Coulombs Law and Electric Fields

1
Coulombs Lawand Electric Fields
Physics 102 Lecture 02

• Todays lecture will cover Textbook Sections
  16.3-6

2
Recall Coulombs Law

• Magnitude of the force between charges q1 and q2
  separated a distance r
• F k q1q2/r2 k 9x109 Nm2/C2
• Force on nucleus of Hydrogen from e-

Example
F
F (9x109)(1.6x10-19)(1.6x10-19)/(10-10)2 N
2.3x10-8 N
(to the right)
3
Three Charges
Example

• Calculate force on 2mC charge due to other two
  charges
• Calculate force from 7mC charge
• Calculate force from 3.5mC charge
• Add (VECTORS!)

Q2.0mC
4 m
6 m
Q7.0mC
Q-3.5 mC
4
Electric Field

• Charged particles create electric fields.
• Direction is the same as for the force that a
  charge would feel at that location.
• Magnitude given by E ? F/q kq/r2

Example
E
Direction?
5
Preflight 2.3

• What is the direction of the electric field at
  point B?
• Left
• Right
• Zero

y
A
B
x
6
ACT E Field

• What is the direction of the electric field at
  point C?
• Left
• Right
• Zero

y
C
x
7
ComparisonElectric Force vs. Electric Field

• Electric Force (F) - the actual force felt by a
  charge at some location.
• Electric Field (E) - found for a location only
  tells what the electric force would be if a
  charge were located there
• F qE
• Both are vectors, with magnitude and direction.
  Add x y components.

8
Preflight 2.2

• What is the direction of the electric field at
  point A?
• Up
• Down
• Left
• Right
• Zero

y
A
B
x
9
ACT E Field II

• What is the direction of the electric field at
  point A, if the two positive charges have equal
  magnitude?
• Up
• Down
• Right
• Left
• Zero

y
A
B
x
10
Preflight 2.5
Charge A is 1) positive 2) negative 3) unknown
Field lines start on positive charge, end on
negative.
11
Preflight 2.6
X
A
B
Y
Compare the ratio of charges QA/ QB 1) QA 0.5QB
2) QA QB 3) QA 2 QB
lines proportional to Q
12
Preflight 2.8
The magnitude of the electric field at point X is
greater than at point Y 1) True 2) False
13
ACT E Field Lines
B
A
Compare the magnitude of the electric field at
point A and B 1) EAgtEB 2) EAEB 3) EAltEB
14
E inside of conductor

• Conductor ? electrons free to move
• Electrons feels electric force - will move until
  they feel no more force (F0)
• FqE if F0 then E0
• E0 inside a conductor (Always!)

15
Preflight 2.10
"Charge A" is actually a small, charged metal
ball (a conductor). The magnitude of the electric
field inside the ball is (1) Negative (2)
Zero (3) Positive
16
Recap

• E Field has magnitude and direction
• E?F/q
• Calculate just like Coulombs law
• Careful when adding vectors
• Electric Field Lines
• Density gives strength ( proportional to
  charge.)
• Arrow gives direction (Start end on -)
• Conductors
• Electrons free to move ? E0

17
To Do

• Read Sections 17.1-3
• Do your preflight.

See you next time!