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Homework Help: Help , with circuits

  1. Sep 3, 2008 #1
    Help plz, with circuits

    Can someone help me solve his circuit. I need to find the Voltage on all the resistors, and current through them. HOw would i start? Can i break this into two loops? How would i implement KVL to this ?

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  3. Sep 3, 2008 #2
    Re: Help plz, with circuits

    Be advised that this thread will probably quickly be moved to the homework section of the boards. You should post questions of this nature there, from now on.

    You can start by recalling that the voltage across the two rightmost resistors will be the same. Can you find the voltage across this pair (combination) of resistors?
  4. Sep 3, 2008 #3


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    Hi th3plan! :smile:

    You can always break any diagram into as many loops as you like.

    In this case, there are 3 loops (the left one, the right one, and the whole outside one).

    But they'll only give you 2 independent equations … in other words, if you apply KVL to all three loops, you'll find that the third equation is just a combination of the first two (in any order :wink:).

    So it doesn't really matter which 2 loops you choose.

    Just mark the (unknown) currents on the diagram where appropriate, and chug happily away! :smile:
  5. Sep 3, 2008 #4
    Re: Help plz, with circuits

    So consider the first loop , its what 10v-V1-V2=0 and 2nd loop is v2-v3=0 ? Is this correct, how do i consider whats positive and negative? Do i pick a node and go from there as current enters and current exits ?
  6. Sep 3, 2008 #5


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    Re: Help plz, with circuits

    The potential drop across a resistor is IR. Designate separate current I1 and I2 for the two branches in your circuit. You can write it like this:
    If the current flows into + terminal of element, potential is IR. If it flows into - terminal of the voltage source, read it as -10V
    So you have
    -Vsource + I1R1 + (I1 + I2)R2 = 0

    where R1 and R2 are the resistors in the loop under consideration.
  7. Sep 4, 2008 #6


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    mark the currents with arrows

    Hi th3plan! :smile:

    To add to what Defennder says … for Kirchhoff's rules you must mark an arrow on each part of the circuit, to indicate the direction of the current.

    (And of course, you must mark the current itself, I1 I2 and I3 … which I suspect you haven't done :frown:)

    You don't need to get the arrow the right way round … if it's the wrong way round, then your I just comes out negative.

    But you must make sure that your arrow directions and Is all obey KVL I at each node. :wink:
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