Q.What is a polar molecule ? Give an example.

A slab where free charges fully cancel the internal field is a conductor; one where bound charges only partially reduce it is a dielectric.

(i) In slab A, the free surface charges $\sigma_{free}$ set up an internal field $E_{in}$ that exactly cancels the external field, $E_0+E_{in}=0$, i.e. the net field inside is zero. This is the defining electrostatic behaviour of a conductor: free charges redistribute on the surface until the interior field vanishes entirely.

In slab B, both a free-type surface charge $\sigma$ and a bound/polarisation charge $\sigma_p$ appear, and the internal field only partly cancels the applied field ($E_0+E_{in}\ne0$, net field inside is reduced but not zero). This is the behaviour of a dielectric: the field polarises the (bound) charges, which produce an opposing field that reduces, but does not eliminate, the net field inside.

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A polar molecule has a permanent electric dipole moment because its centres of positive and negative charge are separated.

In a polar molecule, the constituent atoms have different electronegativities, so the shared bonding electrons are pulled asymmetrically and the centre of positive charge does not coincide with the centre of negative charge. This permanent charge separation gives the molecule a fixed electric dipole moment $\vec p$, which exists even in the absence of an external electric field. When an external field is applied, such molecules additionally tend to orient themselves along the field direction.

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