Understanding Electric Polarization in Dielectric Capacitors

In summary, the electric polarization density of a dielectric inside a capacitor has the same direction as the electrical field, with the induced polarization being proportional to the electric susceptibility and the applied electric field. For time-dependent fields, this relation holds in the frequency domain and is translated into a convolution integral with a retarded Green's function. The susceptibility satisfies a causality constraint and is a holomorphic function in the upper complex frequency-half plane.
  • #1
Nikitin
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Does the electric polarization density of a dielectric inside a capacitor have the same direction as the electrical field? Considering the electric dipole moment vector goes from the - charge to + charge?
 
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  • #2
Yes, because the electrons in the dielectric are dragged a bit out of their equilibrium positions due to the applied electric field in the opposite direction than the field, because the force is [itex]\vec{F}=-e \vec{E}[/itex], where [itex]-e<0[/itex] is the electron's charge. The net effect (in lineare-response approximation) is an induced polarization
[tex]\vec{P}=\chi_{\text{el}} \vec{E}.[/tex] This is for homogeneous isotrophic media and time-independent (static) electric fields.

For time-dependent fields, this relation holds in the frequency domain, i.e., you have
[tex]\vec{P}(t,\vec{x})=\int_{\mathbb{R}} \mathrm{d} t' \chi_{\text{el}}(t-t') \vec{E}(t',\vec{x}).[/tex]
The causality constraint, [itex]\chi_{\text{el}}(t-t') \propto \Theta(t-t')[/itex] makes the susceptibility to a retarded Green's function. In the frequency domain, i.e., for the Fourier transform of the quantities the above convolution integral translates into
[tex]\tilde{\vec{P}}(\omega,\vec{x})=\tilde{\chi}_{\text{el}}(\omega) \tilde{\vec{E}}(\omega,\vec{x}).[/tex]
The retardation condition makes [itex]\tilde{\chi}(\omega)[/itex] a holomorphic function in the upper complex [itex]\omega[/itex]-half plane, where use the usual physicist's convention for Fourier transforms between the time and frequency domain:
[tex]f(t)=\int_{\mathbb{R}} \frac{\mathrm{d} \omega}{2\pi} \tilde{f}(\omega) \exp(-\mathrm{i} \omega t).[/tex]
 

FAQ: Understanding Electric Polarization in Dielectric Capacitors

What is electric polarization in dielectric capacitors?

Electric polarization in dielectric capacitors is the process by which the molecules or atoms of a dielectric material align in response to an applied electric field. This results in the creation of an internal electric dipole moment within the material, which can then store electrical energy.

What is the role of dielectric materials in capacitors?

Dielectric materials play a crucial role in capacitors by increasing the capacitance of the device. This is because the alignment of the molecules in the dielectric leads to an increase in the electric field within the material, allowing for more charge to be stored.

How does electric polarization affect the overall capacitance of a capacitor?

The electric polarization of a dielectric material increases the capacitance of a capacitor by allowing for more charge to be stored on the plates. This is because the electric field within the dielectric is stronger than the electric field in air or a vacuum, allowing for more charge to be attracted to the plates.

What factors can affect the electric polarization in dielectric capacitors?

The electric polarization in dielectric capacitors can be affected by the strength of the applied electric field, the type of dielectric material used, and the temperature of the material. Additionally, the thickness and surface area of the dielectric can also impact the electric polarization.

How is understanding electric polarization in dielectric capacitors important in practical applications?

Understanding electric polarization in dielectric capacitors is crucial in many practical applications, as capacitors are used in a wide range of electronic devices. This knowledge allows for the design and optimization of capacitors for specific purposes, such as in power transmission systems, electronic filters, and energy storage devices.

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