1. Electrostatics
Coulomb’s Law

2. Attraction/Repulsion
Nothing
or Attraction (electrons move) + - + - + + -+ - - +-

3. What is a Charge? neutral atom charged atom (ion)
All materials have neutrons, protons, & electrons
Hard to move neutrons, and protons are stuck to neutrons so they don’t move either
But electrons are far away - they move easily
# of protons = # of electrons
If we remove a few electrons # of protons ≠ # of electrons
Charge is measured in Coulombs
1 proton or electron has a charge of 1.6x10-19C (Quantum of Charge - e)
neutral atom
charged atom (ion)

4. Cu2O
Example
How many electrons are gained if an object has a charge of -1.00C?
Q = N e
Q: Net charge on object (C)
N: Number of charged particles (# of electrons gained or lost)
e: Charge on 1 electron/proton (quantum of charge)
N = Q/e
N = (-1.00C) / (-1.60x10-19C/electron)
N = 6.25x1018 electrons
N is always positive

5. How do we charge an object?
Get Same Charge
Charging by contact - - -
Protons
Don’t
Move + + + 3+ 3+ 3+ 6- - 6- - - - - 3-
NEUTRAL
ELECTRONS TRANSFER
NEGATIVE

6. How do we charge an object?
Get Opposite Charge
Charging by induction - - - + + + 3+ 3+ 3+ 2- - 1- - 3- - 1- -
NEUTRAL
ELECTRONS LEAVE
THROUGH GROUNDING
POSITIVE

7. Coulomb’s Law

8. Where do you recognize this
Coulomb’s Law
Push or Pull
A force between 2 charged particles
Has magnitude & direction (vector)
Depends on:
Distance between objects
Amount of charge on objects
Type (+ or -) of charge on two objects
k = Coulomb’s Constant (9.00x109 Nm²/C²)
q = Charges on the charged particles (C)
r = distance between centres of charged particles (m)
Where do you recognize this
from?

9. Look at diagram for direction
Coulomb’s Law Example
3 charged particles lie on a table, what is net force on “B”?
FAonB = KqAqB / rAB²
FAonB = (9E9)(0.005)(-0.003) / (0.4m)²
FAonB = -843,750N
FConB = KqBqC / rBC²
FConB = (9E9)(-0.003)(-0.008) / (0.5m)²
FConB = 864,000N
FBnet = FAonB + FConB
FBnet = (843,750N [Left]) + (864,000N [Left])
FBnet = 1.7x106N [Left]
Attraction
Repulsion
Look at diagram for direction
5mC
-3mC
-8mC A B C
0.4m
0.5m

10. Coulomb’s Law Example 2
3 particles lie at 90°, what is net force on the 6C particle?
F-3on6 = Kq-3q6 / r-3,6²
F-3on6 = (9E9)(-3)(6) / (0.4m)²
F-3on6 = -1.0E12N
F1.5on6 = Kq1.5q6 / r1.5,6²
F1.5on6 = (9E9)(1.5)(6) / (0.3m)²
F1.5on6 = 9.0E11N
F6net = √[F-3on6² + F1.5on6²]
F6net = √[(-1.0E12N)² + (9.0E11N)²] = 1.35E12N
θ = tan-1[F-3on6 / F1.5on6]
θ = tan-1[(-1.0E12N) / (9.0E11N)] = 48°
F6net = 48°WofS
Attraction
Repulsion
1.5
0.3m -3 6
0.4m

11. Conclusions Opposite charges attract (same repulse)
Quantum of Charge (e) = 1.6x10-19C
Charge (Q) = N*e
Things can be charged by contact and induction
Coulomb’s Law:
FE = Kq1q2/r²
K = 9.00x109 Nm²/C²