Wiktor Wise
Electric displacement, also known as electric flux density, is a fundamental concept in physics that describes the displacement of electric charge across a conductor in an electric field. It is denoted by the letter 'D' and is measured in coulombs per square meter (C/m^2) in the International System of Units (SI). Electric displacement is particularly concerned with free electric charges and excludes the charges present in molecules or neutral atoms. This phenomenon is integral to understanding the behaviour of dielectric materials when exposed to electric fields, as it helps determine the density of electric flux within the charged field.
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What You'll Learn
Electric displacement field
Electric displacement, denoted by D, is the charge per unit area that would be displaced across a layer of conductor placed across an electric field. It is also known as electric flux density. The SI unit of electric displacement is Coulomb per meter square (C/m2).
The electric displacement field is a vector field that appears in Maxwell's equations. It combines the electromagnetic effects of polarization and an electric field in an auxiliary field. It plays a significant role in the physics of phenomena such as the capacitance of a material, the response of dielectrics to an electric field, and the creation of voltages and charge transfer due to elastic strains.
The electric displacement field is used in dielectric materials to determine the response of the materials when an electric field is applied. Dielectric materials are electrical insulators that can be polarized when an electric field is applied, resulting in dielectric polarization. This polarization density is used to express the density of induced electric dipole moments in a dielectric material.
The electric displacement field is related to the electric field (E) and the polarization density (P) through the permittivity of the material (ε). The relation is given by D = εE, where ε is the ability of the material to pass electric flux lines. This equation connects the number density of flux lines, D, with a force per flux line term, E.
The earliest known use of the term "electric displacement field" was in 1864 by James Clerk Maxwell in his paper "A Dynamical Theory of the Electromagnetic Field." However, it was Oliver Heaviside who reformulated Maxwell's complicated equations into the modern form we use today.
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Electric flux density
The term electric displacement was first used in 1864 by James Clerk Maxwell in his paper "A Dynamical Theory of the Electromagnetic Field". Maxwell introduced the term D, specific capacity of electric induction, in a form different from the modern and familiar notations. Oliver Heaviside reformulated Maxwell's complicated equations into the modern form.
Integrating the electric flux density over a given surface gives the electric flux. If the surface is closed and the result is multiplied by ε0, the product coincides with the charge enclosed by the surface due to Gauss's law.
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Dielectric materials
When a dielectric material is placed in an electric field, the electric charges do not flow through the material. Instead, they shift slightly from their average equilibrium positions, causing dielectric polarisation. This means that positive charges are displaced in the direction of the field, while negative charges shift in the opposite direction. This creates an internal electric field that reduces the overall electric field within the dielectric material.
The susceptibility or permittivity of a dielectric material refers to how easily it can be polarised when subjected to an electric field. It also refers to the material's electrical permeability, or the amount of electrical energy stored in the electric field when voltage is applied to it. The lower the dielectric loss, or the energy dissipated in the form of heat, the more effective the substance is as a dielectric material.
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Vacuum permittivity
Electric displacement, denoted by D, is the charge per unit area that is displaced across a layer of the conductor placed across an electric field. It is also known as the electric flux density.
In the 2019 revision of the SI, the elementary charge is fixed at 1.602176634 x 10^-19 C and the value of the vacuum permittivity must be determined experimentally. The requirement is that force should be measured in newtons, distance in meters, and charge to be measured in the engineers' practical unit, the coulomb, which is defined as the charge accumulated when a current of 1 ampere flows for one second. This shows that the parameter ε0 should be allocated the unit C^2 * N^-1 * m^-2 (or an equivalent unit – in practice, farad per meter).
In electrical engineering, ε0 is used as a unit to quantify the permittivity of various dielectric materials. The permittivity is often represented by the relative permittivity εr, which is the ratio of the absolute permittivity ε and the vacuum permittivity ε0. This dimensionless quantity is also often ambiguously referred to as permittivity. The permittivity is a thermodynamic function of state and can depend on the frequency, magnitude, and direction of the applied field. The SI unit for permittivity is farad per meter (F/m).
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Polarization density
Electric displacement, denoted by D, is the charge per unit area that would be displaced across a layer of conductor placed across an electric field. It is also known as electric flux density. In physics, the electric displacement field is a vector field that appears in Maxwell's equations. It accounts for the electromagnetic effects of polarization and that of an electric field, combining the two in an auxiliary field.
The electric dipole moment is a vector quantity with a well-defined direction from the negative charge to the positive charge. When a material is put in an electric field, the material's electronic and ionic positions slightly shift, changing the polarization density. This change in polarization density results in electric susceptibility and permittivity.
In some cases, P is known, but in most cases, the polarization density is not known a priori and is induced by the field. The polarization density can be used to determine the surface polarization charge density at the interface of a material surrounded by free space.
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Frequently asked questions
Electric displacement, denoted by D, is the charge per unit area that is displaced across a layer of conductor placed across an electric field. It is also known as electric flux density.
The SI unit of electric displacement is Coulomb per meter square (C m-2).
Electric displacement plays a major role in the physics of phenomena such as the capacitance of a material, the response of dielectrics to an electric field, and the creation of voltages and charge transfer due to elastic strains.
