# Req problem

Discussion in 'Homework Help' started by freemindbmx, Mar 14, 2014.

1. ### freemindbmx Thread Starter Member

Mar 5, 2014
72
0
Me and a friend got stumped on this, any help would be great. But we need to find the Req at terminals A and B

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2. ### BytetoEat New Member

Mar 5, 2014
25
5
it completely depends on what is going on with C and D!!!!

3. ### WBahn Moderator

Mar 31, 2012
18,085
4,917
Perhaps you and your friend could show your best effort to find Req at terminals A and B?

Perhaps start by bounding the possible results?

What resistance can you guarantee the equivalent resistance will definitely be smaller than? What resistance can you guarantee the equivalent resistance will definitely be larger than than?

At first glance you should be able to obtain a range that is less than a factor of two. A second glance should reduce that range to just a 1Ω.

By bounding the problem, you then will know if your final answer is even possible (and even if you do it wrong and get a wrong answer, you will be close enough for most practical situations).

4. ### shteii01 AAC Fanatic!

Feb 19, 2010
3,500
511
+1

If C and D are not connected, open circuit, then 5 at C and 5 at D can be completely ignored.

So you got 3 and 6 and 3 in series, call it R1. Then R1 is in parallel with 6, call it R2. Then R2 is in series with 5. BAM! You are done.

5. ### freemindbmx Thread Starter Member

Mar 5, 2014
72
0
C and D arn't connected to a load or anything just a open circuit like A and B

6. ### freemindbmx Thread Starter Member

Mar 5, 2014
72
0

Ahh me and my friend for some reason were thinking you short circuit C and D and like you said ignore it, thank you

7. ### shteii01 AAC Fanatic!

Feb 19, 2010
3,500
511
If you short C and D, then there will be current through them when you connect something at AB. Following Ohm's Law, you have current, you have resistance, I*R=V, when you have the two, you have the third, the voltage. Therefore you can not simply ignore them if you short C and D.

That is why the earlier question is so important. If CD is open, you have circuit with 5 resistors. If CD is closed, you have circuit with 7 resistors.