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Two simple questions about equivalent resistance and kirchoff laws

  1. Dec 14, 2012 #1
    two simple questions about equivalent resistance and kirchoff laws!! pleaseeeee! :D

    EQUIVALENT RESISTANCE:
    Hi everybody . greetings from Peru.I know it's a dumb question but help please .please read below
    1. The problem statement, all variables and given/known data
    For the network shown in the figure, show that the resistance Rab = (27/17). Ω
    View attachment er.bmp
    3. The attempt at a solution
    This is the the soltuion I found on the internet ,
    View attachment serway.bmp
    but i don't get WHY rules of equivalent resistances in parallel and series cannot be applied to this problem . obvously using those rules I get another answer which is wrong, can ayone help me please?
    KIRCCHOFF:
    And i have another question can a batery charge another one?
    it looks like i have missed something in class because i don't understand .... like in the solution of this problem :

    A dead battery is charged by connecting it to the live
    battery of another car with jumper cables (Fig. P28.25).
    Determine the current in the starter and in the dead
    battery.
    sol.jpg

    the solution says "the current is backward in the dead baterry tending to charge it up" why? ,I mean ¿what direction of current is needed so that a batery can be charged? ¿from high potntial to less potential ? i just don't get it
    can someone explaian this to me please , if you have links it would be great too. Thank you very much
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     
    Last edited: Dec 14, 2012
  2. jcsd
  3. Dec 14, 2012 #2

    CWatters

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    Re: two simple questions about equivalent resistance and kirchoff laws!!

    Because the circuit doesn't contain resistors that directly "in parallel" with another.
     
  4. Dec 14, 2012 #3

    NascentOxygen

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    Re: two simple questions about equivalent resistance and kirchoff laws!! pleaseeeee!

    Hi carlos125, http://img96.imageshack.us/img96/5725/red5e5etimes5e5e45e5e25.gif [Broken]

    You should be able to see there is no pair of parallel resistors in that figure, nor are there two resistors in series. You need to get it clear in your mind exactly what it means for a pair of resistors to be in parallel.
    If current flows out of the + terminal of a battery into an external circuit, the battery is discharging. If current flows into the + terminal of a battery from an external circuit, the battery is charging. (Assuming we are talking about secondary cells).
     
    Last edited by a moderator: May 6, 2017
  5. Dec 14, 2012 #4

    CWatters

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    Re: two simple questions about equivalent resistance and kirchoff laws!!

    Thats the way batteries work. Current flows one way during discharge and the other way during charge.

    If you want to know why take a look at the chemistry that goes on in a battery. Rechargable batteries use a reversible chemical reaction that creates a flow of electrons. In order to reverse the reaction you have to "push" electrons the other way, typically using a higher voltage than that of the battery.
     
  6. Dec 17, 2012 #5
    Re: two simple questions about equivalent resistance and kirchoff laws!!

    thank you guys for your quick replies ,but aren't the 1-ohm resistance(upper left) and the 3-ohm resistance in series? . The definition for resitance in series is "Two or more elements are in series if they are cascaded or connected sequentially " ,aren't they sharing the same wire? so why aren't they in series? look: res.gif
    thanks . btw the wire i'm talking about doesn't separate in two branches too , i'm confused
     
  7. Dec 19, 2012 #6

    NascentOxygen

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    Re: two simple questions about equivalent resistance and kirchoff laws!!

    Elements are in series iff the exact same current that passes through one also passes through the other/s.

    Elements are in parallel iff both of the ends of those elements connect to the same points.

    The 1Ω and the 3Ω would be in series iff nothing was ever connected at point 'a', because with nothing connected at point 'a' those two resistors would carry the same current. But point 'a' is where the external circuit connects, delivering to those resistors unrelated and different currents.
     
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