1. Introduction

• Static Electricity: The phenomenon of seeing sparks or hearing crackles when taking off synthetic clothes is caused by the discharge of electric charges accumulated through rubbing.

• Definition of Electrostatics: It is the study of forces, fields, and potentials that arise from static charges—charges that do not move or change with time.

• 2. Electric Charge

• Discovery: Thales of Miletus credited the discovery of electrification to amber rubbed with wool or silk around 600 BC.

• Types of Charges: There are only two types of electric charges, named positive and negative by Benjamin Franklin.

• Fundamental Law: Like charges repel each other, while unlike charges attract each other.

   

• Polarity: The property that differentiates the two kinds of charges is called the polarity of charge.

• Neutrality: An object is "electrically neutral" if it possesses no net charge; it is "electrified" or "charged" if it does.

• Detection: A gold-leaf electroscope is a simple apparatus used to detect the presence and amount of charge on a body.

3. Conductors and Insulators

• Conductors: Substances that readily allow electricity to pass through them because they contain "free" electrons (e.g., metals, the human body, and Earth).

   

• Insulators: Substances that offer high resistance to the passage of electricity (e.g., glass, plastic, nylon, and wood).

   

• Charge Distribution: In a conductor, transferred charge distributes itself over the entire surface. In an insulator, the charge stays at the same place where it was put.

   

• Semiconductors: A third category of materials with electrical resistance intermediate between conductors and insulators.

4. Basic Properties of Electric Charge

• Additivity: Total charge in a system is the algebraic sum of all individual point charges, treated as scalars like real numbers.

   

• Conservation: The total charge of an isolated system is always conserved; charges can be redistributed but never created or destroyed.

   

• Quantisation: All free charges are integral multiples of a basic unit of charge, denoted by $e$.

  • The formula is q = ne where n is any integer.

     

     

  • The value of e is approximately 1.602 times 10^{-19} C

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  Macroscopic vs. Microscopic: At a macroscopic scale, the "grainy" nature of charge is ignored, and it is treated as continuous. 

5. Coulomb's Law

• Definition: A quantitative statement describing the force between two point charges.

   

• Mathematical Form: The magnitude of the force between two point charges q1 and q2 separated by distance r in a vacuum is:

   F = k q1 q2 /r^2

   

• Inverse Square Law: The force varies inversely as the square of the distance between the charges and is directly proportional to the product of the magnitudes of the charges.

   

• Permittivity: In SI units, k is usually written as 1 / (4\pi\epsilon0), where epsilon0 is the permittivity of free space 8.854 times 10^{-12} C^2 N^{-1} m^{-2}

   

   

• Vector Nature: The force acts along the line joining the two charges. Coulomb's law is consistent with Newton’s third law (F12= -F21).

   

   

• Strength: Electrical forces are enormously stronger than gravitational forces (e.g., for an electron and proton, the ratio is approximately 2.4 \times 10^39