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#2172 - 06/24/01 07:54 PM Question from the book "Electrical License"  
Mike Shn  Offline
Member
Joined: Jun 2001
Posts: 25
New York
Hello
I'm working as a helper in small electrical firm. Today we had a debate in the work about the following chart:
you have first resistor which has 4 ohm ... than followed two resistors with 6 ohm each (these both resistors are parallel between each other). What is resistance at the end of the chart?
We debate more than an hour at the work. The answer in the book is 5 ohm. However some electrician say that correct answer is 7 ohm. What is correcy answer?
Thanks


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#2173 - 06/24/01 08:21 PM Re: Question from the book "Electrical License"  
sparky  Offline
Member
Joined: Oct 2000
Posts: 5,306


#2174 - 06/24/01 10:59 PM Re: Question from the book "Electrical License"  
Redsy  Offline
Member
Joined: Mar 2001
Posts: 2,056
Bucks County PA
If two 6 ohm resistors are in parallel, the resistance is 3 ohms. Add the 4 ohms in series with this and you have your 7 ohms.


#2175 - 06/25/01 12:17 AM Re: Question from the book "Electrical License"  
Francis Mapile  Offline
Junior Member
Joined: Jun 2001
Posts: 1
Philippines
Quote
Originally posted by Mike Shn:
Hello
I'm working as a helper in small electrical firm. Today we had a debate in the work about the following chart:
you have first resistor which has 4 ohm ... than followed two resistors with 6 ohm each (these both resistors are parallel between each other). What is resistance at the end of the chart?

If the 6 ohm resistors is connected in parallel then the total resitance of these resistors is 3 ohm. If you connect them in series with the 4 ohm resistor, then your total resistance is 7 ohm. However, if you connect the 4 ohm resistor in parallel also with each of 6 ohm reistors, the total reistance is 1.71 ohm.
We debate more than an hour at the work. The answer in the book is 5 ohm. However some electrician say that correct answer is 7 ohm. What is correcy answer?
Thanks



#2176 - 06/25/01 06:43 AM Re: Question from the book "Electrical License"  
sparky  Offline
Member
Joined: Oct 2000
Posts: 5,306
********1***1
RT= R1 + ( - + - )
********R2***R3

[Linked Image]



[This message has been edited by sparky (edited 06-25-2001).]


#2177 - 06/25/01 04:42 PM Re: Question from the book "Electrical License"  
Scott35  Offline

Broom Pusher and
Member
Joined: Oct 2000
Posts: 2,707
Anaheim, CA. USA
Quick answers:

If DC, the total resistance is 7 ohms.

If AC, and one element is in reality some type of Reactance, then total Impedance is 5 ohms.

If AC, and all elements are pure resistance [true power only - no Reactance, or very minimal Reactance], then 7 ohms is the total resistance.

Scott SET


Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!

#2178 - 06/25/01 06:35 PM Re: Question from the book "Electrical License"  
Tom  Offline
Member
Joined: Jan 2001
Posts: 1,044
Shinnston, WV USA
You can't believe everything you read. Every time that I teach an electrical course, I work all the problems & see if my answers agree with the teachers answer guide. It is awful embarassing to be in front of a class trying to tell them they are all wrong when they are, in fact, 100% right.

Tom


Few things are harder to put up with than the annoyance of a good example.

#2179 - 06/25/01 10:39 PM Re: Question from the book "Electrical License"  
SlamTex  Offline
Member
Joined: Apr 2001
Posts: 53
This seems to be a simple combination circuit. In my humble opinion the answer is 7 ohms.

HMEL #688


#2180 - 09/01/01 09:03 PM Re: Question from the book "Electrical License"  
bordew  Offline
Member
Joined: Aug 2001
Posts: 142
Vienna,Ohio, USA
Quote
Originally posted by Scott35:
Quick answers:

If DC, the total resistance is 7 ohms.

If AC, and one element is in reality some type of Reactance, then total Impedance is 5 ohms.

If AC, and all elements are pure resistance [true power only - no Reactance, or very minimal Reactance], then 7 ohms is the total resistance.

Scott SET


I am assuming that they were talking DC, however if the reactance is unknown how do you come up with a 5 ohm impedance and if there is a reactance there should be a corresponding phase angle. ???Or the impedance of a two terminal device is defined as follows; Voltage across the element(v(t))/ current through the element i
(t), when v(t) and i(t)have the sink reference relation and when v(t) and I(t)each vary as eEXPst.



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