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When a dry a cell is connected across...

When a dry a cell is connected across a bulb and the circuit is switched on , then the brightness of bulb goes on decreasing . Explain.

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If the emf of cell is `epsi` and its internal resistance is r, then current through the bulb of resistance R is `l= (epsi)/(R + r)` .
emf of the cell `epsi = V_(+) + V_(-)` is the difference of chemical potentials of electrodes, which is constant for a give cell and is independent of time of use .
Internal resistance of the cell is inversely proportional to charge density). With passage of time of use , ion density in the electrolyte (as resistively is inversely proportional to charge density ). With passage of time of use , ion density goes on decreasing so internal resistance r of cell goes on increasing . Thus,the current l through the bulb goes on decreasing .
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