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Syllabus
🩺 NEET UG
Subject:Physics Syllabus

Electrostatics & Capacitance

Syllabus Unit: Electrodynamics

Why it matters

Chapter overview

Electrostatics studies electric charge at rest — how it exerts force on other charges (Coulomb's law), sets up a field and a potential in the space around it, and how conductors can be arranged to store charge and energy (capacitors). NEET builds this module from a single point charge's force and field, through the shortcut Gauss's law provides for symmetric charge distributions, up to the practical charge-storage device every circuit relies on.

Physics Syllabus Electrodynamics 25%–30% weightage
Live Animation1 / 4
r · F ∝ 1/r²+−F = k·q₁q₂ / r²Coulomb's Law Force Lab

1Coulomb's Law & Electric Field

Coulomb's law gives the force between two point charges; the electric field describes the force per unit charge any charge would feel at a point in space.

Learning Objectives

  • State Coulomb's law and identify the role of the constant k = 1/4πε0 ≈ 9×10^9 N·m²/C².
  • Calculate the force between two point charges and how it changes with distance or medium.
  • Define electric field intensity E = F/q0 and compute E due to a point charge.
  • Sketch field lines for isolated point charges and a dipole, and state the rules that govern them.

Key Terms

Coulomb's Constant (k)k = 1/(4πε0) ≈ 9×10^9 N·m²/C² in vacuum; relates electrostatic force to the product of charges and the square of their separation.
Permittivity (ε0)ε0 = 8.85×10^-12 C²/N·m²; a medium's dielectric constant K tells how much it reduces the force compared to vacuum.
Electric Field (E)Force experienced per unit positive test charge, E = kQ/r², directed radially away from a positive source charge and toward a negative one.
Superposition PrincipleThe net force or field due to several charges is the vector sum of the forces or fields due to each charge acting alone.

**Coulomb's law** states that the electrostatic force between two point charges is directly proportional to the product of their magnitudes and inversely proportional to the square of the distance between them, acting along the line joining them.

Coulomb's law in detail

  • F = kq1q2/r², with k = 1/4πε0 ≈ 9×10^9 N·m²/C² in vacuum/air.
  • Force is repulsive for like charges, attractive for unlike charges — always along the line joining them.
  • It is an inverse-square law: doubling r cuts the force to a quarter.
  • In a medium of dielectric constant K, the force reduces to F/K compared to vacuum.
Key PointNEET often tests the medium change: force in a dielectric medium is F_medium = F_vacuum / K — a frequent numerical trap when combined with a distance change.

Electric field

The electric field at a point is the force per unit positive test charge placed there: **E = F/q0**. For a point charge Q, E = kQ/r², pointing radially outward from a positive charge and radially inward toward a negative one.

  • Field lines originate on positive charges and terminate on negative charges.
  • Two field lines never cross — the field at any point has one unique direction.
  • Denser lines indicate a stronger field; the tangent at any point gives the field's direction there.
Common MistakeCommon mix-up: field lines are a visualisation tool, not physical entities — they never actually cross because the net field at a point is a single vector, not two.
F = k q1 q2 / r² E = F/q0 = kQ/r²