25.10
Potentieel vanwege een gepolariseerd object
Een neutraal atoom bestaat uit een positief geladen kern omgeven door een negatief geladen elektronenwolk. Wanneer ze in een extern elektrisch veld wo…
When a dielectric is placed in an electric field, the dipoles align along the field direction. This effect is called polarization, which is the dipole moment per unit volume.
Consider a long string of aligned dipoles with uniform dipole moments. The equal and opposite charges in adjacent dipoles cancel each other, leaving net negative and positive charges at the dielectric surface, called surface-bound charges.
The surface-bound charge density is the charge per unit area. Using the definition of the dipole moment, the charge can be expressed in terms of polarization.
For an oblique surface, this charge is distributed over an extended area. Defining a normal to this area gives the expression for surface-bound charge density.
When the polarization is non-uniform, the alignment of dipoles accumulates charges inside the dielectric, leading to volume-bound charges that are equal and opposite to the surface-bound charges.
The potential created by a polarized dielectric at a field point can be estimated solely from these bound charges and is the sum of the potentials produced by the surface and volume-bound charges.
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Q1: What happens to atoms when placed in an external electric field?
In an external electric field, the electric force pulls electrons and the nucleus apart, opposing their intrinsic attraction. The opposing forces balance with a slight shift between the nucleus and electron cloud centers, resulting in a polarized atom. This separation creates a dipole moment aligned with the field direction.
Q2: How do surface-bound charges form in a polarized dielectric?
When dipoles align along an electric field in a dielectric, equal and opposite charges in adjacent dipoles cancel each other internally. This leaves net negative and positive charges at the dielectric surfaces, called surface-bound charges. These charges remain bound to specific atoms and cannot move freely within the material.
Q3: What is the difference between surface-bound and volume-bound charges?
Surface-bound charges appear at dielectric surfaces when aligned dipoles cancel internally. Volume-bound charges accumulate inside the dielectric when polarization is non-uniform, causing dipole alignment to create net internal charge. Volume-bound charges are equal and opposite to surface-bound charges, and both contribute to the electric potential created by the polarized dielectric.
Q4: How is polarization defined in a dielectric material?
Polarization is defined as the dipole moment per unit volume in a dielectric. When an external electric field is applied, dipoles align along the field direction, creating this net polarization. Polarization quantifies the degree to which the material's charges separate in response to the applied field.
Q5: Why do molecules like water behave differently from neutral atoms in an electric field?
Molecules like water inherently have separated positive and negative charge centers, unlike neutral atoms. When placed in a uniform electric field, forces on the positive and negative sides cancel, but a net torque exists that rotates the molecule. This torque aligns the dipole along the field direction, producing polarization similar to induced polarization in neutral atoms.
Q6: How can the potential of a polarized dielectric be calculated?
The potential created by a polarized dielectric at any field point is determined by the potentials from surface-bound and volume-bound charges. By knowing the dipole moment and charge distributions, you can calculate contributions from both charge types. Understanding electrostatic boundary conditions in dielectrics helps determine how these charges distribute at interfaces.
Q7: What role does surface charge density play in describing bound charges?
Surface-bound charge density measures charge per unit area on dielectric surfaces. For oblique surfaces, the charge distributes over an extended area, and defining a normal to this area gives the expression for surface-bound charge density. This density quantifies how bound charges concentrate at the dielectric-field interface.