# Electric Circuits Problem!

Joined Feb 18, 2011
16

The Question is in the attachment!

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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

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

Joined Feb 18, 2011
16
how do you get R1???

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.

Joined Feb 18, 2011
16
is the answer of the problem 41.2... i want to check my answer

#### hgmjr

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

hgmjr

Joined Feb 18, 2011
16
well i was thinking V=IR

so V=ix*R3

so V=4*10.3

41.2

Joined Feb 18, 2011
16
V=IR
V=ix*R3
V=4*10.3
V=41.2

#### hgmjr

Joined Jan 28, 2005
9,029
V=IR
V=ix*R3
V=4*10.3
V=41.2
Don't you think that the 5 volt potential and the 5.1 ohm resistor contribute to the votage at the junction the 10.3 and 5.1 ohm resistor?

hgmjr

Joined Feb 18, 2011
16
so what should i do ....im still new to this

#### hgmjr

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

hgmjr

Joined Feb 18, 2011
16
probably the parallel one

#### 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

Joined Feb 18, 2011
16
oh Kirchhoff's

#### 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