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 = ΦL₁

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 = ΦL₂

So, we have E/V = L₁/L₂

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/L₂

Using the above equation, we can find the internal resistance of a given cell.