Finding Vg with Wheatstone Bridge: R1=40, R2=70, R3=50, R4=20

[mjosephh]

1.
I am having trouble finding the Vg for a problem involving a Wheatstone Bridge circuit. If I am labeling the problem correctly, R1=40, R2=70.0 R3=50.0 and R4=20.0.
Voltage - in = 20.0V

I am using the formula: ((R1*R3) - (R2*R4) *V-in) / (R1 + R2) * (R4+R3) to find Vg between points B and D.


R1=40 (resistance between points A and B)
R2=70 (resistance between points B and C)
R4=20 (resistance between points A and D)
R3=50 (resistance between points C and D)
+ -
V-in = 20V






2. I think my problem is that I don't know what to do with the resistance of Vg (between points B and D), which is 60.0 ohms.



3. The answer for Vg in the book is 0.94 V, and I am getting 1.56V. What am I doing wrong?

 

[mjosephh]

I attached a graphic of the problem

Please see image.

 

[mjosephh]

Can anyone help with this? I have tried everything!

 

[bengaltiger14]

Picture is very unclear. Could you try and repost it?

 

[bengaltiger14]

nevermind about reposting. I see it clearly now. You are trying the find the voltage across that 60 ohm resistor?

 

[mjosephh]

I will attach a larger image, but you can try and expand the other one to view it better.
Thanks for looking into this.

 

[mjosephh]

Yes I am trying to find that voltage between B and D. I used the above formula (which is given when all resistances are known, but it didn't work out. I am sure that I need to distribute the 60 ohm resistance across the other resistors somehow, but I am not sure how to. I couldn't find anything online to help me. As most problems are geard to find an unknown resistance.

 

[Astronuc]

Well, I dislike using Wikipedia, but try this

http://en.wikipedia.org/wiki/Wheatstone_bridge

Follow the equations and rewrite according to the orientation of one's problem as necessary, then follow the steps, but realize that the bridge is unbalanced and I_g is not zero.

The input voltage is given, and one needs to determine the potential difference across R_g, which is related to the potential differences V_AB and V_AD.