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Equivalent Resistance question HELP

  • Thread starter Decan
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Equivalent Resistance question HELP!!!

1. Homework Statement
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. Homework 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.
 
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Answers and Replies

berkeman
Mentor
56,115
6,148
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?
 
44
0
Ah, I see...then the 9 ohm resistor would be parallel to the final resistor...thanks!!
 

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