25.10
Potencial debido a un objeto polarizado
Un átomo neutral está compuesto por un núcleo cargado positivamente rodeado por una nube de electrones cargados negativamente. Cuando se coloca en un…
Cuando se coloca un dieléctrico en un campo eléctrico, los dipolos se alinean a lo largo de la dirección del campo. Este efecto se llama polarización, que es el momento dipolar por unidad de volumen.
Considere una larga cadena de dipolos alineados con momentos dipolares uniformes. Las cargas iguales y opuestas en los dipolos adyacentes se cancelan entre sí, dejando cargas netas negativas y positivas en la superficie dieléctrica, llamadas cargas ligadas a la superficie.
La densidad de carga ligada a la superficie es la carga por unidad de área. Usando la definición del momento dipolar, la carga se puede expresar en términos de polarización.
Para una superficie oblicua, esta carga se distribuye sobre un área extendida. Al definir una normal en esta área, se obtiene la expresión de la densidad de carga ligada a la superficie.
Cuando la polarización no es uniforme, la alineación de los dipolos acumula cargas dentro del dieléctrico, lo que da lugar a cargas ligadas al volumen que son iguales y opuestas a las cargas ligadas a la superficie.
El potencial creado por un dieléctrico polarizado en un punto de campo se puede estimar únicamente a partir de estas cargas ligadas y es la suma de los potenciales producidos por las cargas ligadas a la superficie y al volumen.
View the full transcript and gain access to JoVE Core videos
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.