Why this matters for JEE

Coulomb's law is the foundation of electrostatics — a high-weightage, high-scoring topic.

JEE asks 2-3 questions every year on electric charge, Coulomb's law, and electric fields. The inverse-square law is one of the most tested concepts across both JEE Main and Advanced.

Key formulas

Coulomb's Law & Electrostatics Formulas

Coulomb's Law

F = k · q₁q₂/r²

k = 8.988×10⁹ N·m²/C². Like charges repel, opposite charges attract.

Electric field

E = F/q = k·Q/r²

Force per unit charge. Points away from positive, toward negative.

Superposition

F_net = F₁ + F₂ + F₃ + ...

Force from multiple charges = vector sum of individual forces.

Electric flux

Φ = E·A·cos(θ)

For closed surface: Φ = Q_enc/ε₀ (Gauss's law).

Charge quantization

q = ne, e = 1.6×10⁻¹⁹ C

All charges are integer multiples of electron charge.

Coulomb vs Gravity

F_e/F_g = kq₁q₂/Gm₁m₂

Electric force is ~10³⁶ times stronger than gravity. Key comparison!

Step-by-step problem solving

How to solve electrostatics problems in JEE

Identify all charges and positions — list every charge with its sign and coordinates.
Apply Coulomb's law for each pair — F = kq₁q₂/r². Calculate magnitude first, then sign.
Determine direction — repulsion for like charges, attraction for unlike. Draw force vectors.
Use superposition — for 3+ charges, find net force by vector addition.
Consider special techniques — Gauss's law for symmetric charge distributions, symmetry arguments for cancellation.

Common JEE mistakes

What students get wrong

❌ Forgetting direction in superposition

Force is a vector — you MUST add components, not magnitudes. F_net = √(ΣF_x² + ΣF_y²), not |F₁| + |F₂| + |F₃|.

❌ Units and powers of 10

Charge is often in μC or nC. Convert to Coulombs first: 1 μC = 10⁻⁶ C. Distance in cm? Convert to m.

❌ Wrong sign on force

F = k|q₁||q₂|/r² gives magnitude. The sign tells you whether the force is attractive (-) or repulsive (+). Don't just plug negative charges into the formula blindly.

❌ Ignoring equilibrium conditions

If a test charge is placed at a null point, net E = 0. But if displaced, the stable/unstable nature depends on the second derivative. JEE Advanced tests this!

FAQs

Coulomb's Law Questions

Why is the force inversely proportional to r²?

Because electric field lines spread out radially. The surface area of a sphere is 4πr². Doubling r spreads the same field over 4× the area → field strength drops by factor of 4. This is the inverse-square law.

What's the difference between k and 1/(4πε₀)?

They're the same: k = 1/(4πε₀) = 8.988×10⁹ N·m²/C². JEE prefers k notation; some textbooks use ε₀. Both give the same answer.

How do I know if a charge is in equilibrium?

A charge is in equilibrium when net force on it = 0. For three charges on a line, the middle one can have zero net force if the two outer forces cancel. For continuous charge, use ∫dF = 0.

Can I use Coulomb's law for continuous charge distributions?

Not directly — you need to integrate. Break continuous charge into infinitesimal dq, apply dF = k·dq·q/r², then integrate over the entire distribution. For symmetric cases, use Gauss's law instead.

Coulomb's Law Simulator - Electrostatic Force