PHYAP 02 Coulombs Law By Squadron Leader Zahid

PHY-AP -02 Coulomb’s Law By Squadron Leader Zahid Mir CS&IT Department , Superior University

Remember … • Like charges repel one another • Opposite charges attract one another • The force of repulsion/attraction get weaker as the charges are farther apart.

Charles-Augustin de Coulomb (1736 -1806) French Engineer & Physicist

Statement The force between two point charges is directly proportional to the product of their magnitudes and inversely proportional to the square of the distance between them.

Coulomb’s Law : Mathematical Form F α q 1 q 2 r 2 F = k q 1 q 2 r 2 The quantity ε 0 , called the permittivity constant.

Main features • Coulomb’s law holds for point charges only. • It resembles Newton law of gravitation. – Gravitational forces between masses are always attractive. – Electrostatic forces can be either positive or repulsive depending upon the sign of charges in consideration.

Significance of the Law • This law when incorporated with into the structure of quantum physics, correctly describes: – The electrical forces that bind the electrons of an atom to its nucleus. – The force that bind atoms together to form the molecule. – The forces that bind atoms and molecules together to form a solid or liquid. Most of the forces of our daily experience that are not gravitational in nature are electrical.

Example 1 F = k q 1 q 2 d 2 F = (8. 93 x 109) (3. 0 x 10 -6) (2. 0 x 10 -6) (0. 02)2 F= 133. 95 N (repelling force)

Coulomb’s Law – Vector Form The force is the force on the charge q 1 due to the charge q 2 The force is the force on the charge q 2 due to the charge q 1

Coulomb’s Force due to charge distribution

Coulomb’s Force due to charge distribution

Coulomb’s Force due to charge distribution Principle Of Superposition I f more than two charges are present, the resultant force acting on one of the charges is the result of the vector addition of the individual Coulomb forces exerted on this charge by all the other charges.

Sample Problem # 1 1. Two point charges are located on the positive x-axis. Charge q 1 = 1. 0 n. C is 2. 0 cm from the origin, and charge q 2 = -3. 0 n. C is 4. 0 cm from the origin. What is the total force exerted by these charges on a charge q 3 = 5. 0 n. C located at the origin?

Sample Problem 2 2. Two equal positive point charges q 1 = q 2 = 2. 0 C are located at x=0, y=0. 30 m and x=0 , -0. 30 m respectively. What are the magnitude and direction of the total electric force that these charges exert on a third charge Q = 4. 0 C at x=0. 40 m , y=0 ?