# Potentiometer circuit

1. Oct 2, 2015

### Janiceleong26

1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution
The answer is B. But I thought E1 needs to be be known first in order to find E2... And why not the resistance of the wire XY?

E1/ Rxy = E2/ Rxt

Then, in order to find Rxt, we need E1, Rxy and E2?

2. Oct 2, 2015

### Staff: Mentor

Suppose that you know E2. What can you say about the potential at T with respect to X?

Note that you're looking for the potential difference per unit length, not the resistance or resistance per unit length.

3. Oct 2, 2015

E1-E2?

4. Oct 2, 2015

### Staff: Mentor

No. E2 is not connected to point T.

5. Oct 2, 2015

### Janiceleong26

I'm confused .. Why isn't E2 connected to T?

6. Oct 2, 2015

### Staff: Mentor

Because there's the lengths XT and TY of resistive wire between E2 and T. There is some (unknown) current running through those lengths, causing potential differences.

7. Oct 2, 2015

### Janiceleong26

Okay.. So the p.d. across XT is XT/XY x E1?

8. Oct 2, 2015

### Staff: Mentor

No. Forget about E1 for now; you don't have a value for it. What you are to assume is that you know E2, and that the galvanometer reading is zero.

9. Oct 3, 2015

### Janiceleong26

Potential at T is equal to E2

10. Oct 3, 2015

### Staff: Mentor

Right! Note that it doesn't matter what E1 is. As soon as the galvanometer shows zero current, the potential at point T must match E2.

Can you now work out the potential difference per unit length of the wire XY?

11. Oct 3, 2015

### Janiceleong26

Yeah! It's E2 divide by the length XT. Thanks !