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Homework Help: Resistancein a circuit ,,

  1. Mar 17, 2010 #1
    Resistance in a circuit ,,

    1. The problem statement, all variables and given/known data

    Hi guys ,, I have the following picture

    attachment.php?attachmentid=24434&stc=1&d=1268821144.jpg

    3. The attempt at a solution

    OK ,, I know it's easy but my answer's is not like my teacher's answer :

    for (a) I get 6.12 ohm and for (b) I get 2.4 ohm

    but my teacher's answer is :

    a. Rt=4+4+4=12 ohm
    b. Rt= (8//4)+4= 6.667 ohm

    what I don't get for :

    a. how can he just add them together ,, it's not like they are in series and 2 of them are in parallel .. and I don't know why adding only 3 resistors only ?? there are 5 ~.~ ,,

    b.how come 8//4 and there is a resistor between 4 & 4 ,, so don't we have to take a parallel (4//4) then add with 4 then parallel them with the upper resistor ??

    so can someone simplify circuits please -_- ,, (I usually use current direction to know whether they are in parallel or series)
     

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    Last edited: Mar 17, 2010
  2. jcsd
  3. Mar 17, 2010 #2

    Filip Larsen

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    Gold Member

    Think about which wires in the circuit that would actually carry a current if you were to measure R_T by applying a voltage over the two left terminals. It may also help to redraw the diagram so you have the top-left terminal on the left and the bottom left terminal on the right.
     
  4. Mar 17, 2010 #3
    aha ,, now I get it ,, so the arrow (Rt) in the diagram means where the voltage terminals are ,, I get it for (a) and (b) too ,, thanks very much filiplarsen ,, but question , if both circuits where closed my answers were right ??
     
  5. Mar 17, 2010 #4

    Filip Larsen

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    Gold Member

    If you by closed mean that the two terminals to the right are short-circuited, then I get
    a) 4+(8//4)+4 = 10 2/3, and
    b) 4//(4+(4//4)) = 4//6 = 2 2/5
    So we have same result for b) but not a).
     
  6. Mar 17, 2010 #5
    Lol,, got it :)

    thanks very much m8
     
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