Electrostatics - Electric Charges and Field, CLASS 12 CBSE NOTES

Electrostatics - Electrostatics is a fundamental concept in physics, dealing with electric charges at rest and the interactions between them. This article will provide an in-depth exploration of electric charges, their properties, the electric field, and other related principles. Let’s delve into the fascinating world of electrostatics.

What is Electrostatics?

Electrostatics is the branch of physics that studies electric charges at rest and the forces, fields, and potentials associated with them. It primarily focuses on understanding how charges interact in the absence of motion.

Electric Charge (q)

Electric charge is an intrinsic property of matter that enables it to experience electrostatic forces of attraction or repulsion. It is the foundation of electrostatics.

Types of Electric Charges

1. Positive Charge (+): Associated with protons.
2. Negative Charge (-): Associated with electrons.

Charge on a Single Electron

The charge of a single electron is quantified as:

• $e = -1.6 \times 10^{-19} \, C$

Unit of Charge

The standard unit of electric charge is the Coulomb (C).

Properties of Electric Charges

Electric charges exhibit several distinctive properties:

1. Like Charges Repel, Unlike Charges Attract

• Like charges (e.g., two positives or two negatives) repel each other.
• Unlike charges (e.g., a positive and a negative) attract each other.

2. Additive Nature of Charge

The total charge on a body is the algebraic sum of all charges present on it.

3. Quantization of Charge

Electric charge is quantized and expressed as: $Q = n \cdot e$ Where:

• $Q$ = Total charge
• $n$ = Integer (1, 2, 3, ...)
• $e$ = Elementary charge ($1.6 \times 10^{-19} \, C$)

4. Invariability of Charge

Charge does not depend on the state of rest or motion of the object; it is an invariant property of matter.

Principle of Conservation of Charge

The principle states that:

• Charge can neither be created nor destroyed in an isolated system.
• The total charge remains constant over time.

Example:

• Excess electrons on a body result in a negative charge.
• Excess protons on a body result in a positive charge.

Coulomb’s Law

Coulomb’s law defines the force between two point charges. It states:

• The electrostatic force ($F$) is directly proportional to the product of the charges ($q_1$ and $q_2$).
• It is inversely proportional to the square of the distance ($r$) between them.

Mathematical Representation

$F = k \cdot \frac{q_1 \cdot q_2}{r^2}$ Where:

• $k = \frac{1}{4 \pi \epsilon_0} = 9 \times 10^9 \, N \, m^2 \, C^{-2}$
• $\epsilon_0$ = Permittivity of free space ($8.85 \times 10^{-12} \, C^2 \, N^{-1} \, m^{-2}$).

Force Between Multiple Charges

The net force acting on a charge due to multiple other charges is determined by the principle of superposition.

Principle of Superposition

• The total force on a charge is the vector sum of all forces acting on it.
• Individual forces remain unaffected by the presence of other charges.

Electric Field (E)

The electric field is the region around a charged body where another charge experiences a force.

Definition of Electric Field Strength

The electric field strength ($E$) at a point is defined as the force ($F$) experienced by a unit positive charge ($q$) at that point: $E = \frac{F}{q}$

SI Unit of Electric Field

• $N/C$ (Newton per Coulomb) or $V/m$ (Volt per meter).

Characteristics of Electric Field

1. The electric field is independent of the test charge.
2. For a positive charge, the field is directed outward.
3. For a negative charge, the field is directed inward.
4. The magnitude of the electric field decreases with the square of the distance ($E \propto \frac{1}{r^2}$).

Electric Field Lines

Electric field lines are imaginary lines that represent the direction of the electric field in a region.

Properties of Electric Field Lines

1. Originate from positive charges and terminate on negative charges.
2. Never intersect each other.
3. Greater density of lines indicates a stronger electric field.
4. They are continuous but do not form closed loops.

Electric Dipole

An electric dipole consists of two equal and opposite charges separated by a small distance.

Dipole Moment

The dipole moment ($\mathbf{p}$) is a vector quantity that measures the strength of the dipole: $\mathbf{p} = q \cdot d$ Where:

• $q$ = Charge
• $d$ = Distance between the charges.

Applications of Electrostatics

Electrostatics has several practical applications, including:

1. Photocopiers and laser printers.
2. Electrostatic precipitators for pollution control.
3. Working of capacitors in electrical circuits.

FAQs About Electrostatics

1. What is electrostatics?

Electrostatics is the study of electric charges at rest and the forces and fields they produce.

2. How are charges classified?

Charges are classified into two types: positive (proton) and negative (electron).

3. What is Coulomb's law?

Coulomb's law defines the force between two point charges and is proportional to the product of their charges and inversely proportional to the square of their separation.

4. What is the unit of electric charge?

The unit of electric charge is the Coulomb (C).

5. What are electric field lines?

Electric field lines are imaginary lines representing the direction of the electric field around a charge.

6. What is the quantization of charge?

Charge is quantized and exists in integral multiples of the elementary charge ($e = 1.6 \times 10^{-19} \, C$).