2 identical copper sphere are separated by 1m in vaccum,how many electrons would have to be removed from 1 sphere and added to other so that they now attract each other with a force of 0.9N

Since the electrons will be traveling from one sphere to another, so if the charge acquired by one sphere would be +q, the other would be -q.

 

Hence F = kq1q2/r^2

0.9 = |9*10^9 (q)(-q)|/(1)^2

0.9 = 9*10^9 q^2

q^2 = 10^-10

q = 10^-5 

 

Hence, the electrons transferred = q/e = 10^-5 /1.6E-19 = 6.25*10^13