Introduction to Electric Charges and Fields

Electric Charges and Fields is the foundation of electrostatics. This chapter deals with interaction between charged particles, electric field, flux, and Gauss’s law, which are essential for understanding electric forces, energy, and potential in physics systems.

For JEE Main, this chapter is critical because it covers Coulomb’s law, field distribution, and Gauss’s law, which frequently appear in numerical and conceptual problems.

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Importance of Electric Charges and Fields in JEE Main

Studying Electric Charges and Fields helps students:

• Understand forces between charges using Coulomb’s law

• Calculate electric field and flux due to single or multiple charges

• Apply Gauss’s law in symmetric charge distributions

• Solve numericals on point charges, continuous charge distributions, and systems of charges

JEE Main often tests both derivation-based conceptual questions and high-scoring numerical problems, making this chapter a must-prepare topic.

Electric Charge

• Types of charge: Positive (+) and Negative (−)

• Properties of charge:

  • Quantization: q = n e

  • Conservation of charge: Total charge in an isolated system remains constant

  • Additivity: Charges combine algebraically

• Conductors and Insulators:

  • Conductors allow free movement of charges

  • Insulators restrict charge flow

Applications:

• Charging by friction, conduction, and induction

• Understanding static electricity and electrostatics experiments

Coulomb’s Law

• Force between two point charges:
  F = k q₁ q₂ / r²

  • k = 1 / (4πε₀), r = distance between charges

• Vector form: F = k q₁ q₂ r̂ / r²

• Superposition principle: Net force on a charge = vector sum of forces from all other charges

Applications in JEE Main:

• Calculating net force on a charge in systems

• Force between charges in a line, triangle, or square

• Electrostatic equilibrium problems

Electric Field

• Definition: Force per unit positive charge at a point

  • E = F / q

• Due to point charge: E = k q / r²

• Electric field lines:

  • Radial outward for positive, inward for negative charges

  • Number of lines ∝ magnitude of charge

Applications:

• Electric field at a point due to multiple charges

• Field along axis of a ring, disk, or plane

• Direction and magnitude of electric field vectors

Electric Dipole

• Dipole moment: p = q × 2a (q = charge, 2a = separation)

• Field on axial line: E_axial = (1 / 4πε₀) 2p / r³

• Field on equatorial line: E_equatorial = (1 / 4πε₀) p / r³

• Torque on dipole in uniform field: τ = p × E

Applications in JEE Main:

• Force and torque on dipoles

• Electric field along dipole axis and perpendicular bisector

• Energy of dipoles in external field

Electric Flux and Gauss’s Law

• Electric flux (Φ): Total number of field lines passing through a surface

  • Φ = E · A cos θ

• Gauss’s Law: Total flux through closed surface = q_enclosed / ε₀

• Applications:

  • Field due to infinite plane sheet: E = σ / 2ε₀

  • Field of infinite line charge: E = λ / 2πε₀ r

  • Spherical charge distribution: E = q / 4πε₀ r²

JEE Main numericals often test symmetric charge distributions using Gauss’s law.

System of Charges

• Superposition principle for multiple charges

• Net field and potential at a point

• Work done in moving a charge: W = q ΔV

• Potential due to point charges: V = k Σ q / r

Applications:

• Equilibrium positions of charges

• Potential and field calculations in triangular or square configurations

• Energy of systems of charges

Potential Energy and Work

• Electrostatic potential energy:

  • For two charges: U = k q₁ q₂ / r

  • For system of multiple charges: Sum of all pairwise energies

• Work done in moving a charge:

  • W = −ΔU = q ΔV

Applications in JEE Main:

• Energy stored in systems of point charges

• Work done to assemble a system of charges

Applications in Daily Life and Engineering

• Electrostatics in capacitors and insulating materials

• Charge distribution on conductors

• Field mapping and electronic sensors

• Electrostatic precipitators and paint spraying

Understanding real-life applications helps students relate theory with practical JEE Main problems.

Problems on Electric Charges and Fields

• Force between charges in line, triangle, or square

• Electric field at a point due to single and multiple charges

• Field along axis and equatorial line of dipole

• Work done in moving a charge, potential energy calculations

• Flux through surfaces using Gauss’s law

JEE Main numericals often combine Coulomb’s law, electric field, flux, and potential concepts.

Common Mistakes Students Make

• Confusing electric field and electric potential

• Ignoring vector nature of forces and fields

• Wrong choice of Gaussian surface in flux problems

• Miscalculating net force in systems of charges

• Forgetting signs (+ or −) in potential and energy calculations

Avoiding these mistakes ensures accuracy and confidence in solving JEE Main problems.

Weightage of Electric Charges and Fields in JEE Main

• 2–4 questions per exam

• Mix of conceptual and numerical problems

• Moderate difficulty but high scoring for well-prepared students

Preparation Tips for JEE Main Students

• Memorize formulas for Coulomb’s law, electric field, potential, and Gauss’s law

• Solve numericals for system of charges and dipoles

• Practice flux calculations with different surfaces

• Understand vector addition of electric fields

• Draw diagrams for field lines, dipoles, and Gaussian surfaces

Regular practice ensures speed, accuracy, and conceptual clarity.

Why Study Electric Charges and Fields from Studentbro.in

Studentbro.in provides:

• Step-by-step explanations for Coulomb’s law, electric field, potential, and flux

• Solved examples on systems of charges and Gauss’s law

• Conceptual clarity for adva