Is the Calculation of Req Correct in This Circuit Problem?

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Discussion Overview

The discussion centers around a circuit problem involving the calculation of equivalent resistance (Req) and the current (i2) in a circuit with multiple resistors and a voltage source. Participants explore the correct approach to calculating Req and subsequently determining i2, addressing both theoretical and practical aspects of circuit analysis.

Discussion Character

  • Homework-related
  • Mathematical reasoning
  • Technical explanation

Main Points Raised

  • One participant presents a formula for Req based on resistors in parallel but questions its correctness after receiving a different answer.
  • Another participant corrects the initial misunderstanding about the formula for calculating equivalent resistance in parallel circuits.
  • A suggestion is made to analyze the voltages at specific junctions in the circuit to aid in calculating i2.
  • A later reply confirms the voltage values at the junctions and prompts further inquiry about the implications for i2.
  • One participant expresses gratitude for the guidance received and shares their final answer for i2, indicating a resolution to their initial confusion.

Areas of Agreement / Disagreement

There is no clear consensus on the calculation of Req, as one participant believes their approach is correct while another points out a mathematical error. The discussion includes multiple viewpoints on how to analyze the circuit and calculate i2, indicating that some aspects remain unresolved.

Contextual Notes

Participants reference specific voltage values and calculations that depend on the assumptions made about the circuit configuration. The discussion does not resolve all mathematical steps or assumptions regarding the circuit layout.

Who May Find This Useful

Students or individuals studying circuit analysis, particularly those seeking clarification on calculating equivalent resistance and current in electrical circuits.

Mach
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Homework Statement


Calculate i2, given that r1=12, r2=6, r3=12, r4=6.
There is a 20V source.

The first step here is calculate Req, I believe.

http://media.newschoolers.com/uploads/cache/images/1222468444-632813-600x325-1222468266Untitled2423423.jpg

Homework Equations


Req=(r1 ll r2) + (r3 ll r4)

note: ' ll ' means parallel.

The Attempt at a Solution



1/Req = 1/(6+12)+1/(6+12)

Req=9

The answer says that Req should be 8. Is my formula for Req correct?
 
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I think you have the right idea but you're getting the math wrong.

For two resistors in parallel, the equivalent resistance is [tex]\frac{1}{R_{eq}} = \frac{1}{R_1} + \frac{1}{R_2}[/tex]

This is NOT the same as [tex]\frac{1}{R_{eq}} = \frac{1}{(R_1+R_2)}[/tex]! (which is what you have done.)

So take it one step at a time, and solve for the Req of R1 and R2, then for R3 and R4, then the Req of the circuit.
 
Imagine that you remove the wire carrying I2. Assume that the negative side of the voltage source is taken to be a reference (ground). What is the voltage at the junction of R1 and R3? What is the voltage at the junction of R2 and R4? I think that if you calculate those two voltages, you will see that it doesn't take much more calculation to determine I2.
 
Thanks a lot! I got the answer and have a better understanding of circuits!

By the way thanks for not giving me the answer, haha.

For those others trying the question the answer is: i2=0.833A
 
Be careful. It looks to me like if you remove the wire carrying I2, the voltage at the junction of R1 and R3 will be 10 volts (with respect to the negative end of the voltage source), and the voltage at the junction of R2 and R4 will also be 10 volts. Do you agree?

And, if that's the case, what will be the value of I2?
 

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