Calculate the equivalent resistance in the Figure

AI Thread Summary
The discussion revolves around calculating equivalent resistance in a circuit with multiple resistors. Participants identify and clarify the arrangement of resistors, noting that some are in parallel while others are in series. Confusion arises regarding the simplification of the circuit and the role of a diagonal wire, which creates a short circuit affecting one resistor. Ultimately, the correct parallel resistors are identified as R1, R3, and R4, leading to the formula for total resistance. The conversation emphasizes the importance of examining the connections between resistors to determine their configuration accurately.
Kelsi_Jade
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Homework Statement


upload_2017-3-27_9-10-48.png


Homework Equations

/ 3. The Attempt at a Solution
Here is my attempt from the homework assignment. It doesn't appear that I got any points for this question and was hoping to get a little help in correcting my issues or pointed in the right direction to start over. [/B]
upload_2017-3-27_9-16-8.png
 
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Can you spot the resistors in parallel? Parallel resistors will have their ends connected between the same two points.

The red diagram is the correct one. Is that the provided answer?
 
I'm not sure what the correct answer is. I'm also confused in my TA's diagram why it has 4 resistors?
There only 2 resistors that are in parallel, right?
 
Kelsi_Jade said:
There only 2 resistors that are in parallel, right?
No.
There is one more. Look carefully.
 
OK, thinking out loud here:
It looks like he was simplifying the circuit?
I can see that we should be able to combine resistors to make an easier diagram (which is how we go from 5 Resistors to 4 Resistors in the red). I'm a little confused on the orientation and how we have 3 Resis. in parallel.

I doodled it out but I can't see how it is simplified to make 3 resis. in parallel instead of the 2 obvious resistors.
Why isn't R5 in series with the R2 and R3,4 combination now?
upload_2017-3-27_9-54-4.png
 
Kelsi_Jade said:
in parallel instead of the 2 obvious resistors.
How is R3 connected to R4? Check their ends.
 
They're connected in series, so can't you add them together and make it equivalent to a resistor with a sum of their resistances?
eg. R3,4=R3 + R4
and then this new resistor is in series with R1?
 
Kelsi_Jade said:
They're connected in series
No. Check their ends. Aren't they connected between the "same" two points?
 
Is it because of the diagonal line, does that indicate that the first end point of R3 is the same end as the first end point of R4?
 
  • #10
Kelsi_Jade said:
Is it because of the diagonal line, does that indicate that the first end point of R3 is the same end as the first end point of R4?
Exactly!
 
  • #11
Sweet!
OK, so now I have the diagram looking like the figure below, but where does the R5 fit in? It's in series with R2, R3, and R4 also, right?
So, is it R1 and R5 that can be combined in series like I said before?
upload_2017-3-27_10-49-48.png
 
  • #12
Kelsi_Jade said:
Sweet!
OK, so now I have the diagram looking like the figure below, but where does the R5 fit in? It's in series with R2, R3, and R4 also, right?
So, is it R1 and R5 that can be combined in series like I said before?
View attachment 115123
R2 is not in parallel with R3 and R4. In fact, you'll see something interesting about R2.

Which three are in parallel?
 
  • #13
R1 is in series with R2 which is in series with R5, right?
 
  • #14
Kelsi_Jade said:
R1 is in series with R2 which is in series with R5, right?
No.

Which three are in parallel?

What does the diagonal wire do to R2?
 
  • #15
The diagonal would cause a short circuit so there would be no resistance through R2
because the current would go through the path of least resistance, the diagonal wire
 
  • #16
Kelsi_Jade said:
The diagonal would cause a short circuit so there would be no resistance through R2
because the current would go through the path of least resistance, the diagonal wire
Right.
Which three are in parallel?

I believe you'll be able to find the equivalent resistance now.
 
  • #17
Oh gosh, Then R1, R3, and R4 are the parallel resistors
Then, 1/Rtot=1/R1 + 1/R3 + 1/R4
 
  • #18
Kelsi_Jade said:
Oh gosh, Then R1, R3, and R4 are the parallel resistors
Then, 1/Rtot=1/R1 + 1/R3 + 1/R4
No.
(And that leaves only one combination:biggrin:.)

Check the ends of the resistors. If they are connected between the same two points, they are in parallel.
 
  • #19
Oooh so would the points work out equivalently like this?
upload_2017-3-27_11-46-45.png
 
  • #20
Kelsi_Jade said:
Oooh so would the points work out equivalently like this?
View attachment 115124
There you go!
 
  • #21
FINALLY! Thanks for all of your help (and patience).
 
  • #22
Kelsi_Jade said:
FINALLY! Thanks for all of your help (and patience).
You're welcome!
 

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