The force between them will be maximum if the two parts are having the charge.

In which medium the electrostatic force is maximum?

Coulomb force is maximum in air/vaccum because value of dielectric constant of air/vaccum is 1, for other medium its value will always be greater than 1.

What is the maximum force between two charges?

Hint: The force of attraction between two charged particles at a distance is given by Coulomb's law. The force of attraction between two stationary, electrically charged particles is maximum when the first derivative of the maximum force with respect to one of the charges is equal to zero.