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 center behavior center 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 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
M⁰L¹T¹A¹
M⁰L¹T¹A¹.
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,
⇒τ = 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 center of mass of positive charge coincides with the center, charges, s 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. We aim to calculate the maximum torque.
We know that the torque acting on a dipole placed in an external electric field is given by,
⇒τ = 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
FAQs on Electric Dipole Moment
1. What is the electric dipole moment and how is it mathematically defined in physics?
The electric dipole moment is a vector quantity defined as the product of the magnitude of one of the charges and the distance separating the charges in an electric dipole. Mathematically, it is given by P = q × 2a, where q is the magnitude of the charge and 2a is the separation between the charges.
2. What is the direction of the electric dipole moment vector?
The direction of the electric dipole moment vector is always from the negative charge towards the positive charge in the dipole.
3. Which physical quantities are used to express the SI unit and dimensional formula of electric dipole moment?
The SI unit of electric dipole moment is Coulomb-meter (C·m). Its dimensional formula is [M0 L1 T1 A1].
4. How does an electric dipole behave when placed in a uniform external electric field?
When an electric dipole is placed in a uniform external electric field, it experiences torque but no net force. The torque (τ) is given by τ = P × E = PE sinθ, where θ is the angle between the dipole moment and the electric field. The torque is maximum when θ = 90°.
5. What are polar and non-polar molecules with reference to electric dipole moment?
Polar molecules have a permanent electric dipole moment because their centers of positive and negative charge do not coincide. Non-polar molecules have coinciding centers of positive and negative charges, so their net dipole moment is zero.
6. Explain the significance of electric dipole moment in determining the polarity of molecules.
The electric dipole moment helps determine the polarity of a molecule. Molecules with a nonzero dipole moment are polar and can interact with electric fields, while molecules with zero dipole moment are non-polar.
7. Calculate the net electric dipole moment of a water molecule given that the dipole moment of each O–H bond is 0.9187 D.
The net dipole moment of water is the vector sum of the two O–H bond dipole moments. For water, the net dipole moment = 0.9187 D + 0.9187 D = 1.837 D.
8. Why does a system with zero net charge still produce an electric field, with reference to electric dipoles?
Even if a system's net charge is zero, if it has a separation of opposite charges (an electric dipole), it will create an electric field in the surrounding space due to the arrangement of these charges.
9. How is the electric dipole moment applied in understanding chemical bonding and molecular structure?
Electric dipole moment is used to calculate the percentage ionic character of a bond, estimate bond angles, study electric polarization, and determine the geometry of molecules.
10. What are some common misconceptions students have about electric dipole moment and how does understanding it help avoid errors in exams?
Common misconceptions include thinking the dipole moment is zero if net charge is zero, or believing the field from a dipole is always zero. Understanding the definition and vector nature of electric dipole moment helps avoid such errors and prepares students for application questions in board exams.
