1. Forces and Fields Lesson 4
Coulomb’s Law
Forces and Fields Lesson 4

2. Charles Coulomb
Discovered the inverse square law which states that as you move charges apart the force between them decreases exponentially
Also showed that the force between two charged objects is proportional to the product of the charges
Unit of electrical charge is named after him

3. The Torsion Balance
Coulomb used a torsion balance (Physics 20 – universal gravitational constant) to determine “k” a universal value called Coulomb’s Constant.
By measuring the amount of twist when the charges repelled each other, the force could be measured and the constant could be determined.

4. Coulomb’s Law describes the electrostatic forces between two charged objects.
Where:
Fe is the electrostatic force
k is Coulomb’s Constant
q is the charge in Coulombs (C)
( the unit for charge is 1 C which equals the charge of
r is the distance separating the two charges (m)

5. Notice the similarities to Newton’s Law of Universal Gravitation
This shows only attraction while Coulomb’s Law can be used for both attractive and repulsive situations
Gravity is a weak force requiring large masses
Electric forces are very large in comparison and are effective with small charges

6. Using Coulomb’s Law (1 D)
To calculate the electric force between two charges, always solve for the absolute value and ignore the sign on the charge. Determine whether the charges attract or repel at the end. ( A point charge is a small + or – charge.)
Eg) Calculate the magnitude of the electric force between two point charges of uC and uC when they are 4.50 cm apart.

7. Eg 2) A comb with 2. 0 uC of charge is 1
Eg 2) A comb with 2.0 uC of charge is 1.5 m to the left from a hair with -3.0 uC of charge. Determine the force the hair exerts on the comb.
The forces are attractive so the hair is pulling on the comb because it has a larger charge.

8. Two point charges of and
produce a force of
on each other. How far apart are these two charges?

9. NOTE:
Charges are usually in the range of microCoulombs. Only lightning strikes have a charge of 1 or 2 coulombs.
The electron’s charge is so important it is called an elementary charge. Its symbol is “e” and is also on your formula sheet.

10. Net electrical force Determine the net force on object B.
always interpret this to mean that the
object in question is free to move and the
other objects are stationary.

11. Forces on B
Free body diagram

12. F C on B

13. F A on B

14. Fnet

15. AB down on B Example: Find the net electrical force on B. A
2.0 x C
3.20 m
-1.5  C B C
4.1 m
-3.8  C
AB
down on B *

16. BC
left on B *

17. Draw Vector Diagram
3.05 x N
6.67 x N *

18. Example: Calculate the gravitational force between an electron and a proton placed 1.0 cm apart.*

19. Example: Calculate the electrical force between an electron and a proton placed 1.0 cm apart.*

20. Three point charges, A, B, and C, are arranged in a line as shown.
a. Determine the net electrostatic force acting on charge B.

21. b. The charge on object C is negative
b. The charge on object C is negative. Identify the sign of the charge (+ or -) on object i. A. ii. B.

22. 2 D Example:
Three point charged objects are arranged in a right angle as shown in the diagram.
a. The charge on object A is positive. Identify the sign of the charge (+ or -) on object
i. B. (negative)
ii. C. (negative)

23. Read WB p. 85-86 TB p. 532-6 Examples 10.3 to 10.6
b. Determine the net electrostatic force acting on charge B.
Read WB p TB p Examples 10.3 to 10.6
Do WB p ,8,10,14,16,18,19