# Electric Dipole Moment

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## What is an Electric Dipole?

Before we start with the electric dipole moment we must understand what do we mean by the term dipole and dipole moment. An electric dipole is a pair of equal and opposite charges separated by a considerably short distance. In an electric dipole, the magnitude of both the charges will be the same, we can not consider a pair of two charges with different magnitudes.

Since an electric dipole is a pair of equal and opposite charges, therefore the total charge in an electric dipole will be zero. While studying electric dipole we should understand that the total charge of an electric dipole is zero does not mean that field of an electric dipole is zero.

### Dipole Moment

Whenever two equal and opposite charges are brought together there will be either some attraction or repulsion force between them. Some examples of electric dipoles are HCl, H₂O, CH₃COOH, etc. These molecules will have fixed dipole moments because the centre of the positive charge will not be coinciding with the negative charge. The electric dipole moment is a vector quantity, it has a specific direction and magnitude. The electric dipole moment physics plays an important role in order to understand the concept of polarisation. Now, let us define electric dipole moment, or in other words, let us have look at how do we define dipole moment of an electric dipole.

### Define Electric Dipole Moment:

Now, the dipole moment definition is given as the product of the magnitude of charges and the separation between them. The dipole moment determines the strength of an electric dipole to produce the electric field. It is denoted by P and it is a vector quantity.

Mathematically, the electric dipole moment is given by:

Consider two point charges q and -q place on dipole axis separated by a distance 2a, then the electric dipole moment is,

⇒ P = q x 2a………..(1)

Where,

q - The magnitude of the charge

2a - The separation between two charges

Equation (1) is known as the electric dipole moment formula physics.

The direction of the dipole moment is always from the negative charge to the positive charge. The SI unit of the electric dipole moment is Coloumb-meter(C-m). The dimensional formula of an electric dipole is [M⁰ L¹ T¹ A¹] .

### Dipoles in an External Electric Field

Consider an electric dipole placed in an external electric field. The electric dipole will experience some force and is known as the torque. The torque is the force exerted on the dipoles placed in an external electric field and is given by,

⇒ $\tau$  = P x E = PE Sin θ ………(1)

Where,

P - The dipole moment

E - The applied external field

### Significance of Electric Dipole and the Electric Dipole Moment:

• The concept of an electric dipole is not only having importance in physics but it is an equally valid and prominent topic in chemistry as well.

• We know that most of the matter made up of atoms and molecules will be electrically neutral. Depending upon the behaviour of the pair of charges, the molecules are subdivided into two types,

• Polar molecules: If the centre of mass of positive charge doesn’t coincide with the centre of mass of negative charge then it is known as a polar molecule.

• Non-Polar molecules: If the centre of mass of positive charge coincides with the centre of mass of negative charge then it is known as a Non-Polar molecule.

• Polar molecules possess permanent dipole moments. These dipoles are randomly oriented in the absence of an external electric field. On applying an electric field, the polar molecules will align themselves in the direction of the electric field.

• In a system, if the net charge is zero, that does not mean that there will be no electric field or the electric field will be absent. This was more evident by studying the electric dipole moment. Therefore, the study of an electric dipole is important.

• The study of dipoles and the dipole moments will help us understand the concept of polarization.

### Example:

1. When an Electric Dipole P is Placed in a Uniform Electric Field E, at What Angle Between P and E  the Value of Torque will be Maximum?

Ans: Given that an electric dipole is placed in a uniform electric field. Our aim is to calculate the maximum torque.

We know that the torque acting on a dipole placed in an external electric field is given by,

⇒ $\tau$ = P x E = PE Sin θ ………(1)

Where,

P - The dipole moment

E - The applied external field

Therefore, the value torque will be maximum when the angle between the electric field and the dipole moment is 90⁰.