1. Part B: Electric Forces and Fields
Unit 3: Forces and Fields
Part B: Electric Forces and Fields

2. Electric Review
Recall: Objects receive charge from the addition or removal of electrons. (Protons do not move freely between materials as they are in the nucleus).
The law of electric charges states that opposite charges attract and like charges repel. (Neutral objects can also be attracted to other charged objects by induction).
Q= Ne allows us to calculate the charge on charged objects, where Q is the total charge, N is the number of electrons gained or lost and e is the elementary charge = 1.60 x C.

3. Electric Review Example
What is the charge on an object with an excess of one billion electrons?
Q = Ne
= 1 x 109 (-1.60 x C)
= x C

4. Electric Review
When objects rub together (like in a clothes dryer) electrons transfer to the object with a higher electron affinity making them attract as now each object has opposite charges. (Dryer sheets coat the clothes in a chemical preventing electron transfer).
When two charges contact each other, the charge difference is shared equally.
𝑄 𝑎𝑣𝑔 = 𝑞 1 + 𝑞 2 2

5. Electric Review Example
A +10 C charge touches a – 16 C charge, what is the charge on each object once they are separated?
𝑄 𝑎𝑣𝑔 = 𝑞 1 + 𝑞 2 2
= (+10-16) C/2
= -3 C
Each object has a -3 C charge
(Note: A very large charge; lightning uses~25 C)

6. Electric force law
Priestly and Franklin started the idea of an electric force law. Franklin noticed the attraction of cork floating in water in a metal pail to the sides of the pail. Priestly suggested that electric forces behaved similarly to gravitational forces.
Charles Augustine de Coulomb ( ) found that the magnitude of the electric force between two charged objects varied directly with the product of the two charges and inversely with the separation distance from the centres of the charged objects. (He used a torsion balance like Cavendish did with gravitational force).

7. Coulomb’s law
Coulomb’s law results when the results Coulomb found is combined with a proportionality constant: to make it an equation
The constant is called Coulomb’s constant.(k)
k = 8.99 x 109 Nm2/C2.
𝐹 𝑒 = 𝑘 𝑞 1 𝑞 2 𝑟 2