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  4. A negatively charged oil drop is prevented from falling under gravity by applying a vertical electric field 100 text Vm-1 . If the mass of the drop is 1.6× 10-3g, the number of electrons carried by the drop is (g=10 text ms-2)

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Q. A negatively charged oil drop is prevented from falling under gravity by applying a vertical electric field $ 100\text{ }V{{m}^{-1}} $ . If the mass of the drop is $ 1.6\times {{10}^{-3}}g, $ the number of electrons carried by the drop is $ (g=10\text{ }m{{s}^{-2}}) $

KEAMKEAM 2010Electric Charges and Fields

Solution:

$ qE=mg $
or $ ne\times E=mg $
$ n\times 1.6\times {{10}^{-19}}\times 100=1.6\times {{10}^{-6}}\times 10 $
$ n={{10}^{12}} $