How Do You Approach an Irregularly Shaped DC Circuit Problem?

AI Thread Summary
The discussion revolves around solving an irregularly shaped DC circuit problem, where participants express confusion about how to approach the calculation of equivalent resistance. The main issue identified is the misunderstanding of how to apply series and parallel resistor combinations correctly, particularly in a non-symmetrical circuit. Participants clarify that the equivalent resistance should be calculated using the Y-delta transform, which is essential for deriving the correct relationships between the resistors. The conversation emphasizes the importance of recognizing the circuit's configuration to set up the right equations. Understanding these concepts is crucial for accurately solving the problem.
Elbobo
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Homework Statement


http://img144.imageshack.us/img144/9724/tricircuitxv4.jpg
http://g.imageshack.us/img144/tricircuitxv4.jpg/1/


Homework Equations



Re = R1 + R2 + R3

The Attempt at a Solution



I don't really understand this problem. I'm not sure how to approach it. I thought 5 = Rb + Rc, and I did that for all the rest of the resistors to solve the system but that was wrong.

Can someone just help me understand what the problem is asking and what it gives?
 
Last edited by a moderator:
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I would interpret it to mean that

Ra + Rb = 6

etc.

3 equations - 3 unknowns
 
That's exactly what I did, and got a nice rational number - 3.5 (the answer is almost never rational). It's wrong.
 
Elbobo said:
That's exactly what I did, and got a nice rational number - 3.5 (the answer is almost never rational). It's wrong.

I see what's wrong.

Ra + Rb = 6 || (5 + 6)

The equivalent resistance from one to the other includes the alternate series pair in parallel with the direct one.
 
So you're saying Ra + Rb = (1/6 + 1/11)^-1?

Isn't that saying the total resistance of the first circuit equals only the sum of the resistances Ra and Rb?
 
Elbobo said:
So you're saying Ra + Rb = (1/6 + 1/11)^-1?

Isn't that saying the total resistance of the first circuit equals only the sum of the resistances Ra and Rb?

Yes that's the calculation.

No, it's how the resistors are taken. Note the circuit is not symmetrical. You get pair-wise different equations.

There's 6 || 11 and 6 || 11 and 5 || 12
 
Ah ok, thank you.
 
Elbobo said:

Homework Statement


http://img144.imageshack.us/img144/9724/tricircuitxv4.jpg
http://g.imageshack.us/img144/tricircuitxv4.jpg/1/


Homework Equations



Re = R1 + R2 + R3

The Attempt at a Solution



I don't really understand this problem. I'm not sure how to approach it. I thought 5 = Rb + Rc, and I did that for all the rest of the resistors to solve the system but that was wrong.

Can someone just help me understand what the problem is asking and what it gives?

This is known as the Y-delta transform (though this question is probably to get you guys to derive the transformation to be covered in class!):
http://en.wikipedia.org/wiki/Y-Δ_transform
 
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