Question

Explain the use of potentiometer to determine the internal resistance of a cell.

Hint:

A potentiometer can accurately measure the internal resistance of a cell by comparing its emf to the voltage across an external load.

Answer 1

Step 1: Understanding the Setup.
In the potentiometer, the unknown cell is connected in series with a known external resistance. A galvanometer is used to detect the potential difference, and a sliding contact (or jockey) is used to vary the length of the potentiometer wire.
Step 2: Procedure.
1. First, use the potentiometer to measure the potential difference across the cell when no current is drawn from it. This gives the emf \( E \) of the cell. 2. Then, a small known resistance is connected in series with the cell. The sliding contact is adjusted until there is no current in the galvanometer, indicating the potential difference across the cell and the external resistance is equal. 3. Using Ohm's Law, calculate the internal resistance \( r \) of the cell using the relation: \[ r = \frac{E
- V}{I} \] where \( E \) is the emf of the cell, \( V \) is the measured potential difference, and \( I \) is the current through the external resistance.
Step 3: Conclusion.
By determining the emf and the voltage drop across the internal resistance, the internal resistance \( r \) of the cell can be accurately calculated using the potentiometer.