Two parallel plates have equal and opposite charges. When the space between the plates is evacuated, the electric field is E= 3.50×105 V/m . When the space is filled with dielectric, the electric field is E= 2.40×105 V/m .

What is the charge density on each surface of the dielectric?

Explanation:

The given data is as follows.

     Electric field between plates without dielectric,

   Electric field between the plates with dielectric, .

  Permittivity of free space, =

Now, we will determine the charge density as follows.

                 =

                 =

Thus, we can conclude that the charge density on each surface of the dielectric is .

the old price per ounce is c/4.

the new price per ounce is c/3.6.

the ratio of the old price to the new one is

(c/4)/(c/3.6) = (c/c)×(3.6/4) = 0.9 . . = 9/10 . . "nine tenths" in either form