In most molecules, the centres of positive charges and of negative charges lie at the same place. Therefore, their dipole moment is zero. CO2 and CH4 are of this type of molecules. However, they develop a dipole moment when an electric field is applied. But in some molecules, the centres of negative charges and of positive charges do not coincide. Therefore they have a permanent electric dipole moment, even in the absence of an electric field. Such molecules are called polar molecules. Water molecules, H2O,is an example of this type. Various materials give rise to interesting properties and important applications in the presence or absence of electric field.
An electric dipole is a pair of equal and opposite point charges -q and q, separated by a distance 2a. The direction from q to -q is said to be the direction of the dipole. p=q×2a where p is the electric dipole moment pointing from the negative charge to the positive charge.
Force on electric dipole
A small electric dipole having dipole moment p is placed along X-axis, as shown in the figure. A semi-infinite uniformly charged di-electric thin rod placed along x axis, with one end coinciding with origin. If linear charge density rod is +λ and distance of dipole from rod is ′a′, then calculate the electric force acting on dipole. Electric field at A due to dipole, EA=r32KP where K=4πϵo1 EA=(x+a)32KP Force at A due to dipole, FA=qEA=(λdx)×(x+a)32KP FA=(x+a)32λKPdx Total force exerted due to dipole on the rod, FD=∫0∞(x+a)32λKPdx FD=2λKP×−2(x+a)21∣∣∣∣∣0∞ ⟹FD=a2λKP Now force exerted by rod on dipole is equal and opposite to FD (Newton's law) FR=−FD=4πϵoa2−λP