Dipole Moment and Polarity
Dipole moment occurs when atoms in the molecule share electrons unequally. It happens when one atom in the molecule has a higher electronegativity than the other. The electronegative atom pulls the shared pair of electrons more tightly, or the atom with lone pair of electrons can point the electronegativity vector in the same way. Higher the difference in electronegativity means more significant dipole moment. Let's take the example of a water molecule that contains two hydrogen atoms and one oxygen atom to understand this topic more clearly. Oxygen has a lone pair of electrons and has a higher electronegativity than oxygen. It gives hydrogen a partial positive charge and oxygen a partial negative charge.
Insulators and Dielectric Material
Dielectric materials are substances that are very poor conductors of heat or electricity. No current will flow in the dielectric if we place them in the electric field. It is because they do not contain any free or loosely bound electrons like metals. However, electric polarization can still occur in some insulators. In simple words, the insulators that undergo electric polarization when we apply an electric field are known as dielectrics. Hence, it is essential to note that every insulator material is not a dielectric substance. In dielectrics, the charges do not move but show a slight displacement from their equilibrium position. The presence of polar molecules in dielectrics leads to polarization.
Other Electric phenomena also change due to the presence of a dielectric substance. If we compare the force between two electric charges in a vacuum and dielectric medium, it will be less in a dielectric medium. Moreover, the dielectric medium has a higher amount of energy stored in an electric field per unit volume. If you fill any capacitor with a dielectric material, then it would have higher capacitance than a vacuum.
Polarity
The polarity of a molecule refers to the property by which the distribution of charge takes place between its atoms. The separation of electric charge in a molecule results in the formation of an electric dipole. The polarity of the bond arises due to the difference in electronegativity between the atoms taking part in bond formation. In simple words, electronegativity refers to the power of an atom by which it attracts the electrons towards itself during bond formation. If the difference in electronegativity between two atoms is less than 0.4, then they show no dipole moment. The molecule has a dipole moment and polarity. if the difference in electronegativities of its particles is more than 0.4.
Difference Between Polar and Nonpolar Molecules in Physics
The molecules in which there is an equal distribution of electric charges between the atoms are non-polar. The particles present in these molecules have similar levels of attraction due to which their electrical charges coincide in the centre. The lack of polarity in non-polar molecules can be because of two reasons. The first reason can be the presence of a non-polar bond. The other one can be the symmetry of the particles, which can lead to the cancellation of the polar charge. In H2 or O2, the non-polar bond is present, which makes them non-polar molecules. The CO2 comprises the polar bond, but it is still non-polar because of the cancellation of charges due to its symmetrical structure. If the difference in electronegativity of two atoms is too high, then it leads to the formation of the ionic bond. In this type of bond formation, the complete transfer of electrons takes place rather than the sharing of electrons.
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The above image shows the structure of a carbon dioxide molecule. The red part represents the oxygen atom, whereas the black part represents the carbon atom. It is a non-polar molecule because of its linear and symmetric structure.
The polar molecules are the ones that have the centres of positive and negative charges. These opposing charges can result in a net dipole moment and polarity. The asymmetric bonds can also lead to the polarity in the molecules. Some of the popular examples of polar molecules include Ammonia (NH3), Ozone (O3), Water (H2O) etc. The cancellation of the dipole is not possible in polar molecules due to the exertion of force due to charges on atoms. A polar solvent can dissolve other polar molecules easily. The random orientation of dipole moment throughout the molecule leads to the cancellation of net polarity. However, the dipole in the polar molecule aligns itself in the direction of the electric field during its presence.
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The above image shows the polar nature of the water molecule made of oxygen and hydrogen. The electronegativity of oxygen is 3.44, and hydrogen is 2.20. It makes the molecule of water polar. Here, the blue represents the partially positive charged region while the red represents the negatively charged ones.
FAQs on Dielectric Material and Dipole Moment
1. What is a dielectric material and how does it differ from a conductor?
A dielectric material is an electrical insulator that can be polarised when subjected to an external electric field. Unlike a conductor, which possesses a large number of free electrons to conduct electricity, the electrons in a dielectric are tightly bound to their respective atoms. When an electric field is applied, these charges do not flow but rather undergo a slight displacement, creating internal electric dipoles.
2. What is meant by an electric dipole moment?
An electric dipole moment is a measure of the separation of positive and negative charges within a system, essentially quantifying the overall polarity of a molecule. It is a vector quantity, represented by the symbol p (or µ), and is calculated as the product of the magnitude of one of the charges (q) and the distance (d) separating them: p = q × d. The direction of the vector points from the negative charge towards the positive charge.
3. Can you provide some examples of polar and non-polar dielectric materials?
Certainly. Dielectric materials are categorised based on the nature of their molecules:
- Polar Dielectrics: These consist of molecules that have a permanent or intrinsic dipole moment. Common examples include water (H₂O), ammonia (NH₃), and hydrochloric acid (HCl).
- Non-Polar Dielectrics: These consist of molecules that do not have a permanent dipole moment. Examples include hydrogen (H₂), oxygen (O₂), carbon dioxide (CO₂), and materials like mica and paraffin wax.
4. What is dielectric polarisation and how does it occur?
Dielectric polarisation is the process where the atoms or molecules of a dielectric material form induced or aligned dipoles under the influence of an external electric field. This alignment results in the creation of an internal electric field that opposes the external field. In polar dielectrics, the existing permanent dipoles rotate to align with the field. In non-polar dielectrics, a dipole moment is induced by the field as it slightly displaces the electron clouds relative to the atomic nuclei.
5. How does placing a dielectric slab in an external electric field reduce the net field inside it?
When a dielectric is placed in an external electric field (E₀), it becomes polarised. This polarisation generates an internal electric field (Eᵢ) within the dielectric that is directed opposite to the external field. The net electric field (E) inside the material is the vector sum of these two fields: E = E₀ - Eᵢ. Because the induced internal field counteracts the external field, the resultant electric field inside the dielectric is significantly reduced.
6. What is the fundamental difference in how polar and non-polar molecules react to an electric field?
The core difference lies in their initial state and mechanism of polarisation:
- A polar molecule possesses a permanent dipole moment. In an external field, these dipoles experience a torque that aligns them with the field's direction.
- A non-polar molecule has zero permanent dipole moment. An external field induces a temporary dipole moment by distorting the electron cloud, separating the centres of positive and negative charge. This induced moment disappears once the external field is removed.
7. Why is it advantageous to use dielectric materials inside capacitors?
Using a dielectric in a capacitor offers two key advantages. Firstly, it increases the capacitance. The dielectric reduces the net electric field and thus the potential difference between the plates for a given charge, increasing capacitance (C = Q/V). Secondly, it increases the dielectric strength, which is the maximum electric field the capacitor can withstand before the insulating property of the dielectric breaks down. This allows the capacitor to store more energy at a higher voltage.
8. What is the physical significance of the dielectric constant (K)?
The dielectric constant (K), or relative permittivity (εᵣ), is a dimensionless quantity that indicates the extent to which a material can reduce an external electric field. It is defined as the ratio of the field strength in a vacuum (E₀) to the net field strength within the dielectric material (E): K = E₀ / E. A material with a high K value is very effective at insulating and storing electrical energy, making it a superior choice for applications like high-performance capacitors.
