About this Video

• Video Title: Class 12 Chapter 1 || Electric Charges and Fields 01 || Quantisation and Conservation of Charge
• Channel: Physics Wallah - Alakh Pandey
• Speakers: Alakh Pandey
• Duration: 00:40:13

Introduction

This video begins Alakh Pandey's Class 12 Physics lectures. The introductory lesson covers the fundamental concepts of electric charges and fields, focusing on the quantization and conservation of charge. It establishes a foundation for electrostatics, a key component of the class's electromagnetism unit.

Key Takeaways

1. Charge as an Intrinsic Property: Charge is an inherent property of matter, like mass, and cannot be further broken down.
2. Conservation of Charge: The total charge within an isolated system remains constant; charge cannot be created or destroyed, only transferred. This principle is applicable in various contexts, including nuclear reactions.
3. Quantization of Charge: Charge exists in discrete units, multiples of the fundamental charge (e), which is the charge of a single electron (approximately 1.6 x 10^-19 Coulombs). Fractional charges exist (e.g., in quarks), but these are not independent.
4. Methods of Charging a Body: Three primary methods are explained: charging by conduction (direct contact), charging by induction (without contact), and charging by friction (rubbing surfaces together).
5. Charge and Mass Differences: While charge always accompanies mass, the reverse isn't true. Charge can be positive or negative, while mass is always positive. Furthermore, mass is velocity-dependent (relativistic effects), while charge is not.

Understanding Electric Charges and Fields: A Detailed Explanation

This document expands on the key concepts explained in the Physics Wallah video, "Class 12 Chapter 1 || Electric Charges and Fields 01 || Quantisation and Conservation of Charge," aiming for clarity and ease of understanding.

I. Introduction to Electric Charge:

The video begins by introducing the concept of electric charge as a fundamental property of matter, similar to mass. It's an intrinsic property, meaning it's inherent to the object itself and not dependent on any other factors. Just as we can't break down mass into simpler components and understand its fundamental nature, we can't do the same with charge.

II. Types and Properties of Charge:

There are two types of electric charge:

• Positive Charge: Often associated with protons.
• Negative Charge: Often associated with electrons.

Like charges (positive-positive or negative-negative) repel each other. Unlike charges (positive-negative) attract each other. This is a fundamental interaction governing the behavior of charged particles. The SI unit of charge is the Coulomb (C), and the symbol for charge is usually 'q'. Charge is a scalar quantity; it has magnitude but no direction.

III. Conservation of Charge:

A crucial principle is the conservation of charge: the total electric charge in an isolated system remains constant. This means charge cannot be created or destroyed; it can only be transferred from one object to another. This principle is observed consistently in various phenomena, including:

• Charging by Conduction: Direct contact between a charged object and a neutral object results in the transfer of charge until they reach the same potential. If a negatively charged object touches a neutral one, some electrons move to the neutral object, making both negatively charged (though the overall charge remains the same).

• Charging by Induction: A charged object is brought near a neutral conductor, inducing a separation of charge in the conductor. The closer end gets a charge opposite to that of the inducing charge, while the farther end receives the same charge. Connecting the farther end to the earth allows charge to flow to the earth, leaving the conductor with a net charge opposite to that of the inducing object.

• Charging by Friction: Rubbing two objects together can transfer electrons from one to the other, resulting in one object becoming positively charged (losing electrons) and the other negatively charged (gaining electrons). The total charge remains constant.

• Nuclear Reactions: In radioactive decay (such as alpha decay), the total charge before and after the reaction remains the same.

IV. Quantization of Charge:

Electric charge is quantized, meaning it exists in discrete amounts. The smallest unit of charge that can exist independently is the elementary charge, e, which is the magnitude of the charge of a single electron or proton. The value of e is approximately 1.6 × 10^-19 Coulombs.

Any charge on an object is always an integer multiple of e. This means you can have a charge of +e, +2e, +3e, -e, -2e, etc., but never a charge of, for example, +1.5e. While particles like quarks possess fractional charges, they are never found independently; they always combine to form particles with integer multiples of e.

The quantization of charge is mathematically represented as: q = ±ne where:

• q is the charge on the object.
• n is an integer (1, 2, 3,...).
• e is the elementary charge (1.6 × 10^-19 C).

V. Difference Between Charge and Mass:

Though both are fundamental properties of matter, charge and mass differ in several aspects:

• Types: Charge is of two types (positive and negative), while mass is always positive.
• Velocity Dependence: Mass is velocity-dependent according to Einstein's theory of relativity (at high speeds, mass increases). However, charge is independent of velocity.

VI. Example Problem (Quantization):

The video concludes with a problem to illustrate quantization: Can a charge of 8 × 10¹⁸ Coulombs be given to a body? To solve, we use the quantization equation:

q = ne

Solving for n:

n = q/e = (8 × 10<sup>18</sup> C) / (1.6 × 10<sup>-19</sup> C) = 5 × 10<sup>37</sup>

Since n is an integer, this charge can be given to a body. It represents the transfer of a large number (5 × 10³⁷) of elementary charges. If 'n' were not an integer, it would be impossible to transfer that charge because charge exists only in multiples of the fundamental unit.

This detailed explanation complements the video, providing a comprehensive understanding of the fundamental concepts of electric charges and fields. Remember to consult textbooks and other reliable sources for further learning and practice.