Electrostatics is the branch of physics that deals with electric charges at rest and the forces, fields, and potentials produced by them.
We observe electrostatic effects in daily life. A plastic comb rubbed on dry hair attracts small pieces of paper, and lightning is a large-scale electrostatic phenomenon.
Electric charge is a fundamental property of matter responsible for electrical interactions.
There are two types of electric charges:
- Positive charge (+)
- Negative charge (−)
Like charges repel each other, while unlike charges attract each other.
The smallest unit of charge is the elementary charge.
The total charge on a body is always an integral multiple of the elementary charge.
Where:
- Q = total charge
- n = integer
- e = elementary charge
This property is known as quantization of charge.
According to the law of conservation of charge, charge can neither be created nor destroyed.
Methods of Charging
A body can be charged by friction, conduction, or induction.
Charging by Friction
When two neutral bodies are rubbed together, electrons are transferred from one body to another.
Example: Rubbing a glass rod with silk.
Charging by Conduction
In this method, a charged body transfers charge to a neutral body through direct contact.
Charging by Induction
A charged body induces charge separation in a nearby conductor without physical contact.
Coulomb’s Law
Coulomb’s law gives the magnitude of the electrostatic force between two point charges.
Consider two charges q₁ and q₂ separated by a distance r.
Combining the above relations:
Where:
- F = electrostatic force
- q₁ and q₂ = charges
- r = distance between charges
- k = Coulomb’s constant
The force is attractive for unlike charges and repulsive for like charges.
Electric Field
The region around a charge in which another charge experiences force is called an electric field.
Electric field intensity is defined as the force experienced per unit positive test charge.
The SI unit of electric field intensity is:
Electric field is a vector quantity.
Electric Field Due to a Point Charge
Consider a point charge Q.
The electric field at a distance r from the charge is:
The field is directed away from a positive charge and towards a negative charge.
Electric Field Lines
Electric field lines are imaginary lines used to represent the direction and strength of an electric field.
- They originate from positive charges.
- They terminate on negative charges.
- They never intersect.
- A higher density of lines indicates a stronger field.
Electric Flux
Electric flux represents the total electric field passing through a surface.
Its SI unit is:
Gauss’s Law
Gauss’s law relates electric flux through a closed surface to the net charge enclosed within it.
Where:
- Q = enclosed charge
- ε₀ = permittivity of free space
Electric Potential
Electric potential at a point is defined as the work done in bringing a unit positive charge from infinity to that point.
Where:
- V = electric potential
- W = work done
- q = charge
The SI unit of electric potential is Volt (V).
Potential Due to a Point Charge
The electric potential due to a point charge Q at distance r is:
Unlike electric field, electric potential is a scalar quantity.
Potential Difference
The potential difference between two points A and B is:
It represents the work done per unit charge in moving a charge from one point to another.
Capacitors
A capacitor is a device used to store electric charge and electrical energy.
The capacitance of a capacitor is defined as:
Where:
- C = capacitance
- Q = stored charge
- V = potential difference
The SI unit of capacitance is Farad (F).
Parallel Plate Capacitor
A parallel plate capacitor consists of two conducting plates separated by a small distance.
Its capacitance is:
Where:
- A = area of each plate
- d = separation between plates
Energy Stored in a Capacitor
The electrical energy stored in a capacitor is:
Equivalent forms are:
Important Formulas of Electrostatics
Conclusion
Electrostatics forms the foundation of electricity and electronics. Topics such as electric charge, Coulomb’s law, electric field, electric potential, Gauss’s law, and capacitors are essential for Class 12 Physics, JEE Main, JEE Advanced, and NEET examinations.
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