April 13

# Ch1: Electric charges and fields

#### Topics as per CBSE curriculum 2020-21

• Electric Charges
• Conservation of charge
• Coulomb’s law-force between two-point charges, forces between multiple charges
• Superposition principle
• Continuous charge distribution
• Electric field
• Electric field due to a point charge
• Electric field lines
• Electric dipole
• Electric field due to a dipole.
• Torque on a dipole in uniform electric field
• Electric flux
• Gauss’s theorem
• Gauss’s theorem applications to find field due to infinitely long straight wire, uniformly charged infinite plane sheet and uniformly charged thin spherical shell (field inside and outside).

Electric Charge: Electric charge is a basic characteristic that accompanies fundamental particles, wherever they exist.

According to William Gilbert, Charge is something possessed by material objects that make it possible for them to exert electrical force and respond to electrical force.
There are basically two kinds of charges; positive and negative. SI unit of charge is coulomb (C).
The name electricity is coined from the Greek word “elektron” meaning amber. Study of electricity is divided into-

• Electrostatics or Static Electricity : Electricity due to charges at rest.
• Current Electricity : Electricity due to charge in motion.

There are three ways that objects can be given a net charge
Charging by friction : this is useful for charging insulators. If you rub one material with another (say, a plastic ruler with a piece of paper towel), electrons have a tendency to be transferred from one material to the other.
Charging by conduction : useful for charging metals and other conductors. If a charged object touches a conductor, some charge will be transferred between the object and the conductor, charging the conductor with the same sign as the charge on the object.
Charging by induction : also useful for charging metals and other conductors. Again, a charged object is used, but this time it is only brought close to the conductor, and does not touch it. If the conductor is connected to ground (ground is basically anything neutral that can give up electrons to, or take electrons from, an object), electrons will either flow on to it or away from it. When the ground connection is removed, the conductor will have a charge opposite in sign to that of the charged object.

Properties of electric charge

• Charge is a scalar quantity: The like charges repel & unlike charges attract.
• Charges are additive in nature.
• Charges are conserved: The total charge of the isolated system is always conserved. It is not possible to create or destroy net charge carried by any isolated system although the charge carrying particles may be created or destroyed in a process.
• Charges are quantized: Experimentally it is established that all free charges are integral multiples of a basic unit of charge denoted by e. Thus charge q on a body is always given by q=±ne,  where  ‘n’  is an integer, $e=1.602\times{10}^{-19}C$
q2In recent years particles carrying fractional charges were also predicted which are known as “quarks”. Even then also quantization of charge will not get affected as the size of the basic unit of charge is taken as ‘e/3’ instead of ‘e’.