Charges on objects All neutral objects have the

Charges on objects All neutral objects have the same number of + & - charges. Neutral object, Positive object, same no. of + ē removed, & - charges thus excess + charge, Charging objects Negative object, extra ē added, thus object negatively charged 1

Forces between charges Remember: Like charges repel Forces between charged objects While, unlike charges attract Electrostatic forces exist between charged particles/objects 2

Coulomb’s law is the relationship between the forces experienced by charged objects when situated a certain distance apart. Coulomb's law Opposite charges attract Like charges repel 3

Coulomb’s law This relationship enables us to establish the forces that exist between charged objects. Def. : Coulomb’s Law: The force of attraction or repulsion between electric charges at rest is directly proportional to the product of the charges, and inversely proportional to the square of the distance between their centres. Expressed mathematically: Since Q 1 Q 2 F=k 2 r Coulomb's law F ∝ Q 1 F ∝ Q 2 F ∝ 12 r 4

Coulomb’s law In the equation: Coulomb's law F = force between charges (in newton) Q 1 & Q 2 = respective charges (in coulomb) r = distance apart (in m) and k = Coulomb’s constant (9 x 109 N. m 2. C-2) In a calculation, remember to convert units to the correct units for the equation before substituting into the equation. 5

Application of Coulomb’s law Determine the net force on sphere B. A B C +3μC r = 5 cm +3μC -2μC r = 10 cm 1. Find the net force of attraction of C on B, towards C. 2. Find the net force of repulsion of A on B, towards C. 3. Find the net force of A & C on B towards C – by adding the answers to 1 & 2. 6

Application of Coulomb’s law Determine the net force on sphere B. A Solution method 1. Find the net force of attraction of C on B, towards C. B r = 5 cm +3μC -2μC r = 4 cm 2. Find the net force of attraction of A on B, towards A. Resultant C +3μC 3. Find the net resultant force of A & C on B, by adding force vectors BA & BC Application of Coulomb's law 7