1. -Atomic View of Dielectrics -Electric Dipole in an Electric Field -Partially Filled Capacitors
AP Physics C
Mrs. Coyle

2. Atomic View of Dielectrics: The molecules of the dielectric are modeled as dipoles
External E-Field
No External E-Field
Random orientation of molecules
Partial Alignment of Molecules

3. Force Acting by a Uniform External Field on the Dipole
Note: E is not the field produced by the dipole
The force acting on each charge is F = Eq
The net force on the dipole is zero
The forces produce a net torque on the dipole 2a

4. Electric Dipole Moment, p
The electric dipole moment (p) is a vector directed along the line joining the charges from –q to +q ,
A dipole has two equal but opposite sign charges
Assume the distance between the charges is 2a

5. Work Required to Rotate the Dipole will result in an Increase in the Potential Energy of the Dipole-External Field System

6. An external field can polarize the dielectric whether the molecules are polar or nonpolar
The charged edges of the dielectric act as a second pair of plates producing an induced electric field in the direction opposite the original electric field

7. The degree of alignment of the molecules with the field depends on temperature and the magnitude of the field.
Generally:
the alignment increases with increasing field strength
the alignment increases with decreasing temperature

8. Non Polar Dielectrics
The electric field produces some charge separation
This produces an induced dipole moment
The effect is then the same as if the molecules were polar

9. Induced Charge and Field
The electric field due to the plates is directed to the right and it polarizes the dielectric
An induced surface charge results in an induced electric field, Eind, in the dielectric.
The net field at a point inside the dielectric is:
Enet = Eo - Eind
Eind Eo

10. Would the charge on all the surfaces be equal? Find the σ’s.
Eind Eo

11. If the dielectric were replaced by a metal what would be the charge on each surface?
Hint: If the dielectric were replaced with a conductor, the net field between the plates would be zero
E=0

12. Geometry of Some Capacitors

13. Example – Partially Filled Capacitor+Q
What is the capacitance in terms of Co?
Hint: Two capacitors in series.
Ans: d A -Q

14. Example 2 – Partially Filled Capacitor+Q d A -Q
How many capacitors could be considered and in what configuration?