1. ELECTROSTATICS - INTRODUCTION
Fields and Waves
Lesson 3.1
ELECTROSTATICS - INTRODUCTION
Darryl Michael/GE CRD

2. SOURCE of Electrostatic E-Field is CHARGE
ELECTRICAL CHARGES
SOURCE of Electrostatic E-Field is CHARGE
Examples of various charge distributions:
1. Point charge - Q (units of Coulomb)
- model individual particle (eg. Electron) or a well-localized group of charge particles
2. Volume Charge Density - r or rv (units of Coulomb/m3)
- large # of particles - ignore discrete nature to smooth out distribution
Eg. Doped Region of Semiconductor, e-beam in a cathode ray tube
( Beam has finite radius )

3. Other examples of Charge Distribution……..
ELECTRICAL CHARGES
Other examples of Charge Distribution……..
3. Surface Charge Density - r or rs (units of Coulomb/m2)
Eg. Very thin charge layer on conductor surface
4. Line Charge Density - r or rl (units of Coulomb/m)
- not as physically realizable
Eg. Model for a wire, electron beam from far

4. ELECTRICAL CHARGES Maxwell’s equation: More generally, Do Problem 1
Derived from: or
Do Problem 1

5. (force), between point charges
COULOMB’S LAW
(force), between point charges Q1
Unit vector in r-direction
Force on Charge 2 by Charge 1 R Q2

6. COULOMB’S LAW - E Field ,of Q1 is Then,
Unit vector pointing away from Q1
Then,
- we work with E-Field because Maxwell’s equations written in those terms

7. Proportional to Magnitude or strength of E-Field
E-FIELDS
, is a VECTOR Field
How do we represent it?
- Field points in the direction that a +ve test charge would move
Represent using Arrows : Direction and Length
Point charge
Proportional to Magnitude or strength of E-Field

8. Computation of E-fields from multiple charges:
Example: DIPOLE - 2 separated opposite polarity point charges +Q
Apply superposition of Fields x y
Planes of symmetry:
Horizontal axis: Ex cancels, Ey adds -Q
+Q vector
Vertical axis: only Ey component -Q
Resulting vector

9. E-FIELDS - Some examples

10. E-FIELDS - Some examples

11. E-FIELDS - Dipole

12. APPLICATION of SUPERPOSITION
E-FIELDS
How would the DIPOLE field lines change if the charges were the same polarity?
APPLICATION of SUPERPOSITION
Usually text has many examples of setting up this integral
In the course we will do some discrete S cases
Do Problem 2