Increase in internal energy due to polarization of a dielectric

[Fibo112]

I have a plate capacitor with a given charge. It is then dipped in a dielectric fluid and I must determine which height the fluid reaches(all the necessary parameters are given). I can solve this problem if I make the following assumption: the polarization of the dielectric does not( or approximately does not) change its internal energy.

Is this a reasonable assumption? If so why?

 

[DrDu]

Is this a reasonable assumption?

No, it isn't. If you model polarisation P as a density of dipoles, what will be their energy in an electric field?