How Do You Approach an Irregularly Shaped DC Circuit Problem?

[Elbobo]

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?

 

[LowlyPion]

I would interpret it to mean that

R_a + R_b = 6

etc.

3 equations - 3 unknowns

 

[Elbobo]

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

 

[LowlyPion]

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.

R_a + R_b = 6 || (5 + 6)

The equivalent resistance from one to the other includes the alternate series pair in parallel with the direct one.

 

[Elbobo]

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?

 

[LowlyPion]

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

 

[Elbobo]

Ah ok, thank you.

 

[MATLABdude]

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