Dipole

The term electric dipole stands for two charged objects carrying equal but opposite electric charges and are separated by a distance. The field created by a dipole appears as the cube or the third power of the distance from the dipole and its directional variation depends on whether it is being measured along the line separating the charges or perpendicular to it.

If a neutral atom is placed in an electric field, its positively charged particles will be pulled in one direction while the negatively charged particles will be pulled in some other direction. This creates a separation of charge in the direction of the field thus creating a magnetic dipole. A dipole may also undergo a torque in an electric field which tend to rotate in such a way that the axis line up in the direction of the electric field. At instances when the dipole is perpendicular to the electric field, the intensity of torque is the greatest. It is not only affected by the strength of the electric field but also on the separation of the two electric charges as well as their magnitude.

Electric Dipole Moment

Let us assume that each electric charge is of magnitude q and the distance of the negative charge to the positive charge is‘d’ then the electric dipole moment is defined as the product ‘qd’. Electric dipole moment is obviously a vector quantity and always heads from negative charge to positive charge. As we have discussed that dipole moment is qd so its unit is coulomb metre. 

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Induced Electric Dipole

It follows by the name itself that induced dipole means the dipole which has been created. The dipole which is created by displacing the centre of the negative cloud of electrons from the nucleus by an external electric field is called an induced dipole.  

The atoms might lose their dipolarity when their external fields re removed. A water molecule is an example of a permanent electric dipole. Two hydrogen atoms stick on one side and along with the oxygen atom they form an angle of 105° at the vertex. The oxygen side is considered to be slightly negative while the hydrogen side is a bit positive. 

For molecules there can be two more types of dipoles in addition to induced dipoles:

Permanent dipoles: when two atoms in a molecule have drastically different electronegativity then we have permanent dipoles. While one of the atom attracts more electrons than the other becoming more negative, the other becomes more positive. Molecules exhibiting dipole moment of this type are called polar molecules.

Instantaneous dipoles: as the name suggests, these are the dipoles which occur by chance as a result of concentration of electrons in a particular area in a molecule thus creating a temporary dipole. 

We discuss some of the results on dipoles:

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