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A practice problem to find R

  1. Feb 14, 2012 #1
    Hi all ,

    there is a practice problem in my text book which say :

    http://im17.gulfup.com/2012-02-14/1329248560881.jpg [Broken]

    i found after the load is attached that the current through 10 ohm's is 0.2 ampere and through R_L is 0.25 ampere, therefore , the value of R_L is 8 ohm's but the correct answer is 24 ohm's !

    i checked again with the multism program and found my answer is correct !

    i used the current divider method here .

    so i'm a little confused ! which one is correct ?

    BTW , sorry for my bad english :)

    thanks

     
    Last edited by a moderator: May 5, 2017
  2. jcsd
  3. Feb 14, 2012 #2

    berkeman

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    Staff: Mentor

    The current drawn from the source power supply is greater when the Rl resistor is connected.

    To figure this out, just write the equation for the output voltage with the known resistors 40 and 10 and the unknown Rl in parallel with the 10 Ohm resistor.
     
    Last edited by a moderator: May 5, 2017
  4. Feb 14, 2012 #3
    I get 8 Ohms too.
     
  5. Feb 14, 2012 #4

    berkeman

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    Staff: Mentor

    Interesting. I didn't even bother to figure out the answer, but you are right. Plugging in 8 Ohms for the load resistor does give 2V at the output. Hmm...
     
  6. Feb 14, 2012 #5
    I tried to figure out where they could get 24 ohms from, as if I misinterpreted the problem, but I didn't bother with spending the time and assume its an error in the answer book.
     
  7. Feb 15, 2012 #6
    @berkeman
    @DragonPetter
    thank you all of you guys , i really appreciate your help !
    thanks !
     
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