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Thevenin Equivalent

  1. Nov 7, 2014 #1
    Hi everyone. I need some help with regards to a question on Thevenin's equivalent. I attempted to solve it in 2 ways but I got different answers when I tried to do it another way. However, I am not sure of my answer as I do not have the answer key at the moment. The picture below shows one of my attempts at solving it, arriving at the answer, V2 = 2.5V, which gives VOC = -2.5V. My workings are bordered by the red line. Pardon me for the messy working as I was in a rush.

    11j7mvk.jpg

    The question is below: (Please ignore the red markings and the pencil markings.)

    5nj1cg.jpg

    Thank you in advance for all your help. :)
     
  2. jcsd
  3. Nov 8, 2014 #2

    gneill

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    Staff: Mentor

    Hi galaxy_twirl. You need to use the posting template when you create a thread in the homework sections of Physics Forums. Posts without the template will usually be deleted. Please keep that in mind for future posts.
    Since you've shown a good effort in your attempt at solution I'll relax the format requirement this time.

    The result that you've achieved for the Voc in your work looks good.
     
  4. Nov 8, 2014 #3

    NascentOxygen

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    The 2.5V figure for V2 checks okay.

    In your S/C diagram I can't see that 5Ω resistance coming from anywhere.
     
  5. Nov 8, 2014 #4
    Hi gneill. Thank you. I didn't know that we need to follow the template, as I thought as long as I show the question and my attempts, the post should be good. I will keep that in mind in the future. Just wondering, is the template the bolded words shown in the box whenever I create a new thread?

    Oh dear. That means my other working is wrong. Is it okay if I post my other (wrong) working here? I did both workings by myself.

    Thank you! :)
     
  6. Nov 8, 2014 #5
    Hi NascentOxygen. Thank you. What do you mean by the 5Ω resistance? I didn't write any 5Ω resistance in my working, at least for the workings enclosed by the red line. :)
     
  7. Nov 9, 2014 #6

    NascentOxygen

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    The Thévenin voltage is only half the answer. You need to come up with the Thévenin resistance, too.
     
  8. Nov 9, 2014 #7
    Ah yes, I got the RTH during the test, but I got a mistake when doing VTH, hence I re-did the question after my teacher gave me some hints.

    I think I should post my other solution which was erroneous, so I can learn from my mistake as well. :)
     
  9. Nov 9, 2014 #8

    NascentOxygen

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    If you can't find the flaw in your alternative approach, post it here.
     
  10. Nov 9, 2014 #9
    Alright. Thanks! I shall post it below. :D
     
  11. Nov 9, 2014 #10
    Here is my alternative approach, which was what my teacher wrote on the board, but I got a different answer for V2, which is supposed to be 2.5V. May I know what is wrong with my approach? I suspect it has to do with me putting GND between the 2 10 Ohm resistors.

    w6xzs4.jpg

    Thanks! :)

    (Do tell me if the above image is hard to read. I will try to use a better camera to snap my working.)
     
  12. Nov 9, 2014 #11

    NascentOxygen

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    V2 is the voltage at an internal node, it is not the voltage measured across the terminals. You have yet to determine the terminal voltage here.
     
  13. Nov 9, 2014 #12
    Oh ya! I see my mistake now. >< Sorry. Thanks! :D
     
  14. Nov 9, 2014 #13

    gneill

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    Yes.
     
  15. Nov 9, 2014 #14
    Thank you. :)
     
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