# R eqivalence help!

1. Oct 16, 2008

### qwerty321

hello..i am studying CCE and i have some difficulties in electric circuit..
i don not know how to determine a resistance equivalence between 2 terminals..i am unable to know which resistances are in series and which are paralle..can someone help me?

for example i have the following circuit:
(attachement)

and i need to determine Req between terminals ab when:

a)when terminals cd are open -circuited
b)when erminals cd are short-circuited

Thank you and i relly need help:S

I have my quizzes comming up next week

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2. Oct 16, 2008

### Proton Soup

unfold/redraw it

3. Oct 16, 2008

### qwerty321

i do not have to draw it again..it is clear enough..
u have 4 resistance( 6,8,10 and 12) and you have to determine the R eq between terminals ab

4. Oct 16, 2008

### tiny-tim

D'oh! Proton Soup means it would be a lot easier if you re-drew the diagram by picking up bd and turning it round (so it's db from left to right), so that the circuit becomes untwisted.

5. Oct 17, 2008

### qwerty321

idid not understand!what happens then?
how do i turn it around?

6. Oct 17, 2008

### tiny-tim

It's like a letter X, with the lines AD and BC crossed.

Just turn the bottom line (BD) round so that AD and BC are vertical, and not crossing each other.

Then it's much easier to see what's in series and what's in parallel.

7. Oct 17, 2008

### qwerty321

sry but I ain't understanding..can you please draw it for me,or a simple sample..thank you tiny-tim and i really appreciate your help

8. Oct 17, 2008

### qwerty321

anyone there?

9. Oct 17, 2008

### Proton Soup

that would be doing the problem for you. maybe reading the first chapter of an introductory book on graphy theory or topology would help.

10. Oct 17, 2008

### The Electrician

Let's attack part a of the problem first.

If you take away the 6 ohm and 10 ohm resistors, and also delete node d, can you figure out the resistance between nodes a and b? Do you understand whether the remaining 12 ohm and 3 ohm resistors are in parallel or in series?

Then you can replace the 6 and 10 ohm resistors with a single resistor of the same equivalent resistance, put back the 12 and 3 ohm resistors and continue your calculation from there. Does this help your understanding?

11. Oct 17, 2008

### qwerty321

yes i see that 12 and 8(its not 3) are in series and 6 and 10 too..then Req(10+8) and Req(6+10) are parallel..ok but what sould i do when c and d are short-circuited..this is where i do not understand..

12. Oct 17, 2008

### The Electrician

If c and d are shorted together, 12 and 6 ohms are then connected in parallel and can be replaced by their Req. Also, 10 and 8 ohms are then connected in parallel and can be replaced by their Req. Do you see what to do next?

13. Oct 18, 2008

### qwerty321

yes ok i got it..but just a question..how can you know when they are in paralle or in series?
do you use the essential node thing?

14. Oct 18, 2008

### The Electrician

15. Oct 18, 2008

### qwerty321

ok i know that..but i still do not understand th thing i told you about in our case here..
anyway just a question:
are you an electrical engineering?

16. Oct 18, 2008

### The Electrician

Explain again just what it is that you don't understand.

Yes, I'm an electrical engineer.

17. Oct 18, 2008

### qwerty321

how come that 12 and 6 are in parallel..do they share the same pair of essential node?
and btw,i have 2 questions i wana ask you..can I pm you?

thanks for helping me the electriian..i really appreciate it!

18. Oct 18, 2008

### The Electrician

12 and 6 are only in parallel if c and d are shorted together.

One end of 12 and 6 are already connected together at node a. If you short c and d together, then the other ends of 12 and 6 become connected. That's what makes a parallel connection.

Sure, go ahead and PM me. I'll be up for at least another hour.

19. Oct 18, 2008

### qwerty321

ann ok..now i get it
thank due u really rock!
ok ill pm u

20. Oct 20, 2008

### qwerty321

i still do not understand how did you figure oout they were paralle and then in series..please explain to me using essential ndoes