Q.Explain the behaviour of dielectrics in an external field.
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Start your 14-day free trial to unlock the full solution →An external field polarises a dielectric - by aligning existing molecular dipoles (polar dielectrics) or by inducing new ones (non-polar dielectrics) - producing bound surface charges whose field opposes the applied field, reducing the net field inside the material.
Dielectrics are insulators whose molecules may be polar (having a permanent dipole moment, e.g. water, HCl) or non-polar (no permanent dipole moment, e.g. O2, N2, CO2), and their behaviour in an external field differs accordingly, though the end result - polarisation - is the same in kind.
Polar dielectrics: In the absence of a field, the permanent molecular dipoles are randomly oriented due to thermal motion, so the net dipole moment of the material is zero. When an external field E is applied, the torque pE sin(theta) on each dipole tends to align it along the field, so the dipoles become partially aligned along E (not perfectly, due to thermal agitation). This gives the material a net dipole moment along E.
Non-polar dielectrics: These molecules have no permanent dipole moment; the centres of positive and negative charge coincide. When placed in an external field, the field pulls the positive and negative charges of each molecule slightly apart, inducing a small dipole moment in each molecule, aligned along E. This is called induced polarisation.
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