Question

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

• A. Linear momentum of negative point charge is conserved.
• B. Angular momentum of negative point charge about the fixed positive charge is not conserved.
• C. Angular momentum of negative point charge about the fixed positive charge is conserved.
• D. Angular velocity of the charge $q$ is conserved about $+Q$ charge.

Answer 1

Answer: (C)

Since net force on the negative charge is always directed towards fixed positive charge, the torque on negative charge about positive charge is zero. Therefore angular momentum of negative charge about the fixed positive charge is conserved.
$L=\left(m{r}^2\right)\omega$
$\therefore$ As value of r changes, $\omega$ also changes