什么是 Polarisation and bound charges？

A dielectric is an insulator whose molecules acquire (or already own) electric dipole moments. The material’s response is summarised by the polarisation $\vect P$: the dipole moment per unit volume, $\vect P = n\langle\vect p\rangle$ for $n$ molecules per unit volume. A polarised block carries bound charges — not free to leave, but perfectly real: a surface density $\sigma_{\text{b}} = \vect P\cdot\vect n$ where the polarisation meets the surface, and a volume density $\rho_{\text{b}} = -\nabla\cdot\vect P$ wherever it is non-uniform. A uniformly polarised slab is exactly equivalent to two sheets of charge $\pm P$ on its faces: all its interior dipole heads and tails cancel.