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JEE Main & Advanced JEE Main Paper (Held On 26 May 2012)

  • question_answer
    In an experiment of potentiometer for measuring the internal resistance of primary cell a balancing length \[\ell \]is obtained on the potentiometer wire when the cell is open circuit. Now the cell is short circuited by a resistance R, If R is to be equal to the internal resistance of the cell the balancing length on the potentiometer wire will be.   JEE Main Online Paper (Held On 26-May-2012)  

    A) \[\ell \]                

    B)                        \[2\ell \]

    C)                         \[\ell /2\]           

    D)                        \[\ell /4\]

    Correct Answer: C

    Solution :

                    Balancing length \[l\] will give emf of cell \[\therefore \]\[E=Kl\] Here K is potential gradient. If the cell is short circuited by resistance ?R' Let balancing length obtained be V then\[V=kl'\] \[r=\left( \frac{E-V}{V} \right)R\] \[\Rightarrow \]\[V=E-V\]                           \[[\because r=R\,\text{given}]\] \[\Rightarrow \]\[V=E-V\]\[\Rightarrow \]\[2V=E\]or\[2Kl'=Kl\] \[\therefore \]\[l'=\frac{l}{2}\]


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