JEE 2020 Study Notes: Electric Charges, Fields, & Flux

Electrostatics is an important topic from JEE Main / JEE Advanced Exam Point of view. Every year there are 1-3 questions asked from this topic. This short notes on Electrostatics will help you in revising the topic before the JEE Main & IIT JEE Advanced Exam.

Electrostatics Notes

Electrostatics is the branch of physics which deals with the interaction between the stationary and slow-moving charges and their phenomena.

Electric charges and their conservation:

Electric charge is one of the fundamental physical property of matter due to it experiences a force when kept in the Electromagnetic field.

There are three types of charged particles every object can possess:

1. Positive charge (Proton)

2. Negative charge (Electron)

3. Neutral (Neutron)

Conservation of charge

For an isolated system, the net charge of the system remains the same. Therefore charge can't be created and can't be destroyed.

Properties of Electric Charge

1. The electric charge (e) is quantized.

2. The like charges repel each other and unlike attract each other.

Coulomb's law

Coulomb's law states that the force acting between two point charges is directly proportional to the product of the magnitude of the charges and inversely proportional to the square of the distance between them.

where is the constant of proportionality and is the permittivity of free space.

Vector Form of the Coulomb's law is

The force between multiple Charges

The force between the multiple charges can be found using the superposition principle.

Superposition Principle- According to the superposition principle the net force on any charge due to multiple charges is the vector sum of all the forces acting on that charge due to other charges are taken at a time. The individual forces remain unaffected due to other charges presence.

For n charges the net resultant force due to the principle of superposition.

Continuous Charge Distribution

If the charge is continuously distributed over the surface of the conductor it is known as Continuous charge distribution. In continuous charge system, a very large numbers of charges are diligently packed and have insignificant space amid them.

There are three types of Continuous charge Distribution:

1. Line Charge distribution When the charge is consistently spread over the length L of a conductor then it is known as linear charge distribution. It is represented by λ,

2. Surface Charge Distribution When the charge is consistently spread over the surface area of a conductor then it is known as surface charge distribution. It is represented by σ,

3. Volume Charge Distribution When the charge is consistently spread over the volume of a conductor then it is known as volume charge distribution. It is represented by ρ,

Electric field and dipole

Electric field- Electric field is defined as the force experienced per unit positive test charge at a point due to any other charge kept in the vicinity.

Electric field lines- Electric field lines are the hypothetic lines that follow the same direction as the electric field vector at any point. Electric field vector is always tangent to these electric field lines.

The field lines emerge from the positive charge and terminate on a negative charge.

The number of lines drawn leaving a positive charge or approaching a negative charge is proportional to the magnitude of the charge.

No two field lines can cross each other’s path.

Electric Dipole- Electric dipole is a system of two equal in magnitude opposite charges that are separated for a very small distance.

Electric dipole moment: The product of the charge and the separation between them is known as Electric dipole moment. It is a vector quantity which is always directed towards the positive charge.

Electric field due to a dipole

1. On the axial point: Electric field due to the dipole on the axial point P is,

2. On the equatorial plane: Electric field due to the dipole on equatorial plane at a point P is,

Torque Experienced by a dipole in a uniform electric field

The magnitude of the torque is experienced by a dipole in a uniform electric field is,

Electric flux

Electric flux (Φ_E)- Electric flux is defined as the number of electric field lines passing through the surface S. It measures the low of electric field through a given area.

For a uniformly charged conductor, the net flux through the closed surface is