The dielectric constant

The level of polarizability of a substance is characterized by a special value, which is called the permittivity. Consider what this value is.

Let us assume that the intensity of a homogeneous field between two charged plates in a vacuum is equal to E. Now fill the gap between them with any dielectric. Electric charges, which appear on the boundary between the dielectric and the conductor due to its polarization, partially neutralize the effect of charges on the plates. The intensity E of this field will be less than the intensity E.

Experience shows that when the gap between plates is successively filled with equal dielectrics, the field strengths will be different. Therefore, knowing the value of the ratio of the electric field strength between the plates in the absence of an insulator E and in the presence of an insulator E, its polarizability, i.e., can be determined. Its dielectric constant. This value is usually denoted by the Greek letter ԑ (epsilon). Therefore, we can write:

Ԑ = E₀ / E.

The dielectric constant shows how many times the field strength of these charges in a dielectric (homogeneous) will be less than in a vacuum.

The decrease in the interaction force between the charges is due to the processes of polarization of the medium. In an electric field, electrons in atoms and molecules decrease with respect to ions, and a dipole moment arises . Those. Those molecules that have their own dipole moment (in particular water molecules) are oriented in an electric field. These moments create their own electric field, opposing the field that caused their appearance. As a result, the total electric field decreases. In small fields this phenomenon is described by the concept of dielectric permittivity.

Below is the dielectric constant in vacuum of various substances:

Air ......................................... .... 1,0006

Paraffinning .............................. .... 2

Plexiglas (plexiglass) ...... 3-4

Ebonite ................................. .. ... 4

Porcelain ................................. .... 7

Glass ................................................................. .4-7

Mica ................................. ... .4-5

Natural silk ............ 4-5

Slate .............................. 6-7

Amber .............................. ... ...... 12.8

Water .................................... ... .81

These values of dielectric permittivity of substances refer to ambient temperatures in the range of 18-20 ° C. Thus, the dielectric constant of solids varies insignificantly with temperature, except ferroelectrics.

On the contrary, it decreases with gases due to the increase in temperature and increases in connection with the increase in pressure. In practice, the dielectric constant of air is taken as unity.

Impurities in small quantities have little effect on the level of dielectric permittivity of liquids.

If two arbitrary point charges are placed in a dielectric, then the field strength created by each of these charges at the point of finding another charge decreases by a factor of ԑ. From this it follows that the force with which these charges interact with one another is also ԑ times smaller. Therefore , the Coulomb law for charges placed in a dielectric is expressed by the formula:

F = (q₁q₂) / (ԑₐr²).

In the SI system :

F = (q₁q₂) / (4πԑₐr²),

Where F is the force of interaction, q₁ and q₂, are the charges, ԑ is the absolute permittivity of the medium, and r is the distance between the point charges.

The value of ԑ can be numerically shown in relative units (with respect to the absolute dielectric permittivity of vacuum ԑ₀). The quantity ԑ = ԑₐ / ԑ₀ is called the relative permittivity. It reveals how many times the interaction between charges in an infinite homogeneous medium is weaker than in a vacuum; Ԑ = ԑₐ / ԑ₀ is often called the complex dielectric constant. The numerical value of ԑ₀, as well as its dimensionality, are dependent on which system of units is chosen; And the value of ԑ does not depend. Thus, in the CASEC system, ԑ₀ = 1 (this is the fourth basic unit); In the SI system, the dielectric constant of the vacuum is expressed by:

Ԑ₀ = 1 / (4π˖9˖10⁹) Farad / meter = 8.85˖10⁻¹² p / m (in this system ԑ₀ is a derivative value).