Beatrice Lang
Electric polarization is a phenomenon that occurs when there is a relative shift of positive and negative electric charges within an insulator or dielectric, induced by an external electric field. This occurs when the electric field distorts the negative electron cloud surrounding the positive atomic nuclei, causing a slight separation of charges and resulting in one side of the atom becoming somewhat positive and the other side somewhat negative. Electric polarization can be measured using the electric dipole moment, which is the distance between the shifted centres of positive and negative charges multiplied by the amount of charge. The polarization density, denoted as P, describes how a material responds to an applied electric field and can be used to calculate the resulting forces.
| Characteristics | Values |
|---|---|
| Definition | A slight relative shift of positive and negative electric charge in opposite directions within an insulator, or dielectric, induced by an external electric field. |
| Formula | Electric dipole moment divided by volume. |
| SI Units | Coulombs per square meter (C/m2). |
| Other Names | Polarization density. |
| Calculation | Requires choosing a unit cell, which may result in different values for P but not for the microscopic electric field E. |
| Non-linear Media | Described by the field of nonlinear optics. |
| True Resistance | Given by the average of the resistance measured before and after polarity reversal. |
| Effect of Sand | Slows down polarization saturation but enhances polarization. |
| Effect of Silica Fume | Does not slow down polarization saturation but slightly diminishes polarization. |
| Effect of Temperature Increase | Enhances polarization due to increased ionic mobility. |
| Effect on Permittivity | Impacts the Coulomb force between two point charges in a material. |
| Effect on Piezoelectric Materials | Causes polarization due to mechanical stress. |
| Effect on Dipolar Molecules | Thermal agitation results in the cancellation of the net dipole moment. |
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What You'll Learn
Electric dipole moment
Electric polarization refers to the relative displacement of negative and positive charges in atoms or molecules, caused by an external electric field. It is the measure of the extent to which an electric field can induce the separation of charges in a dielectric material. One of the measures of polarization is the electric dipole moment.
The electric dipole moment is a measure of the separation of positive and negative electrical charges within a system, or a measure of the system's overall polarity. It is a vector quantity, with a defined direction from the negative charge to the positive charge. This convention is only followed in physics; in chemistry, the direction is taken to be the opposite. The SI unit for the electric dipole moment is the coulomb-metre (C·m), and another unit of measurement used in atomic physics and chemistry is the debye (D).
The simplest example of an electric dipole is a pair of electric charges of two opposite signs and equal magnitude, separated by a distance. This combination can be called an electric dipole, with the negative charge referred to as "-q" and the positive charge as "+q". The midpoint between the two charges is called the centre of the dipole. The dipole moment is particularly useful when considering an overall neutral system of charges, such as a pair of opposite charges or a neutral conductor in a uniform electric field.
The dipole moment can be calculated from electronic structure theory, either as a response to constant electric fields or from the density matrix. However, these values may not be directly comparable to experimental results due to potential nuclear quantum effects. More accurate dipole moments can be obtained using coupled cluster theory, and reasonable estimates can be calculated using density functional theory. The dipole moment of a molecule can also be calculated based on its molecular structure using group contribution methods.
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Polarization density
Electric polarization is a phenomenon of the relative displacement of the negative and positive charges of atoms or molecules. It can also be described as the orientation of existing dipoles toward the direction of the electric field, or the separation of mobile charge carriers at the interfaces of impurities or other defect boundaries, caused by an external electric field.
The electric dipole moment is a vector quantity, expressed in SI units as coulombs*meters (C*m). Electric polarization is defined as the quotient of the electric dipole moment per unit volume. Polarization density is denoted mathematically by P and is expressed in SI units as coulombs per square meter (C/m2).
The non-uniqueness of P, the polarization density, is not problematic because every measurable consequence of P is a consequence of a continuous change in P. For example, when a material is placed in an electric field, its electronic and ionic positions shift slightly, changing P and resulting in electric susceptibility.
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Dielectric materials
Electric polarization refers to the relative displacement of positive and negative charges in atoms or molecules, caused by an external electric field. Dielectric materials are insulating materials that can be polarized by an applied electric field. They are also known as electrical insulators or non-conducting materials.
When a dielectric material is placed in an electric field, the charges do not flow through the material as they do in a conductor. This is because they have no loosely bound or free electrons that can move through the material. Instead, the charges shift slightly from their average equilibrium positions, causing dielectric polarization. Positive charges are displaced in the direction of the field, and negative charges shift in the opposite direction. This creates an internal electric field that reduces the overall field within the dielectric.
The dielectric constant, or relative permittivity, of a material quantifies its ability to polarize in response to an electric field. It is the ratio of the electric displacement field to the applied electric field. The dielectric constant is an important term because it is related to another term called electronic polarizability, which occurs in all materials and is one of the main mechanisms driving dielectric polarization.
Dielectric polarization can be induced, permanent, or a combination of both. It involves the alignment of electric dipoles within the material in response to an external electric field. These dipoles create an internal electric field that opposes the external field, weakening the overall electric field within the dielectric.
There are different types of polarization mechanisms in dielectric materials, including electronic polarization, ionic polarization, and orientation polarization. Electronic polarization is inherent in dielectric materials, while ionic polarization occurs in ionic crystal elements such as NaCl, KCl, and LiBr when an external field is applied. Orientation polarization results from a permanent dipole, such as the angle between oxygen and hydrogen atoms in a water molecule, which retains polarization even without an external electric field.
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Ionic trapping
Electric polarization refers to the relative displacement of positive and negative charges within an atom or molecule, caused by an external electric field. This phenomenon is observed in insulators or dielectrics, where the electric field causes a slight shift of charges, making one side of the atom or molecule slightly positive and the other slightly negative.
Now, focusing on ionic trapping in the context of electric polarization:
The ions in the electrolytic solution experience a force that drives them towards the electrode, a process known as drift. Simultaneously, they undergo diffusion, a random motion due to thermal energy. The complex interplay between drift and diffusion influences the concentration and distribution of ions around the electrode, affecting the system's electrical properties.
The presence of a structured medium, such as a porous electrode, introduces the concept of ionic trapping. In this scenario, mobile ions interact chemically with the structured medium, potentially becoming trapped within it. This trapping phenomenon significantly influences the electrical impedance of the sample. The standard PNP model, often used to describe ion behaviour, needs to be extended to account for the trapping effect.
The irreversible trapping of ions in the medium leads to a non-solenoidal total electric current, which poses challenges in determining the electric impedance of the cell. However, when the trapping reaction is reversible, the total electric current becomes solenoidal, and the electric impedance can be calculated using standard methods.
In conclusion, ionic trapping plays a significant role in electrode polarization by affecting the drift-diffusion behaviour of ions and influencing the electrical impedance of the system. The reversibility of the trapping reaction is a critical factor in determining the overall electrical behaviour of the system.
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Permittivity
The permittivity of a material is not a constant value. It can vary depending on several factors, including the frequency, magnitude, and direction of the applied electric field, as well as factors like humidity and temperature. At low frequencies, molecules in polar dielectrics are polarized by the electric field, inducing periodic rotations. This phenomenon is known as dielectric absorption, where a capacitor that has been charged for a long time cannot completely discharge when briefly discharged.
The permittivity of a material can be determined through various static electrical measurements, such as dielectric spectroscopy, which covers a wide range of frequencies. Additionally, techniques like cryostats and ovens can be used to study the dielectric properties of a medium over different temperatures.
The permittivity is closely related to the susceptibility of a material, which is a measure of how much an electric field causes polarization in a dielectric material. The susceptibility, in turn, is influenced by the previous electric field at earlier times, and this relationship leads to the frequency dependence of permittivity.
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Frequently asked questions
Electric polarization is the relative displacement of negative and positive charges in atoms or molecules, caused by an external electric field.
Polarization density describes how a material responds to an applied electric field and how the material changes the electric field. It is denoted mathematically by P and expressed in SI units as coulombs per square meter (C/m2).
The electric dipole moment is a vector quantity, expressed in SI units as coulombs times meters (C*m). It is one of the measures of polarization and is calculated by multiplying the distance between the centres of positive and negative charge by the amount of one of the charges.
The measured resistance (apparent resistance) is higher than the true resistance due to polarization. The true resistance can be obtained by reversing the applied electric field and taking the average of the resistance values before and after the reversal.
Orientational polarization is influenced by temperature. In the absence of an electric field, thermal agitation in dipolar molecules can result in the cancellation of the net dipole moment.
