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A couple resistance/power problems

  1. Feb 13, 2007 #1
    1. The problem statement, all variables and given/known data
    http://www.webassign.net/pse/p28-09alt.gif
    We're suppose to find the current on the R resistor(32 ohms) and then find the potential difference between point a and b.

    The attempt at a solution
    For the first problem I'm having a little problem getting this started. I tried getting solving for the total resistance and then assuming voltage is same throughout the circuit, I applied R=V/I to solve for current at that resistor but I'm not getting the right answer. Any help would be appreciated. Thanks!

    EDIT: I solved the other problem
     
    Last edited: Feb 13, 2007
  2. jcsd
  3. Feb 13, 2007 #2

    ranger

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    Gold Member

    For the first problem, how did you go about finding the total resistance?

    As for the second one, do you know any equations for power?
     
  4. Feb 13, 2007 #3
    First off, it would be good to sort of work backwards. First get the total resistance of the 5 ohm, 10 ohm, and R ohm that are in series. (RT = R1 + R2 + R3 = 5 + 10 + 32 = 47).. then when you get that RT value, find the total resistance for the resistors in parallel (1/5 + 1/47 (from the series combination) + 1/10 = 1/RT (for the parallel combination). I got 3.11 ohms for the total resistance of the circuit. After you have the total resistance of the circuit, you find the current by I = V/R since you know V (25) and R (from the series and then parallel calculations) I = 25/3.11 = 8.03A. From there, using current and knowledge of adding currents in parallel and knowing that they the same in series, find the current in each resistor (ultimately yielding to the current in the R resistor) .. once you have the current/resistance in each resistor, then finding the potential difference between A and B will be easy.

    If you need more help.. just ask. I'll do my best to explain it..
     
  5. Feb 13, 2007 #4
    hm I thought that for resistors in parallel its R1+R2 and series it's 1/R1+1/R2? and I did try how you describe it...maybe it's just a lot of things all at once and I'm entering in the wrong number? Ok let me run through what I did in beginning, so I said that the 10(far left) and R resistors are in a series and they are then parallel to the a 5 ohm resistor as well as a 10 ohms one. Next I said that the resistance from all those previous resistors are in series with the 10 ohm one next to battery. That total resistance i put into R=V/I and get current correct? Then through the equations I got that (Assuming I solved for resistance of the middle 10 ohm and 5 ohm resistance and saying there's a current I2 running through it and then I1 running through the 32 ohm resistor, I get something like R(from 10ohm and 5ohm resistors)*(Itotal-I1)=R(32ohm resistor, and 10 ohms)*I1 and then solve for I1 correct?

    EDIT: hm ok so I think I might have had some errors when I was plugging in the numbers because it works now. Could you help me with the second part of this problem? I'm suppose to find the potential difference between a and b. I know it should just be V/I=R but I'm not sure what the numbers for R should be....I should be the amount of current that flows through the 5 ohms and 10 ohms resistor and we neglect that 32 ohms resistor current right?
     
    Last edited: Feb 13, 2007
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