Answers

2014-06-10T17:06:32+05:30
Apply the electric field from the top and drop an electron from top 
force on electron due to field  = qE (upward)
force on electron due to gravity  = mg(downward)
if electron is at rest due to both the forces then 
qE = mg
m = qE/g
we know strength of field , charge on electron , and value of g .so we calculate m.

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2014-06-11T08:29:38+05:30