The net electric force between two charged objects in vacuum with charges `q1` and `q2` is given by the Coulomb law:

`F = (q1*q2)/(4*pi*epsilon0*R^2)`

where `R` is the distance between the two charges.

With the data in the text one has

`F = (7*10^-6*(-3.2*10^-6))/(4*pi*8.854*10^-12 *0.2^2) =-5.032 N`

The negative sign shows that the force is opposing the vector `R(=R_12) =R_2-R_1` drawn from first charge to the second charge and thus it is an attraction force.

Since the charge of an electron is `e =-1.6*10^-16 C` the number of electrons the positive cork lost is

`N_1 = q_1/|e| =(7*10^-6)/(1.6*10^-19) =4.375*10^13 "electrons"`

and the number of excess electrons the negative cork has is

`N_2 = q_2/e =(-3.7*10^-6)/(-1.6*10^-19) =2.3125*10^13 "electrons"`

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