# Potentiometer experiment

1. Jan 4, 2015

### gracy

1. The problem statement, all variables and given/known data
Zero deflection in galvanometer

2. Relevant equations

If potential difference across between two points between which galvanometer is kept is zero galvanometer shows zero deflection.
3. The attempt at a solution
But my doubt starts from this video at 3:33 to 3:44 teacher says that potential at p is same as potential at b,provided that connecting wires (represented in green color in video)are having no resistance.I think it can only happen if internal resistance of galvanometer is same as resistance between length AP.So that potential at p is same as potential at b,and galvanometer will show zero deflection.Am i right?

2. Jan 4, 2015

### Staff: Mentor

Gracy, very few homework helpers will be interested in viewing nearly 4 minute of extraneous material in order to suss out the details of your question. How about you post a circuit diagram and your own analysis of the problem with some details about your concerns?

Also, what does "potential difference across between two points between which galvanometer is kept is zero galvanometer shows zero deflection" mean? My English language parser has failed me on this one, and I'm a native English speaker. Can you clarify your query?

3. Jan 4, 2015

### gracy

It's not about English language.It is just because of my my haste.I didn't check my question once before posting.Actually i am very nervous about my upcoming test.Sorry,I'll do the needed correction.But i don't think it was too difficult to interpret .I am rewriting that sentence
'If potential difference across two point is zero(or if two points are at same potential) ,galvanometer shows zero deflection'.So overall i had made 2 mistakes i used both( across and between ).And second mistake i made was in order to explain the conditions clearly I included 'between which galvanometer is kept'I should have thought that if i really need to explain correctly and clearly i need to post a circuit diagram .Words(that too written in disturbed state of mind)are not going to make up for circuit diagrams which are backbone of electronics.

Last edited: Jan 4, 2015
4. Jan 5, 2015

### gracy

1. The problem statement, all variables and given/known data
Zero deflection in galvanometer

2. Relevant equations

If potential difference across two points is zero ,galvanometer shows zero deflection.
3. The attempt at a solution
But my doubt starts from this video at 3:33 to 3:44 teacher says that potential at p is same as potential at b,provided that connecting wires (represented in green color in video)are having no resistance.I think it can only happen if internal resistance of galvanometer is same as resistance between length AP.So that potential at p is same as potential at b,and galvanometer will show zero deflection.Am i right?

5. Jan 5, 2015

### Staff: Mentor

No thanks. :(

6. Jan 5, 2015

### gracy

a is at same potential as A(as connecting wire is of negligible resistance.And p is particular point is found to be at same potential as b because galvanometer shows zero deflection.My question is why p is a particular point to be at same potential as b,i mean if from s to b the connecting wire is of negligible resistance than any point between length AB will have same potential as b.I think it is because galvanometer internal resistance.So that means galvanometer resistance is same as resistance present in length Ap.Am i right?

7. Jan 5, 2015

### gracy

I didn't understand.But it is certainly not answer of my question.

8. Jan 5, 2015

### Staff: Mentor

The photo is welcome.

AB is a resistance wire; it has resistance all along its length. By sliding the jockey along it, you can set point P to have any potential between the battery's (+) potential and ground. (It might help if you showed the wire AB of a different thickness or color, so as to clearly distinguish it from hookup wire.)
What is that element between points a and b?

Last edited: Jan 5, 2015
9. Jan 5, 2015

### gracy

What is that element between points a and b?[/QUOTE]
It is resistance.

10. Jan 5, 2015

### ehild

11. Jan 5, 2015

### Staff: Mentor

I think you should be considering there is a voltage source between points a and b, not a resistance. The potentiometer allows you to determine what voltage that voltage source is. That is the purpose of a potentiometer. The galvanometer indicates when the voltage at the sliding jockey equals the voltage of the unknown voltage source Vab.

If the unknown voltage source has resistance, too, that doesn't matter, because when the galvanometer indicates a balance point, there is zero current from/to that voltage source, so there is no potential drop across any internal resistance.

It's not surprising you were confused, when you thought it was just a resistance.

Last edited: Jan 5, 2015
12. Jan 5, 2015

### ehild

It was drawn a resistance there but it was said that the potential difference across that resistance is measured. That resistance was part of a circuit, containing voltage source. A very confusing video !

13. Jan 5, 2015

### Staff: Mentor

Ah, so the seed of confusion was set in the video.

All sorted now, I hope.

14. Jan 5, 2015

### gracy

Have you also assumed resistance free or of negligible resistance conducting wire?

15. Jan 5, 2015

### Staff: Mentor

That is always a default assumption, in any circuit. But based on what I wrote, you yourself should now be able to highlight those connecting wires where extra resistance will make no difference to the result. What is the key factor to identifying those wires?

16. Jan 5, 2015

### gracy

I think if two points are at same potential than no current will flow ,so extra resistance will not matter.

17. Jan 5, 2015

### Staff: Mentor

That's correct. So you just need to now identify those two points that will have equal potential.

18. Jan 6, 2015

### gracy

A and a ,p (if zero deflection point is shown by galvanometer at point p)and b.But i am not getting one thing if two points have resistance between them then how can they be at equal potential as presence of resistance results in potential drop.

Last edited: Jan 6, 2015
19. Jan 7, 2015

### Staff: Mentor

At balance, how much current is flowing through the galvanometer and its connecting wires?

Perhaps you are still not appreciating that between a and b there is a battery of some to-be-determined potential? So when the potentiometer slider is moved to the exact same potential as that of the battery, those voltages will be equal.

20. Jan 7, 2015

### gracy

I have a question.,Why there is always one null deflection point?Is it not possible that we slide jockey but there is no such point obtained where galvanometer shows zero deflection?