1. Electric Fields
Chapter 14.1

2. What do you already know about charged particles?
Like charges repel.
Opposite charges attract.
Electric charges exert a FORCE on each other (similar to how masses exert a gravitational force on each other).

3. Inverse Square Law
Recall that gravity follows the inverse square law with respect to the distance between objects (Universal Law of Gravitation)
We see the same behaviour with charges:

4. Coulomb’s Law Is similar to Newton’s Universal Law of Gravitation
Uses the following quantities:
k – Coulomb’s constant (Nm2/C2)
k = 8.99*109 Nm2/C2
q1 & q2 – charge in Coulombs on each object (C)
r – distance between objects (centre to centre) (m)
However, unlike the gravitation law, it is possible to have both positive and negative forces

5. Coulomb’s Law Calculates ELECTROSTATIC force
Electric charges exert forces on each other

6. What exactly is a Coulomb?
A measure of electric charge equal to the charge of 6.25 x 1018 electrons
Therefore 1 electron (or proton)
=1.60x10-19C
A Coulomb is known as elementary or fundamental charge. It is the smallest indivisible amount of charge. All charges are multiples of elementary charge.

7. Prefixes
A Coulomb is a relatively large amount of charge. Often we want to express smaller amounts. You should know the following prefixes for this unit:
1.0 x C = 1 microCoulomb (μC)
1.0 x 10-9 C = 1 nanoCoulomb (nC)
1.0 x C = 1 picoCoulomb (pC)
1.0 x C = 1 femtoCoulomb (fC)

8. Coulomb’s Law Positive force and repulsion will result when you have:
Two positively charged particles
Two negatively charged particles
Negative force and attraction will result when you have:
One positively and negatively charged particle
Electrostatic force is a non-contact force that gets smaller the farther two charges are away from each other.

9. Fg vs Fe
Differences: Fg is only an attractive force but Fe can also repel.
Fg is much weaker than Fe

10. Example A small sphere carrying a charge of
-8.0μC exerts an attractive force of 0.50N on another sphere carrying a charge with a magnitude of 5.0 μC.
a) What is the sign of the second charge?
b) What is the distance between these two spheres? (distance of separation)
a) positive
b) m

11. Practice Problems
Page 638
1-5

12. Question 5 page 638
Two identical objects have charges of +6.0µC and -2.0µC, respectively. When placed a distance d apart, their force of attraction is 2.0N. If the objects are touched together, then moved to a distance of separation 2d, what will be the new force between them?
HINT: When the charges touch they leave with an identical force as the charges transfer.
Alt 230

13. Example 2: 3 Charges Three charges (Charge A: +3.0µC,
B: +5.0µC, C: -3.0µC) are arranged in a right triangle. The distance between A and B is 0.050m, dBC = 0.040m.
What is the net electrostatic force on B?

14. To Solve when 3 Charges Step 1: Free Body Diagram of charge you want
(NOTE: Signs indicate if the force is attractive or repulsive NOT DIRECTION on the x-y plane therefore the diagram is very important).
Step 2: Find the electrostatic forces on charge you want from the other charges.
Step 3: Vector Addition – find the force and direction (tan)
Answer: 1.0 x 102 N [E 32.6’ S]

15. Example 3: 4 Charged Particles
Find Fnet on A:
Fab = 0.01N
Fac = N
Fad = N

16. Fab = 0.01N
Fac = N
Fad = N
Answer: 9.5 x N [70’]

17. Practice
Model Problem
Page 639
Page
Questions 7, 8, 9, 10