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Nilanjan Bhowmick AIR 3, CSIR NET (Earth Science)
Ruby negi
see, when you find e.f outside the cylinder then the e.f is due to whole charge inside the cylinder that's why we take charge is equal to charge density multiply total volume of cylinder, but when you find e.f inside the cylinder then e.f is only due to cylinder of radius r ( less than R) not because of whole cylinder ( of radius R) that's why we take charge is equal to charge density multiply volume of cylinder of radius r..