Lesson 8 of 13 · 7 min
Dielectrics and polarisation
NCERT §2.10
Meera slides a glass sheet between the charged demo plates. Glass has no free electrons, so it cannot cancel the field the way metal does. Does it do anything at all?
The lesson in notes
In short
Dielectrics are insulators with no (or negligible) free charge. A field cannot drive charges through them, but it can shift or turn the charges inside molecules.
A conductor cancels an applied field inside it completely; a dielectric only weakens it.
Non-polar molecules (O₂, H₂) have their positive and negative charge centres together and no permanent dipole moment. A field pulls the centres apart and induces a dipole along the field.
Polar molecules (H₂O, HCl) have a permanent dipole moment. Without a field they point every which way and cancel; a field partly lines them up, against the scrambling of thermal motion.
Polarisation P is the dipole moment per unit volume. For a linear isotropic dielectric P = χₑε₀E, where χₑ is the electric susceptibility.
Inside a uniformly polarised slab the dipole ends cancel, but the two faces normal to the field carry bound surface charges +σₚ and −σₚ.
The field of these bound surface charges opposes the applied field, so the net field inside the dielectric is smaller.