Electrostatics is the branch of physics that deals with electric charges at rest and the forces produced by them.
Every electrostatic phenomenon originates from the presence and interaction of electric charges.
Electric Charge
Electric charge is a fundamental property of matter responsible for electrical interactions.
The SI unit of charge is coulomb (C).
There are two types of charges.
1. Positive charge
2. Negative charge
Like charges repel each other while unlike charges attract each other.
Charge is represented by the symbol q.
Properties of Electric Charge
Electric charge possesses three important properties: additivity, conservation, and quantization.
Additivity of Charge
The total charge on a system is equal to the algebraic sum of all individual charges present in the system.
Positive and negative signs must be considered while adding charges.
For example, if a body contains +5 C and −2 C charges, the net charge becomes +3 C.
Conservation of Charge
Charge can neither be created nor destroyed.
It can only be transferred from one body to another.
The total charge of an isolated system always remains constant.
If one body loses electrons, another body gains the same number of electrons.
Quantization of Charge
Charge exists in discrete packets and not in arbitrary amounts.
The smallest charge that can exist independently is called the elementary charge.
The charge on any body is always an integral multiple of the elementary charge.
where
If a body gains electrons, n becomes negative.
If a body loses electrons, n becomes positive.
Charging of Bodies
A neutral body can be charged by transferring electrons from one body to another.
Charging by Friction
When two suitable materials are rubbed together, electrons transfer from one material to another.
One body becomes positively charged while the other becomes negatively charged.
Example: Rubbing a glass rod with silk.
Charging by Conduction
When a charged conductor touches a neutral conductor, charges flow between them.
The neutral conductor acquires the same type of charge as the charged conductor.
Charging by Induction
In induction, a conductor is charged without direct contact.
A nearby charged object causes redistribution of charges within the conductor.
After grounding and removing connections properly, the conductor acquires charge opposite to the inducing charge.
Conductors and Insulators
Materials are classified according to the ease with which charges move through them.
Conductors
Conductors contain free electrons that can move throughout the material.
Examples: Copper, aluminium, silver, graphite.
Electric charge spreads quickly over the entire conductor.
Insulators
Insulators do not contain free electrons for easy charge movement.
Examples: Rubber, plastic, glass, wood.
Charge remains localized and does not move freely.
Coulomb’s Law
Coulomb’s law gives the electrostatic force between two stationary point charges.
The force is directly proportional to the product of the magnitudes of the two charges.
This means larger charges produce stronger forces.
The force is inversely proportional to the square of the distance between the charges.
This means the force decreases rapidly as the distance increases.
Combining both proportionalities,
Introducing the proportionality constant gives Coulomb’s law.
This equation gives the magnitude of electrostatic force between two point charges in vacuum.
It is commonly written as
where
and
The permittivity of free space is
Physical Meaning of Coulomb’s Law
If one of the charges is doubled, the force doubles.
If both charges are doubled, the force becomes four times.
If the distance becomes twice, the force becomes one-fourth.
If the distance becomes three times, the force becomes one-ninth.
This inverse-square dependence is responsible for the rapid decrease of electrostatic force with distance.
Nature of Coulomb Force
Like charges repel each other.
Unlike charges attract each other.
The force always acts along the straight line joining the two charges.
Vector Form of Coulomb’s Law
The vector form provides both magnitude and direction of the force.
Here
The direction automatically indicates attraction or repulsion depending on the signs of the charges.
Effect of Medium on Electrostatic Force
The force between two charges decreases when a medium is placed between them.
In a medium, Coulomb’s law becomes
where
K is called the dielectric constant or relative permittivity of the medium.
Therefore
A larger dielectric constant reduces the electrostatic force more strongly.
Principle of Superposition
When many charges are present, each charge exerts its force independently of the others.
The resultant force on a charge is the vector sum of all individual forces.
This principle is called the principle of superposition.
It is used extensively in problems involving multiple charges.
Important Formula Summary
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