Question

How many electrons should be removed from a coin of mass 1.6g so that it may just float in an electric field of intensity 10 to the power 9 N/C, directed upward ?

Answer 1

electric force F due to the field E = q E
     we know,F= n e E,      
as because  q = n e,     
       where  e = magnitude of charge on electron
        where n = number of electrons to be removed
         ⇒E = 1 * 10⁹ N/C
         ⇒m = 1.6 gms
we know gravity force = m*g
⇒n*1.6*10⁻¹⁹ *1*10⁹ =1.6*10⁻³ *10 
         n=10⁷=10million 
∴ no of electrons to be removed is 10 million