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By the end of this section, you will be able to:
  • Explain the polarization of a dielectric in a uniform electrical field
  • Describe the effect of a polarized dielectric on the electrical field between capacitor plates
  • Explain dielectric breakdown

We can understand the effect of a dielectric on capacitance by looking at its behavior at the molecular level. As we have seen in earlier chapters, in general, all molecules can be classified as either polar or nonpolar . There is a net separation of positive and negative charges in an isolated polar molecule, whereas there is no charge separation in an isolated nonpolar molecule ( [link] ). In other words, polar molecules have permanent electric-dipole moments and nonpolar molecules do not. For example, a molecule of water is polar, and a molecule of oxygen is nonpolar. Nonpolar molecules can become polar in the presence of an external electrical field, which is called induced polarization .

The figures show a large scale view of an atom. Figure a shows an unpolarized atom, with protons and neutrons in the center and a circular electron cloud surrounding the nucleus. Figure b shows a polarized atom and positive and negative external charges. The atom is oblong in shape with the electron cloud being pulled towards the positive external charge, and the nucleus being pulled towards the negative external charge. Figure c shows another oblong polarized atom.
The concept of polarization: In an unpolarized atom or molecule, a negatively charged electron cloud is evenly distributed around positively charged centers, whereas a polarized atom or molecule has an excess of negative charge at one side so that the other side has an excess of positive charge. However, the entire system remains electrically neutral. The charge polarization may be caused by an external electrical field. Some molecules and atoms are permanently polarized (electric dipoles) even in the absence of an external electrical field (polar molecules and atoms).

Let’s first consider a dielectric composed of polar molecules. In the absence of any external electrical field, the electric dipoles are oriented randomly, as illustrated in [link] (a). However, if the dielectric is placed in an external electrical field E 0 , the polar molecules align with the external field, as shown in part (b) of the figure. Opposite charges on adjacent dipoles within the volume of dielectric neutralize each other, so there is no net charge within the dielectric (see the dashed circles in part (b)). However, this is not the case very close to the upper and lower surfaces that border the dielectric (the region enclosed by the dashed rectangles in part (b)), where the alignment does produce a net charge. Since the external electrical field merely aligns the dipoles, the dielectric as a whole is neutral, and the surface charges induced on its opposite faces are equal and opposite. These induced surface charges     + Q i and Q i produce an additional electrical field E i (an induced electrical field    ), which opposes the external field E 0 , as illustrated in part (c).

Figure a shows a dielectric material with oblong shaped molecules within it. Each molecule has a plus sign on one side and a minus sign on the other. They are all randomly arranged. Figure b shows the same molecules now perfectly aligned in rows and columns, with the negative end of each molecule on the upper side. An external electric field E0 points downwards. A dashed line encompasses the negative signs of all the molecules in the topmost row. Similarly, a dashed line encompasses the positive signs of all the molecules in the bottommost row. Figure c shows negative signs at the top of the dielectric, labeled minus Qi, and positive signs at the bottom, labeled plus Qi. The induced field Ei within the dielectric, points upwards.
A dielectric with polar molecules: (a) In the absence of an external electrical field; (b) in the presence of an external electrical field E 0 . The dashed lines indicate the regions immediately adjacent to the capacitor plates. (c) The induced electrical field E i inside the dielectric produced by the induced surface charge Q i of the dielectric. Note that, in reality, the individual molecules are not perfectly aligned with an external field because of thermal fluctuations; however, the average alignment is along the field lines as shown.
Practice Key Terms 5

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Source:  OpenStax, University physics volume 2. OpenStax CNX. Oct 06, 2016 Download for free at http://cnx.org/content/col12074/1.3
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