The Equivalent Resistance Calculation

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SUMMARY

The equivalent resistance between points a and b in the given circuit is calculated to be 25 Ω. The circuit consists of a 20 Ω resistor in series with a parallel combination of a 10 Ω resistor and the series combination of a 5 Ω and another 10 Ω resistor. The correct formula to find the equivalent resistance is Req = 20 + (10 || (10 + 5) || 10), which simplifies to 23.75 Ω. Visual representations, such as circuit diagrams, are essential for accurately analyzing complex circuits and determining equivalent resistances.

PREREQUISITES
  • Understanding of series and parallel resistor combinations
  • Familiarity with Ohm's Law
  • Basic knowledge of circuit diagrams
  • Ability to perform calculations involving resistances
NEXT STEPS
  • Study the principles of series and parallel circuits in depth
  • Learn how to use circuit simulation software like LTspice
  • Explore advanced techniques for circuit analysis, such as mesh and nodal analysis
  • Investigate the impact of different resistor values on equivalent resistance
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Students studying electrical engineering, circuit designers, and anyone interested in mastering circuit analysis techniques.

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Homework Statement



What is the equivalent resistance between points a and b:

[PLAIN]http://img706.imageshack.us/img706/8615/circuitq.jpg

The Attempt at a Solution



The correct answer is Req=25 Ω. Looking into the circuit the 20 Ω is in series with the 10 Ω which is in parallel with the series combination of 5 Ω and 10 Ω resistos a, which is in parallel with the last 10 Ω resistor:

Req = 20 + [10 || (10+5) || 10]

R_{eq} = 20+ (\frac{1}{\frac{1}{10} + \frac{1}{15} + \frac{1}{10}}) = 23.75 \ \Omega

My answer is slightly different from the model answer, so is something wrong with my calculation?
 
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The 5Ω resistor is not in series with the middle 10Ω resistor; there's another resistor (the rightmost 10Ω resistor) sharing the node where they connect. For components to be in series they must be alone in a line and so pass the same, identical current.

I suggest that in this case you start at the "back end" of the circuit (the rightmost end) and work towards the front.
 
Last edited:
I think it should,

20 + 10 || ( [ 10 || 10] + 5 )
 
With a circuit like this, it's often extremely helpful to isolate your equipotential surfaces. This allows you to determine quickly whether various circuit elements are in series or in parallel.

7wYif.jpg
 
Thank you guys, 20 + 10 || ( [ 10 || 10] + 5 ) worked. And the diagram was very helpful. :)
 
I'm glad the diagram helped. Frankly, I've had my fair share of circuit analysis (including a class devoted to it, as well as my linear algebra text which actually had an entire section devoted to matrix analysis of circuits), and I still do things that a lot of intro-level students often consider "hokey" or otherwise reserved for people who aren't very good at the topic -- like diagramming out all the equipotentials on a circuit in order to aid in evaluating equivalent capacitances and resistances.

The reason is that, simply put, visual representations are almost always helpful if they're rendered correctly. Just remember that the smartest thing you can do as a student isn't to try and solve a problem in a way that's harder for you because other people prefer to solve it that way. The smartest thing you can do as a student is to use whichever techniques help you to solve the problems that are put before you. There is absolutely no shame in setting up a diagram and marking it up, or drawing a picture to help you conceptualize the problem; in fact, the opposite is true. To quote my old intro modern physics instructor, Dr. Zhao:

"I don't get why people don't want to draw a picture. Does it embarrass them or something? PLEEEAAAASE! That's how I know someone knows what he's doing. Why do I want someone who spits numbers at me instead of thinking about what I'm asking him? That's just silly. When I see that picture with those pretty notes on it, I know that guy has his head on straight."
 
WJSwanson said:
I'm glad the diagram helped. Frankly, I've had my fair share of circuit analysis (including a class devoted to it, as well as my linear algebra text which actually had an entire section devoted to matrix analysis of circuits), and I still do things that a lot of intro-level students often consider "hokey" or otherwise reserved for people who aren't very good at the topic -- like diagramming out all the equipotentials on a circuit in order to aid in evaluating equivalent capacitances and resistances.

The reason is that, simply put, visual representations are almost always helpful if they're rendered correctly. Just remember that the smartest thing you can do as a student isn't to try and solve a problem in a way that's harder for you because other people prefer to solve it that way. The smartest thing you can do as a student is to use whichever techniques help you to solve the problems that are put before you. There is absolutely no shame in setting up a diagram and marking it up, or drawing a picture to help you conceptualize the problem; in fact, the opposite is true. To quote my old intro modern physics instructor, Dr. Zhao:

"I don't get why people don't want to draw a picture. Does it embarrass them or something? PLEEEAAAASE! That's how I know someone knows what he's doing. Why do I want someone who spits numbers at me instead of thinking about what I'm asking him? That's just silly. When I see that picture with those pretty notes on it, I know that guy has his head on straight."

That's true. I never really got it until I saw your diagram, it clarified everything. Thanks a lot. :smile:
 

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