A charge particle is moving in a circular path in a magnetic field. A resistive force starts acting on the particle whose direction is oppositely directed to its motion and magnitude is directely proportional to its velocity. Now the particle starts moving in a spiral path, then

Options

A. Angular velocity of the particle decreases continuously.
B. Angular momentum of the particle remains constant
C. Magnetic field at lying only perpendicular to the plane of coil
D. Net force acting on the particle remains constant.

Answer

C

Explanation

Radius `r` of particle in spiral path at any time when velocity is `v`
`r=(mv)/(qB) omega=v/r=(qB)/m. implies` constant
Angular momentum `L=mvr=qBr^(2)` decreases as `r` decreases
If particle is moving in a plane then its velocity and acceleration will be that plane. The force on the particle will also be in that plane. It means magnetic field should be perpendicular to that plane.
Mangnetic force on the particle `=qvB`
Resisitive on the particle `=kv`, where `k` is a constant
As velocity decreases so net force acting on the particle will decreases.