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
     F = n e E,       as  q = n e,     
         e = magnitude of charge on electron,
         n = number of electrons removed.
         E = 1 * 10⁹ N/C
         m = 1.6 gms

    Gravity force = m g

So     n * 1.6 *10⁻¹⁹ * 1* 10⁹  = 1.6 * 10⁻³ * 10 
         n = 10⁷ = 10 million 

Answer 2

Answer:Electric force F due to the field E = q E

    F = n e E,       as  q = n e,     

        e = magnitude of charge on electron,

        n = number of electrons removed.

        E = 1 * 10⁹ N/C

        

   Gravity force = m g

        m = 1.6 gms

         g = 9.8m/s

So     n * 1.6 *10⁻¹⁹ * 1* 10⁹  = 1.6 * 10⁻³ * 9.8

        n = 9.8 × 10⁷ electrons