Electric Circuits Problem!

Thread Starter

sagaradeath

Joined Feb 18, 2011
16
Please look at the attachment!!!

Please look at the attachment!!!

The Question is in the attachment!
 

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addousas

Joined Jul 15, 2010
2
I looked at the question over and over. The problem is not right R1 is necessary to solve this problem. Vb=2*(R1*R3/(R1+R2+R3))+4(R1+R2)*R3/(R1+R2+R3)

To prove my point lets assume that the problem is right
under this assumption pick two values for R1
Vb should be the same but that is not the case, thus R1 is needed to solve this proble
 

hgmjr

Joined Jan 28, 2005
9,029
The reason that R1 is fundamental to the solution of this problem is that it is in parallel with a current source. If it were in parallel with a voltage source then it could be ignored for solving this particular problem.

hgmjr
 

Adjuster

Joined Dec 26, 2010
2,147
The value of R1 is not required explicitly because of the additional information given. The student is actually given the node voltage Va, and so can solve the problem.
 

hgmjr

Joined Jan 28, 2005
9,029
The value of R1 is not required explicitly because of the additional information given. The student is actually given the node voltage Va, and so can solve the problem.
Ooopss!! You are absolutely Adjuster, I completely missed that important clue to the answer. How silly of me.

It is still a good idea to recognize the importance of knowing the value of R1 to the solution of such a problem were the voltage across R1 not provided in the problem statement.

I would not want to lull any newbies into the notion that resistors can be ignored unless a clear explanation of why they can be ignored is provided.

hgmjr
 

Adjuster

Joined Dec 26, 2010
2,147
Well, the problem does not ask for it, but here is how it goes:

You are given that Va = 5Volts. This should allow you to solve for the current in R2.

Having found the current in R2, you will be able to find the current in R1, and of course you already know Va.
 

hgmjr

Joined Jan 28, 2005
9,029
That is not the answer that I obtained.

How about posting your work so that we can see how you obtained your answer.

hgmjr
 

hgmjr

Joined Jan 28, 2005
9,029
What circuit analysis technique were you instructed to use in solving this problem?

hgmjr
 

hgmjr

Joined Jan 28, 2005
9,029
Actually what I was referring to was which circuit analysis method such as:

1. Superposition
2. Kirchhoff's
3. Thevenin's
4. Norton's


hgmjr
 

hgmjr

Joined Jan 28, 2005
9,029
Have a look at this example for a start. Also you should have a couple of examples in your textbook.

Then see if you can form the two equations needed to solve for Vb.

hgmjr
 
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