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Thevenin equivalent resistance problem

  1. Apr 17, 2016 #1
    1. The problem statement, all variables and given/known data

    XUot0kO.png
    2. Relevant equations
    V=IR

    3. The attempt at a solution

    I found that the current around the circuit was 5mA using kirchoffs law ( i think)
    If this is true then the T.E.V should be the voltage across the 120ohm resistor which is .6V (120*5e-3). Now i'm stuck and not sure how to find the T.E.R, thankyou
     
  2. jcsd
  3. Apr 17, 2016 #2

    SammyS

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    Hello glover261. Welcome to PF !

    So far, so good.

    What have you been taught regarding how to find the Thevenin equivalent resistance ?
     
  4. Apr 17, 2016 #3
    Hi thanks, I thought you were supposed to take out all components except resistors in the circuit, current sources by open circuit and voltage sources through short circuit. Then just add up the total resistance of all the resistors, but that method does not seem to work on this problem, as none of the answers are the total resistance.
     
  5. Apr 17, 2016 #4

    SammyS

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    That's not quite right. You don't simply add all the resistances. You find the equivalent resistance across the terminals.
     
  6. Apr 17, 2016 #5

    phinds

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    Is "adding up the total resistance of all the resistors" really what happens in a circuit? Do parallel resistors add?

    EDIT: I see sammy beat me to it.
     
  7. Apr 17, 2016 #6
    Aren't they all in series though? or am i missing something
     
  8. Apr 17, 2016 #7

    SammyS

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    You must be missing something.

    Draw what you have when the voltage source is shorted.

    upload_2016-4-17_19-51-1.png
     
  9. Apr 17, 2016 #8

    phinds

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    Probably be a good idea to go back to basics and study what parallel and series really look like. This is a very common difficulty in beginning study of circuits so it's best to get really solid with it in the beginning.

    EDIT: dammit, sammy, you're too fast for me :smile:
     
  10. Apr 17, 2016 #9
    Sorry I still can't see it
     
  11. Apr 17, 2016 #10

    phinds

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    Probably be a good idea to go back to basics and study what parallel and series really look like. This is a very common difficulty in beginning study of circuits so it's best to get really solid with it in the beginning.
     
  12. Apr 17, 2016 #11

    SammyS

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    See the terminals?

    (phinds beat me this time !)
     
  13. Apr 17, 2016 #12
    So, the 180 and 100 resistors are in parallel. So Rt = 1/((1/180)+(1/100)) + 120 = 184 ohms? Is that right because I thought for a pair of components to be in parallel, both sides had to be connected to the same node.
     
  14. Apr 17, 2016 #13

    SammyS

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    See the terminals?
     
  15. Apr 17, 2016 #14
    What do they mean?
     
  16. Apr 17, 2016 #15

    SammyS

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    Find the equivalent resistance across the terminals.

    Look at the nodes again.

    upload_2016-4-17_20-24-18.png
     
  17. Apr 17, 2016 #16

    SammyS

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    There is an alternate way to find the Thevenin resistance.

    Take the original circuit, and find the current which flows through a wire which is connected to the two terminals. (This is the short circuit current.)

    This is the same current which will flow through the Thevenin equivalent circuit if its terminals are shorted.

    In other words, this current times the Thenenin resistance is equal to the Thevenin voltage.
     
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