Measurement of unknown resistance using potentiometer

[Anind]

[Mentor's Note: Thread moved to homework forums from another forum]

Figure shows a potentiometer circuit for comparison of two resistances. The balance point with a standard resistor R = 10.0 Ω is found to be 58.3 cm, while that with the unknown resistance X is 68.5 cm. Determine the value of X.

In the solution of this problem every book using this method:-

my question is how can we take equal i for each resistance?

 

[NascentOxygen]

We've seen this arrangement presented before, and for a time it had us purplexed. I believe the conclusion was that the lower-drawn source ε[/size] is actually a voltage source having significant resistance. So mark in some fixed internal resistance there, and re-examine the question.

Does your textbook give an explanation of the circuit and its operation, and mentioning that source having internal resistance? Without the recognition of that internal resistance, the circuit makes no sense.

 

[Anind]

We've seen this arrangement presented before, and for a time it had us purplexed. I believe the conclusion was that the lower-drawn source ε is actually a voltage source having significant resistance. So mark in some fixed internal resistance there, and re-examine the question.

Does your textbook give an explanation of the circuit and its operation, and mentioning that source having internal resistance? Without the recognition of that internal resistance, the circuit makes no sense.

i copied the whole question from textbook. nothing was told about internal resistance. in general we have to take an arbitrary internal resistance here.

 

[NascentOxygen]

Yes, arbitrary, but not zero.

 

[Anind]

Yes, arbitrary, but not zero.

ok if there is a non-zero internal resistance, then what about the current through resistor R and X? are they equal? why? please answer it. I need it very badly.

 

[NascentOxygen]

ok if there is a non-zero internal resistance, then what about the current through resistor R and X? are they equal? why? please answer it. I need it very badly.

R and X are different valued resistances, so when separately connected to source ε[/size] will have different currents.

 

[NascentOxygen]

In your first post you provided an "explanation" of how the arrangement works. I don't understand that description.

 

[Anind]

R and X are different valued resistances, so when separately connected to source ε will have different currents.

So you agree that the solution given above is wrong right?

 

[NascentOxygen]

So you agree that the solution given above is wrong right?

It certainly seems wrong.

You are in luck; this exact question has featured in this forum last year. See whether it's illuminating: https://www.physicsforums.com/threa...rison-of-two-resistances.770943/#post-4855402

There is another related to potentiometers, might be worth reading: https://www.physicsforums.com/threads/potentiometer-experiment.790534/page-2

 

[Anind]

It certainly seems wrong.

You are in luck; this exact question has featured in this forum last year. See whether it's illuminating: https://www.physicsforums.com/threa...rison-of-two-resistances.770943/#post-4855402

There is another related to potentiometers, might be worth reading: https://www.physicsforums.com/threads/potentiometer-experiment.790534/page-2

thanks