# PHYS 142 Lecture Notes - Lecture 2: Electric Field

PHYS 142: Electromagnetism and Optics - Lecture 2: Coulomb’s Law and the Electric Field

Coulomb’s Law:

● When two charged particles of respective charges q1 and q2 are a distance r apart, each

particle exerts a force on the other of magnitude:

k∨q1∨¿q2∨¿

r2

F=¿

● K is called the electrostatic constant. In SI units K = 8.99×109 N m2/C2.

● These forces are equal in magnitude and opposite in direction; they are directed along the

line joining the two particles. They are repulsive for two like charges and attractive for two

opposite charges.

● Another way to write Coulomb’s law is:

¿q1∨¿q2∨¿

r2

F=1

4π ϵo

¿

where

ϵo=¿

8.85×10-12

C2/Nm2

●

ϵo

is called the permittivity constant

● Electric forces can be superimposed (addition of vectors)

The Electric Field:

● Consider the space surrounding a charge Q1. Any charge q placed in that space will

experience an electric force. We can calculate the electric force acting on q at any point in

space. The collection of all these force vectors is the electric field created by a charge Q1.

● A charged particle with charge q at a point in space where the electric field is experiences

an electric force:

⃗

Fon q =q

⃗

E

● Thus, the electric field can be represented as:

⃗

E=kq

r2

̂

r=1

4π ϵo

q

r2

̂

r

where

̂

r

represents the direction

● The SI unit of electric field is N/C

## Document Summary

Phys 142: electromagnetism and optics - lecture 2: coulomb"s law and the electric field. When two charged particles of respective charges q1 and q2 are a distance r apart, each particle exerts a force on the other of magnitude: k q1 q2 r2. In si units k = 8. 99 109 n m2/c2. These forces are equal in magnitude and opposite in direction; they are directed along the line joining the two particles. They are repulsive for two like charges and attractive for two opposite charges. Another way to write coulomb"s law is: Electric forces can be superimposed (addition of vectors) is called the permittivity constant. Consider the space surrounding a charge q1. Any charge q placed in that space will experience an electric force. We can calculate the electric force acting on q at any point in space. The collection of all these force vectors is the electric field created by a charge q1.