1. Charge (Q) is measured in coulombs (C) 1C = 6x1018 e
Smallest possible amount of charge:
1 extra electron: q = x C
1 missing electron: q = x C …
For any charge q:
q = ne , where n = 1, 2, 3, etc…
Charge is quantized
Charge is conserved
A positively charged object has lost electrons
A negatively charged objects has gained electrons

2. Example
A solid sphere has a net charge of –4 nC. How many electrons must be added or removed for the sphere to have a net charge of +5 nC?

3. Charging by Contact (rubbing…) - - - - - - - - - - + + + + glass (rub)
fur
plastic
glass
(rub)
silk
(rub)
Charging by
Contact (rubbing…)

4. Separating Charge
A material on the bottom of the list will gain electrons from a material above it on the list…

5. Opposites Charges Attract
Like Charges Repel

6. Charging by Conduction

7. Charging by Induction

10. - - - - - - Grounding Object is discharged or “grounded” +
The earth is a huge reservoir of positive and negative charge + -
© Laura Fellman

11. Coulomb’s constant (k)
Coulomb’s law r
Charge (Q)
Coulombs (C)
1 C = x 1018 e
e = x C
Force (N)
Distance (m)
Coulomb’s constant (k)
k = x 109 Nm2/C2

12. 0= permittivity of free space = 8.85 x 10-12 C2/Nm2
Notes on Coulomb’s Law
1) It has the same form as the Law of Gravitation: Inverse-Square Force
2) But… (can you spot the most basic difference between these two laws?)
3) The electrostatic constant (k) in this law is derived from a more fundamental constant:
0= permittivity of free space = 8.85 x C2/Nm2
Permittivity is the measure of resistance that is encountered when forming an electric field in a medium. 
4) Coulomb’s Law obeys the principle of superposition
© Laura Fellman

14. Tricky!

19. Q 2Q 3Q 4Q L
At each corner of a square of side L there are four point charges. Determine the force on the charge 2Q.