CBSE Class 12-science Answered

Hi,

If a cell of emf E and internal resistance r, connected to an external resistance R, then the circuit has the total resistance (R+r). The current I in the circuit is given by,

This means, V is less than E by an amount equal to the fall of potential inside the cell due to its internal resistance.

From the above equation, 

Using a potentiometer, we can adjust the rheostat to obtain the balancing lengths l₁ and l₂ of the potentiometer for open and closed circuits respectively. 

Then, E= k l₁   and   V = k l₂  ; where k is the potential gradient along the wire. 

Now we can modify the equation for getting the internal resistance of the given cell, by using the above relations as;