Electrostatics

Card # 56 Electrostatics Properties of charges 1. Interaction of charges – Two charges exerts force on each other due to their charges. Like charges repel and unlike charges attract. This force is called electrostatic interaction or electrostatic force. 2. Electrostatic Induction – The induction of electric charge on a conducting body due to presence of a nearby body is called electrostatic induction. 3. Principle of conservation of charge – The charge can neither be created nor destroyed but it may simply be transferred from one body to another. “The net charge of an isolated system remains constant.” 4. Effect of motion on charge – Charge remains unaffected by motion. Example: The charge on electrons remains −1.6 × 10 −19 coulomb whether it is at rest or in motion. 5. Unitization of charge – The charge in a body is produced due to excess or deficiency of electrons. Electron cannot be divided into further smaller parts. Therefore charge on a body is integral multiple of the charge on an electron. q = ± ne Where e = −1.6 × 10−19 - quantum of charge n = 1, 2, 3 – positive or negative integral numbers. Possible charges may be e, 2 e, 3 e… but 0.5 e, 0.7e, 2.5 e, are not allowed.

Coulomb’s law: The force of attraction or repulsion between two point charges is directly proportion to the product of the charges and inversely proportional to the square of distance between them. The direction of this force is along the line joining the two charges.

F=K

q1q2 here K – Is a constant its value depends on medium between the charges and r2

system of units chosen. In SI Unit

1 Fr 2 newton × metre 2 K= = = 4π E0 q1q2 coulomb 2 Here E0 = Permittivity of vacuum or free space

E0 = 8.854 × 10−12 coulomb 2 / newtonmeter 2 1 1 ∴K = = 4π E0 4 × 3.14 × 8.854 ×10−12 ∴ K = 9 × 1010 newtonmetre 2 / coulomb 2 Direction of Coulomb force depends upon the nature of charges

A O

F12

B O

+ q1 A

O

+q1F12

+ q2

r O

F21

B

F21 − q2

Validity of coulomb’s law  Both charges must be point charges. The separation of charges must be greater than nuclear distance (i.e. great than 10 −15 m ).