Question

(a) What is a dielectric?

(b) What are polar and non-polar molecules, give examples.

(c) Define atomic polarisability

Answer 1

(a) Dielectric: A dielectric is a material in which all the electrons are tightly bound to the nuclei of atoms, so that there are no free electrons to carry current. In other words, a dielectric is a substance that does not conduct electric current through it. When an electric charge is given to a dielectric, it remains localised or fixed in the region of their initial distribution or application in contrast to conductors where the charge immediately spread on the surface.

The molecules or atoms of a dielectric can be classified as either (i) polar or (ii) non-polar.

Polar molecules: A polar molecule is one in which the centre of gravity of the positi ve charges (i.e. protons ) does not coincide with the centre of gravity of the negative charges (i.e. electrons). Due to finite separation between the positive and negative charges, polar molecules are permanent electric dipoles and have permanent electric dipole moments, e.g. HCl, NH₃, H₂O, CO₂ are polar molecules. In the absence of electric field, the dipole moment of these polar molecules point in random directions and arrange themselves in closed chains and net dipole moment is zero.

Effect of applied Electric field on polar molecules. An applied electric field merely aligns the polar molecules parallel to itself. Because the molecules are in constant thermal agitation, the alignment is not complete as shown in Fig. However, the alignment increases as the applied electric field is increased or as the temperature is decreased. The dipole moment may also be increased by the applied electric field.

Non-polar molecules: A non-polar molecule is one, in which the center of gravity of the positive charges (i.e. protons) coincides with the center of gravity of the negative charges (i.e. electrons). Thus in these molecules, there is no separation between the centers of positive and negative charges and so they have no permanent electric dipole moments.

Some common non-polar molecules are oxygen (O₂), nitrogen (N₂), hydrogen (H₂) etc.

Effect of applied electric field on non-polar molecules. When a non-polar molecule is placed in an electric field, the center of positive charge moves in the direction of field and center of gravity of the negative charge in an opposite direction. This separation of +ve and -ve charges continues till the force on either of them due to external field is completely balanced by the internal forces arising due to relative displacements. The molecule develops a dipole moment called induced moment. Such a molecule is said to be polarized.

The induced dipole moment disappears as the electric field is removed.

Atomic polarisability. If \(d\vec {l}\) is the separation between centers of positive and negative charge.

q = positive or negative charge on the atom.

Atomic dipole moment \(\vec p = d\vec l\).

The net induced electric dipole moment for an atom is proportional to the strength of applied electric field \(\vec E\)  and its direction is parallel \(\vec E\).

\(\vec P\) ∝  ε₀ 

or \(\vec P\) = α ε₀ \(\vec E\)

where α is called proportionality constant or atomic polarisability. Atomic polarisability is defined as the electric dipole moment induced in the atom by an electric field of unit strength.

Dimensions of α

= \(\frac{\text Dimensions \ of\ p }{ Dim. \ of \ \varepsilon_0\ E}\)

= \(\frac{Cm}{(C^2N^{-1}m^{-2})(NC^{-1})}\)

or Dimension of α = m³

∴ α has dimensions of volume.

Answer 2

a) What is a dielectric?

A dielectric is an insulating material that can be polarized by an applied electric field. When a dielectric is placed in an electric field, it does not conduct electricity but instead undergoes a shift in charge distribution, leading to the alignment of its molecular dipoles. This alignment reduces the overall electric field within the material. Dielectrics are used in various applications, including capacitors, where they increase the capacitance by reducing the effective electric field and thereby allowing more charge to be stored for a given voltage.

 (b) What are polar and non-polar molecules?

 Polar Molecules Polar

 molecules have a permanent electric dipole moment due to the uneven distribution of electrons between atoms with different electronegativities. This results in partial positive and negative charges within the molecule.

Examples:

Water (H₂O): Oxygen is more electronegative than hydrogen, creating a partial negative charge on the oxygen atom and partial positive charges on the hydrogen atoms.

Ammonia (NH₃):

Nitrogen is more electronegative than hydrogen, leading to a dipole moment where the nitrogen has a partial negative charge and the hydrogen atoms have partial positive charges.

 Non-Polar Molecules

Non-polar molecules either have an even distribution of electrons or their dipole moments cancel out due to symmetric arrangements. These molecules do not have permanent dipole moments.

 Examples:

Methane (CH₄):

The molecule has a symmetric tetrahedral structure with equal distribution of electrons, resulting in no net dipole moment.

Carbon Dioxide (CO₂):

Although the C=O bonds are polar, the linear geometry causes the dipole moments to cancel each other out, resulting in a non-polar molecule.

 (c) Define Atomic

 Polarizability  Atomic polarizability refers to the ability of an atom to be distorted by an external electric field, leading to the formation of an induced dipole moment. When an electric field is applied, the electron cloud around the nucleus can be displaced relative to the nucleus, creating a temporary dipole. The extent of this displacement is characterized by the polarizability, which depends on the electron cloud's size and the binding strength of the electrons to the nucleus. Atomic polarizability plays a crucial role in various phenomena, such as the dielectric behavior of materials, van der Waals forces, and the refractive index of substances