Electric Dipole
An electric dipole consists of a pair of equal and opposite point charges separated by some small distance.
The line connecting the two charges defines a direction in space. The middle point of locations of – q and + q is called the centre of the dipole. It is represented by O.
This does not mean that the field of the electric dipole is zero.
Atoms or molecules of ammonia water, alcohol carbon dioxide HC lets. Are some of the examples of electric dipoles because in their cases the centers of positive and negative charge distributions are separated by some small distance shows molecule of water (H2O) with their nuclei represented by dots? The electric dipole moment p points from the (negative) oxygen side to the (positive) hydrogen side of the molecule.
Dipole moment
Dipole moment (p) is a measure of the strength of electric dipole. It is a vector quantity whose magnitude is equal to product of the magnitude of both charge and the distance between them.
P = q (2a)
|p| = q (2a)
By conventions the direction of p is from negative charge to positive charge.
The SI unit of dipole moment is coulomb –meter (c – m).
If charge q gets larger, and the distance 2a gets smaller and smaller keeping the product |p| = q x 2a = constant, we get what is called and ideal dipole or point dipole. Thus an ideal dipole is the smallest dipole having almost no size.
Physical significance of electric dipoles
The study of electric dipoles is important for electrical phenomena in matter. Matter, as we know consists of atoms or molecules which are electrically neutral. In a molecule there are positively charged nuclei and negatively charged electrons. If the centre of mass of positive charges coincides with the centre of mass of negative charges, the molecule behaves as a non – polar molecule. On the contrary, if the center of mass of positive charges does not coincide with the centre of mass of negative charges the molecule behaves as a polar molecule and it possesses some intrinsic or permanent dipole moment. In the absence of an external electric field the dipole moments of different, molecules in a piece of matter are randomly oriented, so that net dipole moment of the piece is zero. When an external electric field is applied the polar molecules tend to align themselves along the field and some net dipole moment develops. The piece of matter is said to have been polarized.
When non-polar molecules are subjected to the action of an external electric field, the centre of mass of positive and negative charges in the molecule get displaced in opposite direction. Thus, the external field induces some dipole moment in the molecule in the direction of the field. The induced dipole moments of different molecules in the sample add up vectroially to produce some net total dipole moment.
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