Beatrice Lang
Electric polarization is a phenomenon that occurs when there is a relative displacement of negative and positive charges in atoms or molecules due to an external electric field. It is also known as polarization density and is denoted by P. This process involves the redistribution of electrons, leading to a temporary separation of charges and the creation of a dipole moment. The dipole moment measures the separation of positive and negative charges within a system. Electric polarization is influenced by factors such as temperature and the addition of certain materials, and it plays a crucial role in various scientific fields, including chemistry, physics, and materials science.
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What You'll Learn
Ionic polarization
Electric polarization is a phenomenon that occurs when there is a relative displacement of negative and positive charges in atoms or molecules due to an external electric field. This displacement results in a slight shift of charges in opposite directions, causing one side of the atom to become slightly positive and the other side slightly negative. This is often referred to as dielectric polarization, as it involves the polarization of dielectric materials.
Dielectric materials, such as ionic crystal elements (e.g. NaCl, KCl, LiBr), can exhibit a type of polarization known as ionic polarization. In these materials, there is no net polarization without an external electric field because the dipole moments of negative ions are cancelled out by positive ions. However, when an external electric field is applied, the ions become displaced, leading to induced polarization.
Furthermore, temperature plays a role in ionic polarization. An increase in temperature enhances polarization due to increased ionic mobility. This relationship between temperature and polarization results in a property known as pyroelectricity.
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Electronic polarization
In simple terms, polarization occurs when an electric field distorts the negative cloud of electrons around positive atomic nuclei in a direction opposite to the field. This slight separation of charges gives one side of the atom a slightly positive character and the other side a slightly negative character. This separation of charges is what we refer to as an electric dipole moment. The electric dipole moment is a measure of polarization and is calculated by multiplying the distance between the shifted centres of positive and negative charges by the amount of one of the charges.
The concept of electronic polarization is particularly important in the study of dielectric materials. A dielectric material is one that does not easily conduct electric current and is an insulator. When a dielectric material is placed in an external electric field, its molecules gain an electric dipole moment, and the material becomes polarized. This process of polarization involves the reorientation of existing dipoles toward the direction of the field or the separation of mobile charge carriers at the interfaces of impurities or other defect boundaries.
The degree of polarization in a dielectric material depends on the time variation of the electric field. The polarization due to the elastic displacement of electron clouds is relatively quick, while polarization involving the movement of particles, such as the migration of charge carriers or the sluggish collective orientation of dipoles, takes a much longer time. The time required for polarization can also depend on the material's properties, such as its temperature or the addition of other substances. For example, an increase in temperature enhances polarization due to increased ionic mobility.
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Electric dipole moment
The electric dipole moment is particularly useful in the context of an overall neutral system of charges, such as a pair of opposite charges or a neutral conductor in a uniform electric field. A dipole moment occurs when there is a separation of charge. This can occur between two ions in an ionic bond or between atoms in a covalent bond. Dipole moments arise from differences in electronegativity—the larger the difference in electronegativity, the larger the dipole moment. The distance between the charge separation is also a factor in the size of the dipole moment.
Theoretically, an electric dipole is defined by the first-order term of the multipole expansion. It consists of two equal and opposite charges that are infinitesimally close together, although real dipoles have separated charges. Two point charges, one with charge +q and the other with charge −q, separated by a distance d, constitute an electric dipole. The dipole moment of a molecule can be calculated using the equation:
> \(\vec{\mu} = \sum_i q_i \, \vec{r}_i\)
The dipole moment acts in the direction of the vector quantity.
The dipole moment of a molecule can be calculated based on the molecular structure using the concept of group contribution methods. It can also be calculated from electronic structure theory, either as a response to constant electric fields or from the density matrix.
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Dielectric materials
Electric polarization refers to the relative displacement of negative and positive charges in atoms or molecules caused by an external electric field. This phenomenon is observed in dielectric materials, which are electrical insulators that can be polarised by an applied electric field. Dielectric materials are characterised by their high polarisability, or relative permittivity, which indicates their ability to store energy in an electric field.
The polarisation of dielectric materials is influenced by factors such as temperature and chemical composition. The relative permittivity or dielectric constant of a dielectric material is a crucial parameter that characterises its ability to store energy. The dielectric constant is dependent on the frequency of the applied electric field, with higher frequencies resulting in lower polarisation effectiveness.
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Polarization density
Electric polarization refers to the relative displacement of negative and positive charges in atoms or molecules, caused by an external electric field. This phenomenon results in a slight separation of charges, making one side of the atom or molecule slightly positive and the other slightly negative. This process is also known as charge redistribution in a material.
The concept of polarization density is particularly relevant when studying the behaviour of materials in external electric fields. It helps describe how a material responds to the applied field and how it modifies the field. By understanding polarization density, we can calculate the forces resulting from these interactions. Additionally, polarization density is crucial in the study of anisotropic materials, where the polarization density and the electric field are not aligned in the same direction. In such cases, the relationship between the polarization and electric field components becomes more complex.
It is important to note that the value of polarization density ("P") within a bulk solid is not always uniquely defined. Two observers of the same solid may calculate different values of "P" by choosing different unit cells for their computations. However, this non-uniqueness is not problematic because the measurable consequences of "P" are associated with continuous changes in its value. For example, when a material is subjected to an increasing electric field, its electronic and ionic positions shift slightly, resulting in a change in "P" and, consequently, electric susceptibility and 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, also known as electric polarization, is the vector field that expresses the volumetric density of permanent or induced electric dipole moments in a dielectric material.
Electric polarization occurs when an external electric field is applied to a dielectric material, causing a displacement of bound charged elements. The positive charged elements are displaced in the direction of the field, while the negative charged elements are displaced in the opposite direction.
An example of electric polarization is the polarization of water molecules. Water molecules have a permanent electric dipole moment due to the unequal distribution of electrons, which causes them to align with the electric field.
Electric polarization and magnetization are both processes that describe the response of materials to external fields, with electric polarization involving the response to an electric field and magnetization involving the response to a magnetic field.
