# Circuit Analysis. Finding voltage. Possible trick question?

Discussion in 'Homework Help' started by VinceClortho, Mar 31, 2016.

1. ### VinceClortho Thread Starter New Member

Mar 30, 2016
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This is an extra credit question from my homework. Can someone spot a trick here or anything? My instructor likes to throw curveballs at us sometimes. How do I go about finding the voltage from A to B?

2. ### Jony130 AAC Fanatic!

Feb 17, 2009
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First find voltage at VA (voltage drop across R4) next find voltage at VB (voltage across R6) and final VAB = VA - VB.

EDIT
Also notice that R4/(R3+R4) = R6/(R5+R6) (voltage divider) so from there we can conclude that Vab = ??

Last edited: Mar 31, 2016
3. ### VinceClortho Thread Starter New Member

Mar 30, 2016
20
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Thanks for the help. I came up with 200mV.

4. ### Jony130 AAC Fanatic!

Feb 17, 2009
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How ?? This is wrong see my additional note.

5. ### MrChips Moderator

Oct 2, 2009
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No. It is a trick question. Google Wheatstone Bridge.

6. ### VinceClortho Thread Starter New Member

Mar 30, 2016
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Here are my calculations. Would you mind showing me where I went wrong?

7. ### VinceClortho Thread Starter New Member

Mar 30, 2016
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Disregard that "or 1.5V." It's not related.

8. ### Jony130 AAC Fanatic!

Feb 17, 2009
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Yep, (R5 + R6) are connected in parallel with (R3+R4). As I says early please notice that this two voltage divider (R3,R4) and (R5,R6) are the same. They have the same voltage ratio R4/(R3+R4) = R6/(R5+R6).
https://en.wikipedia.org/wiki/Voltage_divider#Resistive_divider

9. ### WBahn Moderator

Mar 31, 2012
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Just remember that Vxy is short for Vx - Vy.

So find the voltage at Node A (Va), Find the voltage at Node B (Vb). Then compute Vab = Va - Vb.

10. ### WBahn Moderator

Mar 31, 2012
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In this particular case, if you look at the values closely, you can answer the question by inspection. But, if not, just analyze the circuit as described above and you will get the answer. Once you do, then ask yourself if, in hindsight, there are things you could have spotted that would have led you to the same result with less work.

11. ### VinceClortho Thread Starter New Member

Mar 30, 2016
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Is the answer 0v? I calculated 72.2V across R4 and across R6. Another calculation came up with 79.38V across both branches (R3 and R5 included). Either way it suggests The voltage across the branches are the same which is how it should behave according to what was suggested earlier about the ratios (not to mention the nature of parallel circuits in general). I hope I'm not missing something.

12. ### crutschow Expert

Mar 14, 2008
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3,369
You are missing the fact that the answer can be determined by inspection.
You don't need to do any calculations, just observe the the voltage division ratio of R3 and R4 is the same as R5 and R6 so the voltage difference is 0V.
If you did the calculations and came up with some small voltage due to rounding errors in your calculation, some profs will take points off your (not quite correct) answer because you didn't observe that it should be 0V.

13. ### VinceClortho Thread Starter New Member

Mar 30, 2016
20
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Thanks for the help everyone.

14. ### MrAl Distinguished Member

Jun 17, 2014
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Hello,

This is not a trick question...balanced bridges are a fact of nature...it happens. In practice there may be a small difference, but in theory we can theorize that there is absolutely zero difference. Another way of putting this is that a balanced bridge is "unobservable".

If you have never dealt with resistive bridges however this may be new to you, so you just have to carefully calculate the two voltages and then subtract. Sometimes polarity matters, so Vab=Va-Vb and Vba=Vb-Va, although there may be cases where they are both the same (ideally). I dont want to give out too much just yet though.