Question

A positive point charge +Q is fixed in space. A negative point charge -q of mass m revolves around a fixed charge in elliptical orbit. The fixed charge +Q is at one focus of the ellipse. The only force acting on negative charge is the electrostatic force due to positive charge. Then which of the following satement is true.

• A. Linear momentum of negative point charge is conserved
• B. Angular momentum of negative point charge about fixed positive charge is conserved
• C. Total kinetic energy of negative point charge is conserved
• D. Electrostatic potential energy of system of both point charges is conserved

Answer 1

The correct option is B

Angular momentum of negative point charge about fixed positive charge is conserved

This is similar to a planet revolving around it's star. It's not in uniform circular motion. So its magnitude of velocity and hence linear momentum and KE keep changing. Only angular momentum X & total energy are conserved but not kinetic energy