Electric Charge & Its Nature- Work Done by Electric Dipole

An electric dipole consists of a pair of positive and negative charges separated by a small distance. Here, the charges are of equal magnitude but opposite in nature. Several fundamental particles of matter, or subatomic particles, are electrically charged. An electron, for instance, is negatively charged, while a proton is positively charged. We can find the formation of an electric dipole in nature. It has several practical applications. This article explains the meaning of an electric dipole, its properties, and its effects. Some of the practical examples of dipole systems are HCl(Hydrochloric Acid), H2O(Water), CH3COOH(Acetic acid), etc. The couple is defined as the combination of two forces having equal magnitude but opposite directions. The direction of the electric force field within a dipole system is from the negative charge to the positive charge within the dipole.SI unit of Electric dipole moment is C-m(Coulomb Metre).The dimension of the Electric dipole moment is M⁰L¹T¹A¹.

What is an electric dipole?

An electric dipole consists of equal and opposite charges, which are separated at some distance apart.

Electric charges are of two types: positive and negative. They attract each other. We can interpret different behaviours of opposing charges by keeping them at a distance.

An electric dipole is caused due to the non-uniform charge distribution of protons and electrons in a molecular structure. If the centres of positive and negative charges do not coincide, an electric dipole is formed.

Work done by an electric dipole

The couple in physics is defined as a pair of forces having equal magnitude but opposite directions. When a dipole system is present, they are associated with a couple of forces because the positive charge exerts the same force which the negative charge offers but in the opposite direction.

Let us assume T be the torque acting on a dipole system. E be the electric field within the dipole system-generated due to charge separation.

So as per the definition, we have:

T=PXE (Cross product of P and E)

T=P*E*sinθ

Now work done(DW) in rotating the dipole by a small angle, say dθ is given by 

dw=Tdθ

Putting the value of T, we get 

dw=P*E*sinθ*dθ

On integrating both sides, we get 

W=-P*E[cosθ2-cosθ1]

θ2 and θ1 are the final and initial rotation angles of the dipole direction and the electric field, respectively.

Special case:

• Now, if θ1 is 0, the expression becomes 

          W=-P*E[cosθ2-1]

• If a dipole is rotated by 180, then the work done is

W=2*P*W

Electric dipole moment

Qualitative definition:

In physics, the electric dipole moment is simply defined as the product of the distance between the charges and their magnitude.

Whenever two charges of equal magnitude but opposite in charge are brought closer, they experience an attractive force. This system is known as an electric dipole. Some examples of electric dipoles which exist in practical life include HCl(Hydrochloric Acid), H2O(Water), CH3COOH(Acetic acid), etc.

Mathematical definition:

Let us assume that q1 and q2 be two charges present .d be the distance between the charges. Hence by definition, the electric dipole moment of the two charged particles is defined as :

P=q*2*d

Here P represents the electric dipole moment of the system. (Note that the electric dipole system is defined only for two charged particles, but the overall calculations related to the dipole, like the dipole moment calculation, can be extended to multiple charged particles too.)

• The dipole moment is directed from a negative charge to a positive charge.

• C-m is the SI unit of the Electric dipole moment(Coulomb Meter).

Putting the respective dimensions in the formulae, we can see the dimension of the Electric dipole moment is 

M⁰L¹T¹A¹.

The dipole moment is a vector quantity since it has both magnitudes and directions.

Dipole moment-Its physical significance

The concept of an electric dipole is important in physics, but it has importance in other branches of science like chemistry, bioscience, etc. In most of the molecules where the centre of positive charges coincides with the centre of negative charges, the molecule experiences no dipole moment. Such molecules are known as non-polar molecules, and they have distinct properties like solubility in water but insoluble in organic solvents etc. One example of such molecules is CO2 (Carbon Dioxide).On the other hand, those molecules whose centre of positive and negative charge does not coincide are known as polar molecules, and hence they are soluble in water but insoluble in organic solvents.

Conclusion

The dipole moment is a fundamental physics concept that involves two charged particles. It is defined as the product of the charges of each charged particle and the separation distance between them. Even though it is only described for two charged particles, the theory can be applied to numerous particle systems. Some realistic dipole system examples include HCl (Hydrochloric Acid), H2O(Water), CH3COOH(Acetic acid), and so on. The couple is defined as the interaction of two forces of equal magnitude but opposing orientations. In a dipole system, the force, or electric field, is directed from the negative to the positive charge within the dipole. C-m is the SI unit for electric dipole moment (Coulomb Meter).The dimension of the Electric dipole moment is M⁰L¹T¹A¹. The key physical significance of the dipole system is that in those molecules in which the centre of positive charge coincides with the centre of the negative charge, they are known as non-polar molecules. They’re insoluble in water but soluble in organic solvents. On the other hand, those molecules in which the centre of positive and the negative charge do not coincide are polar. They dissolve in water and are insoluble in organic solvents.