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| Dielectrics and Polarisation |
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| Dielectrics are non-conducting substances, they have no charge carriers or no free electrons. If an external field is applied, it turns out that charges are induced on the surface which in turn produces a field and opposes the external field. The opposing field does not exactly cancel the external field but only reduces it. |
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| To understand this concept, one has to look into the charge distribution of a dielectric in its molecular level. The molecules of a dielectric are classified as polar or non-polar. |
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| The "Polar molecules" is one in which the centre of gravity of protons (positive charges) and electrons (negative charges) do not coincide. They have an asymmetrical distribution of charge and have permanent dipole moments in the range 10-30 cm. |
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| Examples : H2O, CO2, NO2 etc. |
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| In the absence of an external electric field, the electric dipole moment of molecules is in random direction and cancels with each other and the average electric dipole moment per unit volume of a dielectric is zero. |
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| In the presence of an external electric field, the molecules align in the direction of the electric field, and is complete when working at a low temperature and stronger electric field. |
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| The "Non-polar" is that molecule in which the centre of gravity of protons (positive charges) and electrons (negative charges) coincide. In normal state, they have no dipole moment. |
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| Example O2, N2, H2 etc. |
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| When an external electric field is applied, the protons as a whole are pulled in the direction of the electric field, while electrons are pulled in the opposite direction. The separation of charges continues till the external electric field are balanced by the internal forces. This creates two new centres of charge, the molecule is said to be polarised and is known as induced electric dipole. The dipole moment so acquired is known as induced electric dipole moment. |
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The induced dipole moment is proportional to the applied field and is almost independent of the temperature. The direction of induced
dipole moment  is parallel to the direction of electric field  and for
a single polar atom |
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 |
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| where 'a' is known as atomic polarisability |
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| The dimension of 'a' are the same as that of volume. S.I. unit of 'a' is m3. |
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| Polarisation is the alignment of the dipole moments of the permanent or induced dipoles in the direction of the applied electric field. |
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| A uniformly polarized dielectric amounts to induced surface |
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| charge density, but no volume charge density |
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| The induced charges on the surface of a dielectric are due to thin surface layers as shown in figure. There is an excess of negative charge in one layer and excess of positive charge in the other layer. The induced charges are on the surface of the dielectric and are bound to their respective molecules. So these charges are called 'bound charges'. Net charge per unit volume is zero. Thus, the dielectric as a whole remains electrically neutral. |
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| It is clear from the above that polarisation of dielectric does not result due to net increase of charge by merely due to relative displacement of charges. |
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