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Explain , using suitable diagrams , the difference in the behaviour of a (i) conductor and (ii) dielectric in the presence of external electric field . Define the terms polarisation of a dielectric and write its relation with susceptibility. |
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Answer» Solution :(i) When a conductor is placed in a uniform electric field `vecE_(0)` , free electrons present inside conductor drift in a direction opposite to the external electric field , till the electric field `vecE` , produced by drift of free electrons within the conductor just becomes equal and opposite to the external field `vecE_(0)` . Thus the net electric field `E_0 - E.` at any point inside the conductor is zero . (ii) When a dielectric slab is placed in the electric field `vecE_(0)` . the electrons in atoms / molecules of dielectric get PULLED in a direction opposite to the applied electric field. The separation between the charges is such that the force due to the external electric field is just balanced by the restoring force due to internal electric fields in the atom/molecule . The molecules , thus develop an induced DIPOLE moment and the dielectric is said to be polarised . Let `vecE_(p)` be the internal electric field in the dielectric due to polarisation of charge . Thus , net electric field in the presence of dielectric becomes `E = E_0 - E_p` It is observed that `E = (E_(0))/(E_(0) - E_(p)) = (E_0)/(K)` , where K is the dielectric constant of the given dielectric . The polarisation vector `vecp` of a dielectric is DEFINED as the dipole moment developed in the dielectric per unit VOLUME when placed in an external electric field . Thus , polarisation vector `vecp = ("Net dipole moment")/("Volume of dielectric")` For linear isotropic dielectrics , the polarisation vector `vecp` is found to be directly proportional to external electric field `vecE_(0) `i.e, `vecp prop vecE_(0)` or `vecP = x_(e) vec E_0` Here `X_(e)` is a constant which is characteristic of a dielectric and is known as the electric SUSCEPTIBILITY of given dielectric . |
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