This subtopic covers the fundamental concepts and historical development of electric charge, essential for understanding the nature of electric charge and solving related exam questions confidently.

• Electric Charge: A fundamental property of matter that causes it to experience a force when placed in an electric and magnetic field.
• Benjamin Franklin's Convention: He assigned the terms positive (+) and negative (−) to electric charges arbitrarily. According to Franklin, the charge on a glass rod rubbed with silk is positive, and the charge on the rubber rod rubbed with fur is negative.
• Fundamental Unit of Charge (Elementary Charge, \( e \)): The smallest stable unit of electric charge, carried by a single proton or electron. Its value is \( e = 1.6 \times 10^{-19} \) coulombs (C).
• Charge Quantisation: Electric charge exists only in discrete amounts which are integral multiples of the elementary charge \( e \). This means charge can be expressed as \( q = n \times e \), where \( n \) is an integer.
• Significance of Charge Quantisation: It explains why charges are always found in whole number multiples of \( e \) and never in fractional amounts.
• Historical Contributions:
  1. Benjamin Franklin: Named positive and negative charges.
  2. J.J. Thomson: Discovered the electron, a negatively charged particle.
  3. Millikan: Determined the value of the elementary charge through the oil drop experiment.
• Properties of Subatomic Particles:
  • Proton: Positive charge \( +e \)
  • Electron: Negative charge \( -e \)
  • Neutron: Electrically neutral (charge = 0)
  • Positron: Positive charge \( +e \), antiparticle of electron
• SI Unit of Charge: Coulomb (C), a derived SI unit defined as the charge transported by a current of one ampere in one second.

Tips to Remember:

1. Always use \( e = 1.6 \times 10^{-19} \) C for calculations involving elementary charge.
2. Charge quantisation means charge can only be in multiples of \( e \), never fractions.
3. Franklin’s convention is historical; modern convention aligns with it but the naming was arbitrary.
4. Neutrons have zero charge because they do not carry elementary charge units.