CBSE Class 12-science Answered
Principle of working: The voltage drop along the wire is directly proportional to the length of the wire. The potentiometer works without drawing any current from the voltage source
We can also use a potentiometer to measure internal resistance of a cell. For this the cell (emf E) whose internal resistance (r) is to be determined is connected across a resistance box through a key K2, as shown in the figure.
With key K2 open, balance is obtained at length L1 (AN1).
Then,
E = L1
When key K2 is closed, the cell sends a current (I ) through the resistance box (R). If V is the terminal potential difference of the cell and balance is obtained at length L2 (AN2), then
V = L2
So, we have E/V = L1/L 2
But, E= I (r + R) and V = IR. This gives
E/V = (r+R)/R
On comparing the above equations, we get,
(R+r)/R = L1/L 2
Using the above equation, we can find the internal resistance of a given cell.