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    Question

    The balance point in a potentiometer for a cell of emf

    1.5 V is 60 cm. If a resistance of 5 ╬й is connected in parallel with the cell and the new balance point shifts to 50 cm, what is the internal resistance of the cell?
    A 1 ╬й Correct Answer Incorrect Answer
    B 2 ╬й Correct Answer Incorrect Answer
    C 3 ╬й Correct Answer Incorrect Answer
    D 4 ╬й Correct Answer Incorrect Answer

    Solution

    Let E be the emf of the cell and r be its internal resistance. In the first case, the balance point is at l тВБ тАЛ= 60 cm. The potential difference across the balance point is proportional to the emf of the cell: E тИЭ l тВБ E = kl┬н тВБ , where k is the potential gradient. 1.5 = k ├Ч 60 k = 1.5/60 = 1/40 тАж(1) In the second case, a resistance R=5╬й is connected in parallel with the cell. The effective voltage across the potentiometer wire is now the terminal voltage V of the cell. The internal resistance of the cell is 1╬й.

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