The dielectric constant

The level of polarizability of a substance is characterized by a special quantity called dielectric constant. Consider what this value is.

Assume that the uniform field strength between two charged plates in a void is equal to ₀. Now fill the gap between them with any dielectric. The electric charges that appear at the boundary between the dielectric and the conductor due to its polarization partially neutralize the effect of charges on the plates. The field strength E of this field will become less than the field strength E₀.

Experience finds that when the gap between the plates is successively filled with equal dielectrics, the field strengths will turn out to be different. Therefore, knowing the magnitude of the ratio of the electric field strength between the plates in the absence of dielectric E₀ and in the presence of dielectric E, it is possible to determine its polarizability, i.e. its dielectric constant. This value is usually denoted by the Greek letter ԑ (epsilon). Therefore, you can write:

ԑ = E₀ / E.

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

The decrease in the interaction force between charges is caused by the 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, counteracting the field that caused their appearance. As a result, the total electric field decreases. In small fields, this phenomenon is described using the concept of dielectric constant.

The following is the dielectric constant in vacuum of various substances:

Air …………………………… .... 1,0006

Paraffin ...................... 2

Plexiglass (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 the dielectric constant of substances refer to ambient temperatures in the range of 18–20 ° . Thus, the dielectric constant of solids varies slightly with temperature, with the exception of ferroelectrics.

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

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

If two arbitrary point charges are placed in a dielectric, then the field strength created by each of these charges at the location of the other charge decreases by a factor of.. It follows from this that the force with which these charges interact with each other is also ԑ times less. Therefore , the Coulomb law for charges placed in a dielectric is expressed by the formula:

F = (q₁q₂) / (ԑₐr²).

in SI system :

F = (q₁q₂) / (4πԑₐr²),

where F - is the interaction force, q₁ and q₂, - the magnitude of the charges, ԑ - is the absolute dielectric constant of the medium, g - the distance between the point charges.

The value of ԑ can be shown numerically in relative units (relative to the value of the absolute dielectric constant of vacuum ԑ₀). The value 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 vacuum; ԑ = ԑₐ / ԑ₀ is often called the complex permittivity. The numerical value of , as well as its dimension, depend on which system of units is selected; and the value ԑ is independent. So, in the system of SSES ԑ₀ = 1 (this fourth basic unit); in the SI system, the dielectric constant of the vacuum is expressed:

ԑ₀ = 1 / (4π˖9˖10⁹) farad / meter = 8.85˖10⁻¹² f / m (in this system, ԑ₀ is a derived quantity).