1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Thevenin equivalent resistance

  1. Dec 8, 2013 #1
    ImageUploadedByPhysics Forums1386493104.979236.jpg i am confused at the 20v voltage since it becomes short circuit.i had started reading the circuit from the left side 1M ohm and 2k ohm are in parallel and in series with the 3k.knowing that current wants less resistant ways it will go to the killed voltage source...after that im wasnt able to decide in what way the current will go
     
  2. jcsd
  3. Dec 8, 2013 #2

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

  4. Dec 8, 2013 #3
    Doing as you suggested i am getting Rt(thevenin equivalent resistance) as 4.2k ohms while book solution is 4.0k and i havent done any calculation error.at least I think I haven't.
     
  5. Dec 8, 2013 #4

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    I get 4.0k - please show your working and describe your reasoning.
    i.e. which resistors are left and what is the circuit?
     
  6. Dec 8, 2013 #5
    Is the eq res. of 1M and 2k parallel to 3k? If yes can you please tell me why?because calculating it as parallel i got the result right
     
  7. Dec 8, 2013 #6

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    The 1M, 2k and 3k are, indeed, parallel.
    But they are also disconnected from the rest of the circuit by the short.
    Even if you did include it, you would not get exactly 4.0k. Be careful about rounding off too soon.

    Actually physically draw the circuit (without the load) with the short in place of the 20V source.
    If you put an ohmmeter between a and b, what would it read?
     
  8. Dec 8, 2013 #7
    Well if i calculate only the right side of the circuit I'm getting 4.4 considering the two 6k's are in series then in parallel with 3k then in series with 2k.
     
  9. Dec 8, 2013 #8

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    One of the 6k resistors is in parallel with a short circuit though.
     
  10. Dec 8, 2013 #9
    So i should ignore every resistance affiliated with the short circuit,draw the circuit with only three resistances(6k in parallel with 3k then in series with 2k) and get 4.0k
     
  11. Dec 8, 2013 #10

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    You technically use all the resistances - but you have to include the effect of the short circuit.
    The effect of the short is to give a large chunk of the circuit an equivalent resistance of zero.

    That leaves only the three by the terminals having any effect on the resistance seen at the terminals.

    Sometimes it helps to see which resistors have any effect if you imagine you replace the load with a small current source and trace where the current would go. All the resistors the current pass through are the ones that you use.
     
  12. Dec 9, 2013 #11
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: Thevenin equivalent resistance
  1. Thevenin equivalent (Replies: 13)

  2. Thevenin equivalent (Replies: 3)

  3. Thevenin Equivalent (Replies: 13)

Loading...