Energy of dielectric systems

Question

In Griffith's book "introduction to electrodynamics" third edition, page 193, in the section of "Energy in Dielectric Systems" he states that this method of calculating the total energy of the system consists of three parts: the electrostatic energy of the free charge, the electrostatic energy of the bound charge, and the 'spring' energy associated with the dielectric material: $$W_{tot} = W_{free} + W_{bound} + W_{spring}.$$ He states that the last two are equal and opposite since the bound charges are always in equilibrium, and hence the net work done on them is zero.

Could someone elaborate on what is meant by this equilibrium and why this implies that the net work on them is zero?

Thanks.

Answer 1

I think what he is saying is that when you have separated the bound charge that bound charge is able to do some work for you that is the electrostatic energy of the bound charge.
However to move the bound charge to that new position you have had to do an amount of work pulling the springs equal to the electrostatic energy gained due to separating the bound charges.

Contrast this with the free charges which are not attached to anything (there are no springs) and all the work that you did is stored as the electrostatic energy of the free charge.

Contrast this again to assembling just free and bound charges where, in a sense, no work has to be done to "create" the bound charges by distorting the molecules of which the dielectric is made.