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

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

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

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

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

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

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

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

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

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

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

Geometry of Some Capacitors

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

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