The Dipole

Application: Molecular Dipoles and Dielectrics

Matter in nature is made up of molecules, and those molecules are made up of atoms. Molecules are usually neutral - they have no net charge. The fact that they are neutral overall does not prevent molecules from having a charge distribution within them, so that one side is positively charged and one side is negatively charged. These are known as "permanent molecular dipoles." Generally, this charge distribution is very slight and difficult to observe - it's much less powerful than the attraction that binds a molecule together. Water is a good example of a permanent electric dipole.

Other materials have no permanent molecular dipole, such as nitrogen gas. The electrons in nitrogen have no preference for one side of the molecule over the other, whereas those in water tend toward the oxygen rather than hydrogen atoms.

Molecular dipoles are randomly aligned in most materials. Their respective fields are also randomly aligned, and cancel each other out.

However, in the presence of an electric field, we can see what's called an "induced molecular dipole." The electric field pulls on the electrons in the molecule, forcing a slight positive charge on one side and a negative charge on the other side. The molecules begin to align along the electric fields. In the picture below, a capacitor provides our electric field.

The positive and negative charges near each other in the material can be ignored - they will cancel each other out. We can treat the material as if the whole chunk of it had a positive charge on one side, and a negative charge on the other. Thus a dielectric begins generating its own electric field! This new field opposes the existing electric field that aligned the dipoles in the first place.

Capacitors are governed by the simple equation Q = eCV. The variable Q represents the charge on the capacitor, V is the voltage across the capacitor, and C is an intrinsic value of the capacitor, known as the "capacitance." "e" in this case is the "dielectric constant" of the material - an expression of how much it can be polarized. For vacuum and air, e = 1. For other materials, this number increases. Silicon has e = 11.8, and KTaNbO3 has e = 34,000 (one of the highest known values). Note that the higher our value of e is, the more charge can be stored in the capacitor.

Dielectric materials (or just "dielectrics") are very important to the manufacture of capacitors. The electric field that opposes the original field decreases the overall field. This in turn means that the electric potential decreases. This lets the capacitor store more energy, allowing smaller capacitors to be used and smaller electronics to be built.

Now that we've learned so much about point charges, let's take a look at line charges.


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