Q.Explain the difference between a polar and a nonpolar molecule, and describe briefly how each responds when a dielectric made of such molecules is placed in an external electric field.

Nonpolar molecules (e.g. $\text{N}_2$) have coincident positive/negative charge centres with no field present; an applied field $E_0$ separates them slightly, inducing a small dipole moment along the field. Polar molecules (e.g. $\text{H}_2\text{O}$) already carry a permanent dipole moment, randomly oriented by thermal motion with no field, partially aligning under an applied field via the same $U=-\vec{p}\cdot\vec{E}$ torque mechanism. Either way, uncancelled bound surface charge appears at the dielectric's faces perpendicular to the field -- negative near the field's positive source, positi …