Determining Resistance Equivalence in Electric Circuits: Troubleshooting Guide

[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

 

[Proton Soup]

unfold/redraw it

 

[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

 

[tiny-tim]

unfold/redraw it

i do not have to draw it again..it is clear enough..

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.

 

[qwerty321]

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

 

[tiny-tim]

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

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.

 

[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

 

[qwerty321]

anyone there?
please just redraw the figure and I will try to figure out the answers by myself please help!

 

[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.

 

[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?

 

[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..

 

[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?

 

[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?
please tell me how

 

[The Electrician]

Have a look here:

http://en.wikipedia.org/wiki/Parallel_circuits#Parallel_circuits

It's explained very well there.

 

[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?

 

[The Electrician]

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

Yes, I'm an electrical engineer.

 

[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!

 

[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.

 

[qwerty321]

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

 

[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

 

[Proton Soup]

let's say you've got a two-node element like a resistor. if two elements share a single node, they're in series. if they share two different nodes, they're in parallel.