CATAGORY: Electricity & Magnetism
QUESTION: When a piece of dielectric material is placed in an electric
field, the dielectric produces an "induced" electric field
which modifies the total electric field within the material.
How does its magnitude and direction compare to the original
field? Explain the origin of this induced field.

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The magnitude of the field is less than the original electric field by
a factor of 1/k where k is the dielectric constant.  k is always
greater than one so 1/k is less than one.  The direction of the
induced field is opposite that of the applied field.  This is because
the molecules of the dielectric are or act like they are randomly
oriented freely turning dipole molecules with a positive charge at one
end and a negative charge at the other.  When an external electric
field is applied the dipoles experience a torque.  The positive ends
feel a force along the applied field direction and the negative
charges feel a force opposite the applied field direction.  The result
is that the dipoles rotate and align themselves with the applied
field.  The electric fields of the dipole itself however points from
the positive charges to the negative charges which is in the opposite
direction of the applied field.  Inside the dielectric the fields 
from adjacent dipoles cancel but at the surface on one side there are
uncancelled positive charges and on the other there are uncancelled
negative charges. So a field is established in the dielectric from the
induced surface charge density.

         direction of the external applied field
    --------------------------------------------->
    
          +|-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+|-
          +|-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+|-
          +|-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+|-
          +|-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+|-
          +|-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+|-
          +|-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+|-
          +|-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+|-
          +|-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+|-
boundry of |   interior of the dielectric     | boundary of 
dielectric |                                  | dielectric
            <-----------------------------
              direction of induced field
              from the induced SURFACE charge
              distribution.  Inside the 
              dielectric the fields from opposite 
              ends dipoles cancel.