An electret is a slab of insulator that keeps a fixed polarisation with no applied field
PHYSICS · Multi-part · Fermi University Physics · Electricity, Magnetism & Electromagnetism · Electric fields in matter · Electric fields in matter
Electricity, Magnetism and Electromagnetism, Ch Dielectrics: polarisation, bound charge and the displacement field
Problem
An electret is a slab of insulator that keeps a fixed polarisation with no applied field. A slab of face area A=5.0cm2 and thickness 1.0mm carries a uniform polarisation of magnitude P=6.0×10−5Cm−2 directed along the normal to its large faces. The slab is alone in vacuum, carries no free charge, and its thickness is small compared with the width of its faces, so the field inside may be treated as that of two large charged planes.
((a)) Find the bound surface charge density on each large face, with its sign, and the total bound charge on the face that P points towards.
((b)) State the bound volume charge density inside the slab.
((c)) Find the magnitude and direction of E inside the slab.
((d)) The same material is used elsewhere as a linear dielectric: a field of 1.0×106Vm−1 inside it produces a polarisation of 6.0×10−5Cm−2. Find its susceptibility χe and its relative permittivity εr.