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!

Rank in order, from largest to smallest, the resistance (eq)

  1. Mar 29, 2017 #1
    1. The problem statement, all variables and given/known data
    There is a figure, I'll try my best to draw/describe.
    1. All three resisters are in parallel
    ___R____
    !___R____!
    !___R____ !

    2. 2 resisters are parallel and one in series, after the parallel (ignore the dots)
    ___R___
    ............... _____R___
    !___R___ !

    3. 2 resisters are in series and one in parallel.

    ____R____R____

    ______R_______!
    2. Relevant equations
    For resistors in parallel: 1/R(eq)= 1/R1+ 1/R2+1/R3.....
    For resistors in series: R1+R2+R3......

    3. The attempt at a solution
    Ok!
    So, for the first one the resistance (eq)= 1/R+1/R+1/R= 1/3R
    for 2. I first added R in parallel= 1/2R and with added R separately, that gave me, 1.5R
    for 3. I first added two resistors in series R+R= 2R, then I added this 2R using parallel resistance equation. 1/2R + 1/1R = 1.5 R.
    I know for a fact this is wrong, resistance of #3 should be less than that of #2. Since it is in parallel.
    Can someone please explain what I'm doing wrong here?
    Thank you
    Ali
     
  2. jcsd
  3. Mar 29, 2017 #2
    I think I got it. for #3. 1/R(eq)= 3/2R. R(eq) must be equal to 2/3R.
     
  4. Mar 29, 2017 #3

    epenguin

    User Avatar
    Homework Helper
    Gold Member

    I am beginning to think that conductance is an under-used concept often more helpful than resistance. Also that ppl would do well to use more physical less formulaic thinking for these electrical problems - note here the question was only qualitative!

    So I'd say the first case 1 has 3 conducting each the least resistance possible, so that must have the highest conductance, lowest resistance.
    Comparing 2 and 3, well notice that part of the paths are the same. You could say in both cases at top a current is going through
    __ R__R__ so that's the same for both. Then at the bottom in 2 current goes through __ R__R__ , in 3 it only goes through __R__ ...

    Well may be you did think that, since you did somehow know your calculated result was wrong . :approve:
    Intuition and calculation should each be used as a check and correction of the other!
     
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: Rank in order, from largest to smallest, the resistance (eq)
  1. Rank the resistances (Replies: 1)

Loading...