On one hand, in this “Measurement of polarisation” lab manual, polarization is surface charge density, $P=frac{Q}{A}$, in other words, charge an polarization are essentially the same thing.
On the other hand, in this “Ferroelectric Hysteresis Measurement & Analysis” report, the authors say:
Charge and Polarisation are strictly speaking different but for
materials with a high relative permittivity we can assume they are
equal.
So if “charge”, Q, and “polarization”, P, are different (by not just the area involved, A), what is the difference? And how the value of the permittivity is related?
EDIT:
According to Wikipedia, the related equation is for charge displacement field, $mathbf{D} equiv varepsilon_{0} mathbf{E} + mathbf{P}$, from which the polarization can be extracted: $mathbf{P} = mathbf{D} – varepsilon_{0} mathbf{E}$.
However, according to Hall et al., $mathbf{P}simeq mathbf{D} = varepsilon_0 varepsilon_r^* mathbf{E}$, which seems contrary to the previous equation.
Anyway, how the charge Q and the relative permittivity $varepsilon_r$ of the sample are related and what is the correct equation?