# Homework Help: REALLY hard equivalent resistor problem!

1. May 10, 2014

### jaredvert

Please don't say this is homework because it is not. I am simply studying problems for the physics c exam and I can't figure out this textbook problem. Please show me how to do this! The equivalent resistance part in the most depth because I dont understand. Is it right to say no current runs through the middle r because it would be repelled by the other current? And if that's the case this problem becomes easy but in not sure that's the case... Thanks .

Last edited: May 10, 2014
2. May 11, 2014

### ModusPwnd

Even thought its not "real" homework, it can still go in the homework section because its of that format.

Anyway, looks like it might be a case of the ol' delta-wye. Does that mean anything to you?
https://en.wikipedia.org/wiki/Y-Δ_transform

Your tried and true series and parallel addition formulas wont work here as you have found. The delta-wye formulas can be derived, or just blindly used, depending on what you want.

3. May 11, 2014

### Drakkith

Staff Emeritus
This is a homework style question and as such should be posted in the homework section of the forums per PF rules.

4. May 11, 2014

### jaredvert

Ok van u just tell me why the current wouldn't go through the middle resistor and then I will delete it. I had no other way of conveying this question

5. May 11, 2014

### UltrafastPED

6. May 11, 2014

### ModusPwnd

Current will go through all those resistors.

7. May 11, 2014

### jaredvert

Guys this was in my textbook that says absolutely nothing about delta y transformation or anything of the sort. Can you think of anything else??? They wouldn't put something in the problems at the back if it wasn't applicable to the knowledge learned through the book and this obviously wo t come up on the ap exam but I still am curious and would like to know

8. May 11, 2014

### ModusPwnd

I would guess that there is a small section on delta-wye in there. My second guess is that the book editor just messed up. It happens sometimes.

9. May 11, 2014

### jaredvert

Giancoli physics for scientists and engineers vol 2. I read the whole chapter and nothing on delta wye? How do u know when to use delta wye?

10. May 11, 2014

### ModusPwnd

I consider it when I can't figure out how to use parallel or series addition. :tongue: The resistors are not in parallel or series with any other resistor.

UltrafastPED's link is good. You can see the basic geometry of the delta and they wye, they literally look like a delta and a wye. (Of course your wires may be aranged differently making it hard to see. Practice is needed)

Using the formulas in that link you can turn one into another as needed and then, hopefully, use your parallel and series rules to reduce the circuit.

11. May 11, 2014

### jaredvert

K thanks dudio

12. May 11, 2014

Staff Emeritus
Closed because it's in the wrong section (since moved) and doesn't use the template.

13. May 11, 2014

### Staff: Mentor

Just to add an important comment: you do not need a Y-Delta transformation. This is way beyond the scope of your homework.

You can draw the setup in a different way to see how symmetric it is, and then you can see why there is no current in the middle resistor. Afterwards it is easy to continue.