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!

Finding the absolute error for equation of total resistance

  1. May 29, 2017 #1
    1. The problem statement, all variables and given/known data
    Prove the equation for ΔRC. (See the first attachment with the circuit schematic inside)

    2. Relevant equations
    (The equation for RC, in both attachments)

    3. The attempt at a solution
    You can see my attempt at a solution in the second attachment. I'm stuck at the step shown in the picture. Maybe that needs a bit algebra, or my attempt was incorrect.

    Attached Files:

    • 111.png
      File size:
      19.7 KB
    • 222.png
      File size:
      23.5 KB
  2. jcsd
  3. May 29, 2017 #2


    User Avatar
    Science Advisor
    Homework Helper
    2017 Award

    I already have difficulty with the first step: If ##R_C = R_{//} + R_3 ## then it certainly is not so that ##\log R_C = \log R_{//} + \log R_3 ## !
    And then the ##\Rightarrow## is also unjustified.

    I suggest you try to find ##\Delta R_{//}## on its own...
  4. May 29, 2017 #3


    User Avatar
    2017 Award

    Staff: Mentor

    ln(a+b) and ln(a) + ln(b) are not the same (first -> second line and also within the second line) and the result is wrong.
  5. May 30, 2017 #4
    Thanks a lot, dear BvU! I tried your suggestion and it worked! :smile:
    Yes, I had a big mistake in my first attempt as you mentioned.

    What do you mean by this:
    Is that because of my mistake in taking logarithms or it's generally not considered right to use in these equations?
  6. May 30, 2017 #5


    User Avatar
    Science Advisor
    Homework Helper
    2017 Award

    Transition from first line in attachment to second line was in error.
    The transition from before the ##\Rightarrow## to after was also in error, hence my remark.
    $$\ln{R_1 R_2\over R_1+R_2} =\ln R_1+\ln R_2-\ln(R_1+R_2) $$
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted