Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Two Resistors

  1. Mar 15, 2010 #1
    1. The problem statement, all variables and given/known data

    Two resistors of resistances R1 and R2 , with R2>R1 , are connected to a voltage source with voltage V0 . When the resistors are connected in series, the current is Is . When the resistors are connected in parallel, the current Ip from the source is equal to 10Is

    Let r be the ratio R1/R2

    Find r

    2. Relevant equations



    3. The attempt at a solution

    First I found an expression for voltage in each circuit and then equated them to give me

    Is*(R1+R2) = 10Is*((R1*R2)/(R1+R2))

    Although from here I'm kind of stuck. I'm not sure how to, or even if I am able to manipulate my equation to get R1/R2. Unfortunately it seems my algebra is letting me down in a lot of physics work.
     
    Last edited: Mar 15, 2010
  2. jcsd
  3. Mar 15, 2010 #2

    tiny-tim

    User Avatar
    Science Advisor
    Homework Helper

    Hi Yaaaldi! :smile:

    (Remember, they're also both equal to V0)

    There's various ways of solving this, but one is to replace R1 by rR2, to get a quadratic equation in R2. :wink:
     
  4. Mar 15, 2010 #3
    r^2.R2^2 + rR2^2 + R2^2 = 0

    For the quadratic equation in terms of r equation I got a=1, b=1, c=1

    if I try to solve this I'll get complex roots..

    Have I done something wrong?
     
  5. Mar 15, 2010 #4

    tiny-tim

    User Avatar
    Science Advisor
    Homework Helper

    Hi Yaaaldi! :smile:

    (try using the X2 tag just above the Reply box :wink:)
    How did you get that? :confused:
     
  6. Mar 15, 2010 #5
    Nevermind.. somehow managed to forget to write the 10 infront of the rR2^2 on the RHS earlier.

    now have a=1 b=-8 and c=1

    Solved to get 7.873 and 0.127 for r

    and as I know R2>R1

    r=0.127

    Thanks!
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook