me is to figure out the R4
     
  4. Jan 18, 2012 #3

    berkeman

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    Staff: Mentor

    Ground the bottom of the voltage source and write the KCL at the node between the left resistor and the right parallel resistors. What does that equation do for you?
     
  5. Jan 18, 2012 #4
    How do I ground it! I did not learn that...
     
  6. Jan 18, 2012 #5

    berkeman

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    Staff: Mentor

    It just means draw a little ground symbol on that bottom wire, to indicate that you are considering it to be the zero volts potential for the circuit. That way your voltage source output is the value of the source (since it is referenced to zero volts). You could arbitrarily set the bottom of the voltage source to something else like "10 volts", but that just introduces extra complexity into the KCL.
     
  7. Jan 18, 2012 #6
    64. (a) R4 5 2.50 x 102
    I1 = 0.250 A
    I2 = 0.150 A
    I4 = 0.100 A
    (b) V2 = 16.7 V
    V3 = 8.30 V

    This is the answer from the back of the textbook... Also my teacher said to use the basics aka... I am not suppose to ground!
     
  8. Jan 18, 2012 #7

    berkeman

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    Staff: Mentor

    Okay, no problemo. Nothing fancy.

    Just label that bottom wire with "0 volts", and write the one KCL equation for the top middle node. Simple! :smile:
     
  9. Jan 18, 2012 #8
    Can you show me step by step, because I still am confused!
     
  10. Jan 18, 2012 #9

    berkeman

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    Staff: Mentor

    No, sorry, that's now how we handle Homework Help at the PF (which you know).

    Tell us how to write KCL (Kirchoff's Current Law) equations. Then write the KCL for the node in the middle of the upper branch as I mentioned several times already.

    http://www.wiziq.com/tutorial/32583-Solving-problems-using-Kirchoff-s-Law-I

    .
     
  11. Jan 18, 2012 #10
    Just a quick question, How come you have to put 0 volts at the bottom in the parallel...
     
  12. Jan 18, 2012 #11

    berkeman

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    Staff: Mentor

    With the KCL, you will write the sum of all currents leaving that central node = 0 (amps or milliamps). When you write the currents as V/R for each branch, the delta-V is the voltage at the node minus the voltage at the node on the other side of the resistance. If you label the bottom wire as 0 volts, then that second voltage in the subtraction goes away in the equation (because it is 0). It's mostly a convenience thing for writing the KCL equations -- pick a logical node to label as 0 volts before starting to write out the KCL equation(s).

    If a node is already labeled with a ground symbol in a schematic, you automatically take that node as being at 0 volts.

    Can you take a cut at writing the KCL at that node now?
     
  13. Jan 18, 2012 #12
    I am sorry, but I personally feel more confused! I don`t feel that you are helping me as much. Thank you for trying to help!
     
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