JKBOSE Class 12th Physics | Electric Charges and Fields | ONE-SHOT | JKBOSE Wallah

TLDR;

Alright, so this entire video is basically a crash course on the first chapter of 12th-grade physics: Electric Charges and Fields. Sir's going to cover everything from the basics of charge to Gauss's Law, plus important derivations and numericals. He also shares tips on how to approach the exam and stay motivated.

Covers electrostatics, electric charge, Coulomb's law, electric fields, dipoles, Gauss's law, and their applications.
Focuses on understanding concepts, derivations, and numerical problem-solving.
Provides tips for exam preparation and staying motivated.

Introduction [0:00]

Sir is just starting off the class 12th journey and promises to teach from scratch, covering everything from basic concepts to important exam questions. He assures that the notes will be available on the PW app. The syllabus will be covered in about 2.5 hours, focusing on small questions, derivations, and numericals.

Class Notes [2:52]

Sir mentions that all the notes that he will write on the board will be available on Physics Wallah (PW) app.

JKBOSE Syllabus [3:03]

Sir plans to cover the entire syllabus, pointing out where to focus on short questions, derivations, and numericals.

Topics to be covered [3:50]

The first unit, Electrostatics, will be covered in two chapters, carrying a total of nine marks. The discussion will cover all possible questions from zero to hundred marks.

Electrostatics [5:02]

Electrostatics is the study of charges at rest. When a charge is at rest, it produces an electric field. When a charge moves with a constant velocity, it produces both electric and magnetic fields. When a charge moves with acceleration, it produces electric and magnetic fields along with electromagnetic waves.

Electric charge & its properties [12:01]

Electric charge is a basic property of matter that causes it to experience an electric force. There are two types of charges: positive (protons) and negative (electrons). The SI unit of charge is the coulomb (C). The CGS units are statcoulomb (esu) or Franklin (Fr). The smallest unit of charge is the elementary charge (electron), 1.6 x 10^-19 C. The largest unit of charge is Faraday, 1 Faraday = 96500 Coulombs. Charge is a scalar quantity and its dimension is AT.

Practice Questions [28:01]

Quantization of charge means charge is always an integral multiple of the fundamental charge (electron), represented by q = ne. Conservation of charge states that for an isolated system, charge can neither be created nor destroyed.

Method of charging [30:10]

Generally, bodies are neutral because they have equal numbers of electrons and protons. Charging occurs when two bodies are rubbed together, causing charge to transfer from one body to another. The body that donates electrons becomes positive, and the body that accepts electrons becomes negative. When a glass rod is rubbed with silk, the glass rod becomes positive because it donates electrons.

Practice Questions [40:25]

The value of charge in coulombs on 7N14 is calculated using q = ne, where n is 7 and e is 1.6 x 10^-19 C. The number of electrons present in 1 coulomb charge is 6.25 x 10^18. The CGS unit of charge is statcoulomb, and the SI unit is coulomb, with the relation 1 coulomb = 3 x 10^9 esu.

Coulomb’s law, Permittivity [46:44]

Coulomb's Law explains the force between two charges. The force is directly proportional to the product of the magnitudes of the charges and inversely proportional to the square of the distance between them: F = k * q1 * q2 / r^2. K is a constant that depends on the medium. In air, k = 1 / (4πε0) = 9 x 10^9 Nm^2/C^2, where ε0 is the permittivity of free space (8.85 x 10^-12 C^2/Nm^2).

Vector form of Coulomb’s law [1:02:13]

Coulomb's law in vector form considers the direction of forces. The force on charge 2 due to charge 1 is F21 = k * q1 * q2 / r^2 * r12 (unit vector from 1 to 2). Similarly, the force on charge 1 due to charge 2 is F12 = -F21. Coulomb's law follows Newton's third law of motion. Coulomb's force is a conservative and central force, following the inverse square law, and acts along the line joining the charges.

Principle of superposition [1:07:55]

The principle of superposition states that the net force on a charge is the vector sum of the forces exerted by individual charges. If there are multiple charges Q1, Q2, Q3... acting on a charge Q0, the net force on Q0 is the vector sum of individual forces.

Electric field, Intensity of Electric field [1:11:32]

The electric field is the space around a charge in which other charges experience a force of attraction or repulsion. A source charge creates the electric field, and a test charge (positive and very small) is used to detect and measure the field. The electric field intensity is the force per unit test charge: E = F/q0. It is a vector quantity with the unit N/C.

Electric field due to a point charge [1:17:20]

The electric field intensity due to a point charge is derived using Coulomb's law and the definition of electric field intensity. The electric field E = kQ/r^2, where Q is the source charge and r is the distance from the charge.

Electric field lines [1:20:36]

Electric field lines are imaginary lines that represent the direction and strength of the electric field. They originate from positive charges and terminate at negative charges. Electric field lines never intersect each other. Electric field lines are open curves, starting from positive and ending at negative charges. Uniform electric field lines are parallel to each other, while non-uniform electric field lines are not parallel.

Force on a charge in an electric field [1:30:01]

The force on a charge in an electric field is given by F = qE. A positive charge experiences a force in the direction of the electric field, while a negative charge experiences a force in the opposite direction.

Electric Dipole & Dipole Moment [1:32:58]

An electric dipole consists of two equal and opposite charges (+q and -q) separated by a small distance (2a). The dipole moment (P) is a vector quantity defined as the product of the magnitude of either charge and the distance between them: P = q * 2a. The direction of the dipole moment is from the negative to the positive charge. The unit of dipole moment is Coulomb-meter (Cm), and its dimension is ALT.

Electric field due to dipole [1:40:21]

The electric field due to a dipole is calculated at two points: on the axial line and on the equatorial line. For a dipole, the electric field on the axial line is E = 2kPr/ (r^2 - a^2)^2. If a is much smaller than r, then E = 2kP/r^3. The electric field on the equatorial line is E = kP/ (r^2 + a^2)^(3/2). If a is much smaller than r, then E = kP/r^3. The relationship between the electric field on the axial line and the equatorial line is E_axial = 2 * E_equatorial.

Practice Questions [2:19:44]

Sir discusses the condition for an ideal electric dipole, where the charge is very large (approaching infinity) and the distance between the charges is very small (approaching zero).

Area Vector, Electric flux, Charge densities, Gauss’s law [2:22:50]

Area is a vector quantity with a direction perpendicular to the surface. Electric flux is the number of electric field lines passing through an area, given by Φ = E * A = EAcosθ. The unit of electric flux is Nm²/C. Charge density is discussed for linear (λ = Q/L), surface (σ = Q/A), and volume (ρ = Q/V) distributions. Gauss's Law states that the total electric flux through a closed surface is proportional to the charge enclosed: Φ = Q_enclosed / ε0.

Application of Gauss’s law [2:36:12]

Gauss's Law is applied to calculate the electric field due to a uniformly charged infinitely long straight wire. A cylindrical Gaussian surface is considered, and the electric field is found to be E = λ / (2πε0r) = 2kλ/r. Gauss's Law is also applied to calculate the electric field due to an infinite plane sheet, resulting in E = σ / (2ε0).

Practice Questions [3:05:48]

If a dipole is placed inside a closed surface, the net flux is zero. The value of electric flux leaving out a unit positive charge is 1/ε0, or ε0^(-1).