Basic Circuit Problem: Find Req, v1, v2, i4

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In summary, the equivalent resistance of the circuit at the dashed line is 139.612 ohms. The voltage at v1 is 35.58 V, the voltage at v2 is 29.867 V, and the current through i4 is 0.103 A. The steps taken to find the equivalent resistance are combining R3 and R4, then combining that with R2, then adding R1, then combining it with the parallel resistor, and finally adding the last resistor in parallel. However, there may be an alternative interpretation of the question that is not clear.
  • #1
EngnrMatt
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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.
 

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  • #2
Can you show the steps you took to find the equivalent resistance? I'm not seeing the same result.
 
  • #3
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
EngnrMatt said:
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
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
EngnrMatt said:
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
28.373 ohms
 
  • #8
EngnrMatt said:
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
Definitely not, considering this is the homework for just day 3 of circuits 1.
 
  • #10
EngnrMatt said:
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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1. What is Req in a basic circuit problem?

Req, or equivalent resistance, is the single resistance value that can replace multiple resistors in a circuit while maintaining the same current flow and voltage drop.

2. How do I calculate Req in a basic circuit problem?

To calculate Req, you need to first identify all the resistors in the circuit and their values. Then, use the formula 1/Req = 1/R1 + 1/R2 + ... + 1/Rn, where R1, R2, etc. are the individual resistances. Finally, take the reciprocal of the sum to get the equivalent resistance.

3. What are v1 and v2 in a basic circuit problem?

v1 and v2 refer to the voltage drops across two specific points in the circuit. These points can be chosen arbitrarily, but they are typically chosen to simplify the calculations.

4. How do I calculate v1 and v2 in a basic circuit problem?

To calculate v1 and v2, you need to use Ohm's Law, which states that voltage (V) equals current (I) multiplied by resistance (R). Use the values of the resistors and the current flowing through them to find the voltage drops at the chosen points in the circuit.

5. What is i4 in a basic circuit problem?

i4 refers to the current flowing through a specific resistor in the circuit. It is important to note that the direction of the current is determined by the direction of the voltage drop, not necessarily the direction of the current source.

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