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Kirchoff's Law help

  1. Apr 29, 2012 #1
    1. The problem statement, all variables and given/known data
    http://imgur.com/DuBgK


    2. Relevant equations
    I1=I2+I3

    Sum of the directed potential differences around any closed loop equals zero

    3. The attempt at a solution
    I worked out the total resistance which came out to be 4.5Ω [3 + 1/(1/6+1/2)]
    But now I don't know which voltage to use to calculate the total current flow in the circuit. The confusion I have is the +18V, I don't understand where it comes from hence I am confused as to what amount of potential to use.. Thanks in advance.
     
  2. jcsd
  3. Apr 29, 2012 #2
    Remember that potential is, in some sense, arbitrary. The loop rule says that if you start at some point, and traverse one whole loop and come back to your starting point, the total change to your potential is zero. So taking any loop starting at 18 V and coming back to that point will leave you with a potential of 18 V, so the 18 V doesn't actually matter for figuring out the relative currents. What it is useful for is working out the absolute numbers; e.g., once you figure out the current [itex]i[/itex] across the top left resistor, you can subtract [itex]iR[/itex] from 18 V to get the potential at point A.
     
  4. Apr 29, 2012 #3
    Oh thanks for the advice there :D
    So I worked out the current flow to be 4/3 A (Vsupply/Rtotal) and found that the voltage drop across the first resistor to be 4V so 18-4 = 14V right? :P
    Here is how I'm finishing it:
    Since the supply has lost 4V it is left with 2V across the parallel section. Since the voltage across parallel is constant, I worked out the current for each of the branches to be 1/3 A and 1 A so now I can just subtract iR from the remaining electrical potential to find the solution to the rest of the points. Hope I am doing this right, thank you for your help.
     
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