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Homework Help: Equivalent Resistance question HELP

  1. Jan 29, 2007 #1
    Equivalent Resistance question HELP!!!

    1. The problem statement, all variables and given/known data
    The following diagram shows part of an electrical circuit. Find the equivalent resistance between points A and B of the resistor network.

    Here is the circuit in question : http://img182.imageshack.us/img182/8742/physicsyf0.th.png [Broken]

    2. Relevant equations
    R_s = R_1 + R_2 + R_n
    R_p = (1/R_1 + 1/R_2 + 1/R_n)^-1

    3. The attempt at a solution
    I figured out the equations listed above...we have not discussed this in lecture yet (and the hw is due tomorrow before our recitation so I cant get help) so I tried the best I could by reading the text. Here's what I think...

    The 28 ohm, 5 ohm, and 23 ohm resistors are in a series so they can be replaced by a 56 ohm resistor. The same can be said about the 38, 8, and 7 ohm resistors being replaced with a 73 ohm resistor. Now, the 9 ohm, the 73 ohm, and 56 ohm resistors are parallel so using the R_p equation:

    R_p = (1/9 + 1/73 + 1/56)^-1 = 7.00 ohms

    The correct answer is 8 ohms...what am I doing wrong? Do I not understand the concept of series and parallel circuits correctly? Thanks in advance to anyone who helps.
    Last edited by a moderator: May 2, 2017
  2. jcsd
  3. Jan 30, 2007 #2


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    Staff: Mentor

    You have the right idea, but as you collapse each combination of resistors into one resstor, keep that one resistance as you do further combinations.

    -- combine 28 5 and 23 in series

    -- combine that in parallel with 8

    -- add that in series with 38 and 27

    -- and then what do you do with the 9 Ohms?
  4. Jan 30, 2007 #3
    Ah, I see...then the 9 ohm resistor would be parallel to the final resistor...thanks!!
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