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Basic circuit problem

  • Engineering
  • Thread starter EngnrMatt
  • Start date
  • #1
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Homework Statement



In the circuit below, VS = 65 V, R1 = 64 Ω, R2 = 190 Ω, R3 = 90 Ω, R4 = 290 Ω.
Find:

(a) The equivalent resistance (Req) at the dashed line
(b) v1
(c) v2
(d) i4



Homework Equations





The Attempt at a Solution



I found the equivalent resistance of the circuit (139.612 [itex]\Omega[/itex]) by collapsing the circuit step-by-step, but that didn't seem to work. Apparently this line is referring to something else, and I'm not sure what to do.

Homework Statement





Homework Equations





The Attempt at a Solution

 

Attachments

Answers and Replies

  • #2
gneill
Mentor
20,792
2,770
Can you show the steps you took to find the equivalent resistance? I'm not seeing the same result.
 
  • #3
34
0
First I combine R3 and R4 (290+90) which gives 380. Then I combine that with its parallel R2 (190) which gives 1/[(1/380)+(1/190)] = 126.66, then I add that to R1 (64) which gives 190.66, then combine it with its parallel (50), giving 39.612, which I then add to the last resistor, 100, to get 139.612 ohms. If I am making a mistake, I'd like to know where.
 
  • #4
gneill
Mentor
20,792
2,770
First I combine R3 and R4 (290+90) which gives 380. Then I combine that with its parallel R2 (190) which gives 1/[(1/380)+(1/190)] = 126.66, then I add that to R1 (64) which gives 190.66, then combine it with its parallel (50), giving 39.612, which I then add to the last resistor, 100, to get 139.612 ohms. If I am making a mistake, I'd like to know where.
Ah. That last resistor, 100 Ω, is also in parallel, not in series; You're want to look at the circuit from the point of view of where the dotted line is, not from where VS is connected.
 
  • #5
34
0
Okay, well I'm not sure how to handle it at that point then. I did everything the same as before, only with 100 in parallel, but that didn't seem to work, so I'm all out of ideas.
 
  • #6
gneill
Mentor
20,792
2,770
Okay, well I'm not sure how to handle it at that point then. I did everything the same as before, only with 100 in parallel, but that didn't seem to work, so I'm all out of ideas.
What value did you get?
 
  • #7
34
0
28.373 ohms
 
  • #8
gneill
Mentor
20,792
2,770
28.373 ohms
That's what I'm seeing, too. So...

It's just possible that they want the equivalent resistance for the network to the left of the dotted line only. This would be a common step that one might do if one wanted to replace that piece of the circuit with a Thevenin equivalent for the source and network (you may or may not have learned about Thevenin equivalents yet).
 
  • #9
34
0
Definitely not, considering this is the homework for just day 3 of circuits 1.
 
  • #10
gneill
Mentor
20,792
2,770
Definitely not, considering this is the homework for just day 3 of circuits 1.
Well, I don't know what else to suggest. That's the only likely alternative interpretation of the question that springs to mind.
 

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