# 物理代写|电磁学代写electromagnetism代考|ELEC3104

## 物理代写|电磁学代写electromagnetism代考|Comparison Between Dielectric Materials

To know about the differences between dielectric and conducting materials, we can consider their behavior in electric fields. In particular, we have shown in Fig. $4.7$ a conducting and dielectric sheet between the parallel plates in which a potential difference exists. That is, there are an equal amount of opposite charges on the two plates.

In the conducting sheet, the conducting electrons are free to move, and they establish a surface charge which exactly cancels the electric field within the conductor, as shown in Fig. 4.7. That is, the surface charge density of the plates and conducting sheet is the same but with opposite sign. On the other hand, the electrons in the dielectric material are bound to atoms, and the external electric field causes only a displacement of the electronic configuration of atoms (see Fig. 4.7). However, it is sufficient to produce some surface charge with density $\sigma_{i n d}$ (called an induced charge). We say that the dielectric is polarized. Note that the surface charge is not able to cancel the external electric field within the sheet; however, it does reduce. In the following, we will introduce a simplified molecular theory of dielectrics to understand the behavior of dielectric materials in the presence of an external electric field. ${ }^1$ A more complicated, but more precise theory, will be introduced in the following sections, accounting for electric polarization of the ponderable media. ${ }^2$

## 物理代写|电磁学代写electromagnetism代考|Molecular Theory of Dielectrics

Consider a dielectric material placed in the electric field between the plates of a capacitor. The dielectric material is made up of polar molecules. Note that dielectric material can also be made up of the nonpolar molecules, which can be polarized in the presence of the external electric fields. In the dielectric made up of polar molecules. the permanent dipoles of molecules are also called permanent dipoles. For dielectrics made up of nonpolar molecules, the dipoles created due to polarization are called induced dipoles.

The permanent dipoles in the dielectric arising from the polar molecules of the dielectric material are randomly distributed and oriented in the absence of an electric field, as shown in Fig. 4.8. When an external field $\mathbf{E}_0$ applies, for example, created by charges on the capacitor plates, the forces exerted on the dipoles produce torques, causing them to align almost in the direction of the field. We can say that the dielectric is polarized; that is, macroscopic charge separation occurs. The degree of alignment of molecules with an electric field depends on temperature and the magnitude of the field. In general, the alignment increases with decreasing temperature and with increasing electric field. If the molecules of the dielectric are nonpolar, then the electric field due to the plates produces some charge separation and an induced dipole moment. These induced dipole moments tend to align with the external field, and the dielectric is polarized.

# 电磁学代考

## 物理代写|电磁学代写electromagnetism代考|Molecular Theory of Dielectrics

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