P142 Lecture 19

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P142
Lecture 19

SUMMARY
Vector derivatives: Cartesian Coordinates
Vector derivatives: Spherical Coordinates

See Appendix A in “Introduction to Electrodynamics” by Griffiths

P142 Preview; Subject Matter

P142
Lecture 19

• Electric dipole • Dielectric polarization • Electric fields in dielectrics • Electric displacement field, D • Summary

Dielectrics: Electric dipole

The electric dipole moment p is defined as

p = q d

where d is the separation distance between the charges q pointing from the negative to positive charge.

Lecture 5 showed that

Forces on an electric dipole in E field

Torque:

Forces on an electric dipole in E field

Translational force:

Dielectric polarization: Microscopic

Atomic polarization:

d ~ 10^-15m

Molecular polarization:

d ~ 10^-14m

Align polar molecules:

d ~ 10^-10m

Competition between alignment torque and thermal motion or elastic forces

Linear dielectrics:

Three mechanisms typically lead to

Electrostatic precipitator

Linear dielectric: Translational force:

Electrostatic precipitator

Extract > 99% of ash and dust from gases at power, cement, and ore-processing plants

Electric field in dielectrics: Macroscopic

Electric field in dielectrics: Macroscopic
Polarization density P in dielectrics:
Polarization density in dielectrics:
Polarization density in dielectrics:

The dielectric constant κ_e

• Net field in a dielectric

E_net= E_Ext/κ_e

• Most dielectrics linear up to dielectric strength

Volume polarization density
Electric Displacement Field
Electric Displacement Field
Electric Displacement Field
Electric Displacement Field
Summary; 1
Summary; 2

Refraction of the electric field at boundaries

with dielectrics
Capacitance with dielectrics Capacitance with dielectrics
Electric energy storage

• Showed last lecture that the energy stored in a capacitor is given by where for a dielectric
• Typically it is more useful to express energy in terms of magnitude of the electric field E. • It can be shown that the energy density in the E field is • Thus the total stored energy is given by integrating the energy density over all space

Note that the energy stored is proportional to κ_e

Electric energy storage

Note that the energy stored is proportional to κ_e

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