# Parallel-Series equivalent resistance

Thread Starter

#### SilverKing

Joined Feb 2, 2014
72
Hi everyone,

In the circuit uploaded as an attachment:

Is R1//R2, R3//R4, R5//R6//R7//R8 and their sum is RT?

and how can I compute the voltage at R4 and R5? and the current at R3 and R5?

P.S. Do you any textbook include these types of problems?

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#### #12

Joined Nov 30, 2010
18,210
Do you? Meaning, "Is this a homework problem?"

First thing to do is re-draw it to remove the intentional confusion.

Last edited:

#### tubeguy

Joined Nov 3, 2012
1,157
Hi everyone,

Is R1//R2, R3//R4, R5//R6//R7//R8 and their sum is RT?
To the OP.
After you do as #12 suggested.
Do you have any series resistances ?

Thread Starter

#### SilverKing

Joined Feb 2, 2014
72
To #12:
Sorry, I meant "Do you have any textbooks that includes similar problems?"

To tubeguy:

I've tried to re-draw the circuit and came up with two thoughts

In the first one, I see that (R5//R6//R7/R8) in series with (R3//R4)
but in the second, I see that they're in parallel

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#### #12

Joined Nov 30, 2010
18,210
Is this a homework problem?

#### studiot

Joined Nov 9, 2007
4,998
In the first one, I see that (R5//R6//R7/R8) in series with (R3//R4)
Well I'd say that (R5//R6//R7/R8) looks more like someone hanging out the washing from a window in the first one. How can it be in series when it is not fully connected to anything?

However if you connect it you will find something interesting.

Further look at the other parts of your first drawing and compare with your circuit original.

This should help you spot another parallel combination.

edit No#12 is right this looks as though it belongs in homework help.

Thread Starter

#### SilverKing

Joined Feb 2, 2014
72
So basically all the resistors are in parallel.

and shall I use the basic current divider rule (Rx=IT*RT/Rx) and the voltage divider rule (Rx=E*Rx/RT)?

And no, this isn't a homework problem, it's previous exam problem.
Our "prof" does not have the time to give us "homework" ~__~

#### studiot

Joined Nov 9, 2007
4,998
and shall I use the basic current divider rule (Rx=IT*RT/Rx) and the voltage divider rule (Rx=E*Rx/RT)?

No I think you should use your (scientific) brain.

If the resistors are all in parallel, what does that mean for the voltage across them?

Thread Starter

#### SilverKing

Joined Feb 2, 2014
72
I know that the voltage across resistors in parallel is the same, but I meant the voltage across R4 and R5 in the original circuit. Can I use the voltage divider as I mentioned above or do I have to find the total resistance with exception of R4 and R5 and THEN use the voltage divider?

#### tubeguy

Joined Nov 3, 2012
1,157
What does your re-draw #2 in post #4 show?

#### studiot

Joined Nov 9, 2007
4,998
I know that the voltage across resistors in parallel is the same
And what basic law of electricity do you know that tells you the current in any resistor if you know the voltage across it?

Thread Starter

#### SilverKing

Joined Feb 2, 2014
72
It shows that all resistors are in parallel with the voltage source, so the voltage across all the resistors is 15V --> oh my

and for the current, I'd first "factor out" R3 and use the current divider, isn't?

P.S. Which book do recommend me to use? Hayt, Boylestad, Alexander and Sadiki, or Miller and Robbins? or any other?

#### studiot

Joined Nov 9, 2007
4,998
It shows that all resistors are in parallel with the voltage source, so the voltage across all the resistors is 15V --> oh my
Good show.

Now your question did not ask for the total current did it?
So why are you wedded to a current divider?

So if you have 15 volts across a 1Ω resistor what is the current through it and what law do you use ?

Thread Starter

#### SilverKing

Joined Feb 2, 2014
72
Aha, I just have to stick to ohm law I=E/R
no need for current divider, I don't know what's my problem with current divider

Case closed, the problem is solved

thank you for your patience, studiot, and thank you for you approach that you didn't gave me a straight answer ^^

but you didn't answer me, which textbook is better? specially for complicated equivalent circuits

#### studiot

Joined Nov 9, 2007
4,998
Well I don't know what subject you are studying or where in the world you are, so will you have American or European texts?

I'm glad the penny finally dropped because blind application of formulae is not recommended.

Thread Starter

#### SilverKing

Joined Feb 2, 2014
72
My subject is "Basic Circuit Analysis", and I guess there are no much differences since it's a general subject, but I'll stick to the American texts, which one do you prefer?

#### studiot

Joined Nov 9, 2007
4,998
I would go with

Electric Circuits by Nilsson.

Thread Starter

#### SilverKing

Joined Feb 2, 2014
72
Thank you very much, and have a nice day

#### #12

Joined Nov 30, 2010
18,210
Hey, I get a thanks too.
I was stuck helping inwo and jegues build a fail-safe for almost 2 hours.

#### studiot

Joined Nov 9, 2007
4,998
I think this thread is an excellent example of cooperation all round, valuable contributions were made by all concerned.

SilverKing please note that even though this was not set as 'homework' we regard this as belonging in the 'homework section' - after all the section has to have some sort of descriptive name.
The point of this section is that special rules apply to provide correct teaching help for learners.
This situation is common to most technical help sites.