A long needle-shaped electret of length and radius is uniformly polarised along its…

PHYSICS · Multi-part · Fermi University Physics · Electricity, Magnetism & Electromagnetism · Electric fields in matter · Electric fields in matter

The Boundary-Value Problem, Ch Polarisation and bound charge, completed

Problem

A long needle-shaped electret of length

L = 10\,\mathrm{cm}

and radius

R = 1.0\,\mathrm{mm}

is uniformly polarised along its own axis,

P = 2.0\times10^{-5}\,\mathrm{C\,m^{-2}}

. ((a)) Find

\rho_b

, the bound charge on the curved side surface, and the bound charge on each flat end. Check that the total bound charge is zero. ((b)) Using the depolarising factor for a long needle polarised along its axis, find the depolarising field inside the needle, away from its ends.

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