Coulomb's LawJEE Main

Force between two point charges — interactive Physics simulation for IIT-JEE.

Concept

Two point charges push or pull along the line joining them with a force that grows with the product of the charges and falls off as the inverse square of their separation. Like charges repel; unlike charges attract.

Key formula

F = \frac{1}{4\pi\varepsilon_0}\frac{|q_1 q_2|}{r^2} = k\frac{|q_1 q_2|}{r^2}, \quad k \approx 8.99\times10^9

Derivation

The force is proportional to each charge and inversely proportional to $r^2$ , mirroring Newton's gravitation but vastly stronger and able to attract or repel.

Doubling the distance quarters the force; doubling either charge doubles it. The (signed) potential energy $U = kq_1q_2/r$ is positive for like charges and negative for unlike ones.

Scenarios to explore

Coulomb's Law — Inverse-square force between two point charges.

Real-world applications

Atomic and molecular bonding.
Capacitors and electrostatic shielding.
Millikan's oil-drop and particle-trap experiments.

JEE exam tips

Use the superposition principle for more than two charges (vector sum).
In a medium, divide the vacuum force by the dielectric constant $\varepsilon_r$ .
Force is along the line joining the charges — always a vector.

Common mistakes

Forgetting the inverse-square (using $1/r$ instead of $1/r^2$ ).
Mismatched units — charge in coulombs, distance in metres.
Dropping the sign when computing potential energy.

Exam traps to avoid

Coulomb force is enormously stronger than gravity between the same particles.
Potential energy is signed; force magnitude is not.