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what are the formula in finding internal resistance in potentiometer

Asked by anu Mishra 25th January 2016, 6:20 PM
Answered by Expert
Answer:

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,

begin mathsize 12px style straight I equals fraction numerator straight E over denominator straight R plus straight r end fraction Hence comma space straight V equals IR equals straight E minus Ir end style

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, 

begin mathsize 12px style straight r over straight R equals fraction numerator straight E minus straight V over denominator straight V end fraction THe space internal space resistance space of space the space cell straight r equals straight R fraction numerator straight E minus straight V over denominator straight V end fraction end style

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;

begin mathsize 12px style straight r equals straight R fraction numerator open parentheses straight l subscript 1 minus straight l subscript 2 close parentheses over denominator straight l subscript 2 end fraction end style



Answered by Expert 25th January 2016, 6:25 PM
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