(3347) Electric charge and field Oneshot in 30 minutes | Class 12th Physics Chapter 1 Oneshot CBSE JEE NEET

Electric Charge

• Definition: Electric charge is a property of materials that allows them to exert electrostatic forces on other materials, causing attraction or repulsion.

• Types of Charge:

  • Negative Charge: Occurs when a body has excess electrons.

  • Positive Charge: Occurs when there is a deficiency of electrons compared to protons in a body.

• Effect on Mass:

  • A negatively charged body will have an increase in mass due to the additional electrons.

  • A positively charged body will have a decrease in mass due to the loss of electrons.

• Charge Quantization:

  • Charge on electrons: -1.6 × 10^-19 Coulombs.

  • Charge on protons: +1.6 × 10^-19 Coulombs.

Charging Methods

• Charging by Friction: When two bodies are rubbed together, electrons transfer from one to another, resulting in a net charge.

• Charging by Conduction: Involves direct contact between charged and uncharged bodies, allowing electrons to transfer.

• Charging by Induction: A charged body is brought close to an uncharged body without contact, inducing a charge in the latter. Once the uncharged body is grounded, it retains the charge after the inducing body is removed.

Properties of Charge

• Additivity of Charge: The net charge in a system is calculated by algebraic addition, regardless of the direction (positive or negative).

• Conservation of Charge: Charge cannot be created or destroyed, only transferred.

• Quantization of Charge: Charge occurs in discrete amounts, specifically integral multiples of the charge on an electron.

Coulomb's Law

• Definition: Coulomb's Law calculates the force between two point charges.

• Formula: F = (k * |q1 * q2|) / r²

  • Where k = 1 / (4πε₀) = 9 × 10^9 N m²/C² (Coulomb's Constant)

• Vector Form: Indicates the direction of the electrostatic force between two charges.

Electric Field

• Definition: Electric field lines illustrate the direction of force on a positive charge in the vicinity of other charges.

• Characteristics:

  • Lines originate from positive charges and terminate at negative charges.

  • Electric field lines never intersect.

• Mathematical Expression: E = F / q₀, where F is the force on a test charge q₀.

Electric Dipole

• Definition: A pair of equal and opposite charges separated by a distance.

• Dipole Moment (p): Defined as the product of one charge and the distance between charges: p = q * d.

• Electric Field Due to a Dipole:

  • The field varies depending on the observer's position: axial, equatorial, or general point calculation exists.

Applications of Gauss's Law

• Closed Surfaces: The net electric flux through a closed surface is directly proportional to the charge enclosed.

• Relationship: Φ_E = Q_enc/ε₀, where Φ_E is the electric flux and Q_enc is the enclosed charge.

• Spherical Symmetry: For uniformly charged spheres or cylindrical distributions, electric fields can be deduced using Gauss's Law effectively.

• Charge Distribution: When considering a charged conducting sphere, the electric field outside acts as if all the charge were located at the center; inside, it is zero.

Electric Field from Different Charge Geometries

• Point Charge: Electric field E = k * q / r², pointing away from positive and towards negative charges.

• Ring: Electric field on the axis is derived from a formula relating charge, distance, and constants.

• Conductors and Insulators: Inside an ideal conductor, the electric field is always zero, while surface charges create electric fields outside.