# WheatStone bridge

Discussion in 'Electronics Homework Help' started by Dabu, Aug 19, 2018.

1. ### Dabu

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Mar 25, 2018
Recently i studied the wheatstone bridge in me physics book in class but i could not understand it properly. I asked the teacher about my confusion but he also dont know. i have posted the picture of topic(wheatstone bridge). My question is ;
1.Why current I1 is following through point A to D in loop ADBA.
2.Why current I2 is following through point D to C in loop DCBD.

My confusion is that why current are following through these two paths. As current always flow through level of a higher potential to a level of lower potential but here it seems opposite.

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2. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,419
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Jan 21, 2010
So you understand that there are 4 current paths from C to A, and that at any time only 2 or 3 of them are active?

3. ### WHONOES

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May 20, 2017
Essentially, 13.27 at the bottom of the page says it all.

4. ### Laplace

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Apr 4, 2010
This question suggests an unfamiliarity with the method of mesh current analysis. Suffice it to say that mesh currents are not real currents but merely an analysis tool for deriving the equations that govern a circuit's operation. The other prime method for deriving circuit equations is node voltage analysis, except in this case the node voltages are the real voltages. If your physics book has a section describing mesh current analysis method, study it.

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5. ### Ratch

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Mar 10, 2013
Because that is the direction defined by I1. The total current is I3 - I1 . Since I3 is greater than I1, the total current direction will be from point D to point A.

Same type of answer as above. You have to consider the total current, which includes I3.

Yes, higher voltage to lower voltage is the mathematical convention for current direction. But, don't forget to take the total current into consideration.

By the way, charge does not flow twice. Current flow means "charge flow flow". You should instead say current is present, current exists, or charge flow.

Ratch

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6. ### Ratch

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Mar 10, 2013
I aver that calculated mesh current does represent the real current that is present in a particular mesh.

Ratch

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7. ### Laplace

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Apr 4, 2010
Of course, you would. But you can only make that statement after having done the mesh analysis and solved the circuit equations. One might find that the mesh current flows in the opposite direction of the real current.

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8. ### Ratch

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Mar 10, 2013
I did specify calculated current in my remark above. Anyway, the calculated mesh current will always give the "real" current direction according to the mathematical convention to which all ammeters are marked. If a negative current is encountered in the calculations, it means that the assumed direction is opposite to the mathematical convention. Then, the assumed direction is simply reversed.

Ratch

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9. ### mrinal

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Aug 23, 2019
I m also confused with the same thing, like in below figure

I know that at balance point the current will be same in R1, R2 and in R3,R4 and the current through ammeter will zero. but no body explains why the current through ameter is zero at one point of time, I means current should always flow through the ammeter even in smaller amount. If there is resistance in ammeter, which obviously it should have, then there would be voltage drop and the current should flow. Then how it is possible to achieve the balance point.
May be I have missed something, but i couldn't find the answer. Above image is taken from this Wheatstone bridge and they also just said that "at balance point this will happen...." but why??

10. ### iimagine

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Oct 23, 2013
In order for current to flow, there must be a difference in voltage. Since both junctions are at the same voltage current cannot flow. It does not matter what resistance is in between.

11. ### Ratch

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Mar 10, 2013
To iimagine:
"Current flow" means "charge flow flow" which is redundant and ridiculous. Don't use technical slang when explaining a precise concept. You should instead say current exists, or current is present, or charge flow, or sometimes just plain current. It reduces the confusion and bemusement of the gleaning from the meaning of the word.
Ratch

12. ### Martaine2005

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May 12, 2015
@Ratch , he didn’t say “current flow”.
He said “for current to flow”.
If ‘current’ by itself meant ‘current flow’ it would be one word ‘currentflow’.
However, current to flow, or charge to flow is acceptable in the English language.
I think pedantic comes to mind. Afterall, we know what it is.

Martin

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13. ### Ratch

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Mar 10, 2013
Martaine2003,
You cannot have current without something flowing (water, charge, syrup, etc) Therefore, "current to flow" is as redundant as "current flow". "Charge to flow" is OK, but "current to flow" is not. Knowing what something means even if it is presented wrong is gauche. Ever see a astronaut "walk" in space? Not much traction, is there?

Ratch

14. ### Martaine2005

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May 12, 2015
Firstly, it’s an astronaut

15. ### Martaine2005

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May 12, 2015
Sorry, hit the send button.
I will respond tomorrow.

16. ### Martaine2005

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May 12, 2015
@Ratch
I know technically you are correct.
But you have to remember that this is not a physics forum. Every member here is probably guilty of answering an OP question by "current flow"..
This is a 'help' forum. And we

17. ### Ratch

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Mar 10, 2013
Don't stop now.

You can be Lord Byron the poet and say "The stream flowed through the meadow."
Or you can be be Sir Isaac Newton and say "A stream exists in the meadow".
I believe the more technical approach is appropriate for this forum.

Wouldn't it be clearer if the OP said something like this below without the "current flow phraseology"?

"... and the current will be zero, but nobody explains why the current is zero at this one point in time. I mean, current should always be present even in a small amount. Obviously there is some resistance in the ammeter, therefore a voltage drop exists, and a current should be present. ..."

Ratch

18. ### iimagine

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Oct 23, 2013
Are you serious? lol

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1,088
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Mar 10, 2013

20. ### Martaine2005

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May 12, 2015
@Ratch I really want to say that you have too much time on your hands. You can hi-light a physics question and tell the forum they are wrong.
Current flow in the physics world is incorrect.
However, current being a quantity, can flow.
So, the current to flow (quantity) is acceptable. IMHO.

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