"Basic research is like shooting an arrow into the air and, where it lands, painting a target."

Homer Burton Adkins

• In all our discussions to date we have implicitly assumed that our charges have been located in a vacuum or on the surface of conductors.  We now need to consider how to take into account the presence of non-conducting material in the real world.  Dielectric material is simply another way of saying non-conducting material.

• Imagine a parallel plate capacitor in which a dielectric material is placed between the plates (at right below).  The dielectric is composed of atoms/molecules which contain positive and negative charges.  The applied electric field between the plates, E₀, will cause the positive and negative charges of the constituent atoms/molecules to move slightly in opposite directions (right).  Electric dipole moments will be "induced" in the material, as shown.  The net effect will be for charge to appear on the surface of the dielectric material as shown.  The dielectric is said to have been polarized, leading to a polarization electric field, E_P.
  

  In conductors (metals) there are (almost) free electrons which will move through the material when an electric field is applied, generating an electric current.

• The net E field between the plates has been reduced,
  

where κ is called the dielectric constant or relative permittivity of the medium.

Note that for a vacuum, since E_P = 0,  κ = 1 and since  E_P < E₀ for all other materials κ > 1.

• It is easy to show that for the parallel plate capacitor the voltage (p.d) between the plates and the energy stored are reduced by a factor κ, whereas the capacitance is increased by a factor of κ.

• By application of Gauss's Law to a parallel plate capacitor with a dielectric between the plates it can be shown that to account for the presence of the dielectric Gauss's Law becomes,

As a general rule when dielectric media is present wherever ε₀ appears it must be replaced by ε₀κ.

A chemist, a biologist and an electrical engineer were on death row waiting to go in the electric chair.

The chemist was brought forward first. "Do you have anything you want to say?" asked the executioner, strapping him in. "No," replied the chemist. The executioner flicked the switch and nothing happened. Under this particular State's law, if an execution attempt fails, the prisoner is to be released, so the chemist was released.

Then the biologist was brought forward. "Do you have anything you want to say?" "No, just get on with it." The executioner flicked the switch, and again nothing happened, so the biologist was released.

Then the electrical engineer was brought forward. "Do you have anything you want to say?" asked the executioner. "Yes," replied the engineer. "If you swap the red and the blue wires over, you might make this thing work."

 

Dr. C. L. Davis
Physics Department
University of Louisville
email: c.l.davis@louisville.edu