1. Not finding help here? Sign up for a free 30min 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!

Hard Circuit simplification problem

  1. Aug 11, 2016 #1
    1. The problem statement, all variables and given/known data
    Everything is in the picture. Question 3(a)
    This is for preparation for a contest.

    FtiKaYw.png
    http://i.imgur.com/FtiKaYw.jpg

    2. Relevant equations
    V=IR

    3. The attempt at a solution
    I calculated the total resistance of the first circuit and used that to evaluate V(AB). But this is in terms of the resistance of the load. I tried using kirchoffs laws to eliminate it but i cannot seem to do so. Also i do not understand why there is a unique combination of R(th) and E(th) that works.
    I also noticed that R(th) is in face the value of the resistance of the first circuit if the load has zero resistance but i dont understand why this is so
     
    Last edited by a moderator: Aug 15, 2016
  2. jcsd
  3. Aug 11, 2016 #2

    BvU

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Look at it from the point of view of the load. And post your work to see where you get stuck...
     
  4. Aug 11, 2016 #3

    cnh1995

    User Avatar
    Homework Helper

    Look up Thevenin's theorem. Eth is the Thevenin equivalent voltage and and rth is the Thevenin resistance.
     
  5. Aug 11, 2016 #4
    Thanks ! i saw it. However i still don't understand how i'm supposed to figure that out in the problem
     
  6. Aug 11, 2016 #5

    BvU

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    You have two equations (already filling in ##\ \ V_{AB} = V_{PG} = V_L \ \ ## and ##\ \ I=I_0={V_L\over R_L}\ ##) : $$
    V_L = V_{th}\; {r_{th} \over r_{th} + R_L}$$ in the right diagram; and in the left diagram you have $$
    V_1 \; {R_3 \over R_1 + R_3} = V_L + R_2 {V_L \over R_L} $$and your task was to work around these two equations with four unknowns to two relationships by eliminating ##V_L## and ## R_L##. Not so easy, I grant you. But thereby you prove Thevenin's theorem (as opposed to making use of it)
     
  7. Aug 11, 2016 #6
    Thanks ! i tried something similar.
    The second equation come from equating the pd accross R3 right ?
     
  8. Aug 11, 2016 #7

    BvU

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Across R3 and across the series Rload and R2 as well
     
  9. Aug 11, 2016 #8
    The right hand side of the equation represents the pd across the load and R2( right ? ) shouldnt the left hand side be v1- v1*R1/(total resistance for the circuit) ?
     
  10. Aug 11, 2016 #9

    BvU

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    My bad ! Sorry. Well observed !
     
  11. Aug 11, 2016 #10
    No problem!
    God so i knew how to do it its just the maths part thats killing me. I guess i'll have to work that out slowly. Thanks!
     
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: Hard Circuit simplification problem
Loading...