Draw a circuit for determining internal resistance of a cell using a potentiometer. Explain the principle on which this method is based.


The circuit to determine the internal resistance of a cell using a potentiometer of is shown below: -



The potentiometer is a device used to measure the internal resistance of a cell and is used to compare the e.m.f of two cells and potential difference across a resistor. The relation between potential difference, emf, and internal resistance of a cell is given by




Here, the V=IR;


So we can write the above equation as;



The above equation reflects that the potential drop V is less than E by the amount of fall in potential inside the cell due to internal resistance.


From the above equation,



The internal resistance of the cell is given as;



where r is the internal resistance of the cell and R is the resistance for the potentiometer.


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