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When a dry a cell is connected across...

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

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