Question

A particle of charge $q$ and mass $m$ moves in a circular path of radius $r$ in a uniform magnetic field $B$ at a speed $v$.
The particle is replaced with a particle of identical mass and charge $q$, moving at speed $v$.
The radius of the new circular path the particle makes is:

• A. $\frac{1}{4}r$
• B. $\frac{1}{2}r$
• C. $r$
• D. $2r$
• E. $4r$

Answer 1

Answer: (C)

$r$

The radius of charged particle q (mass=m), moving with velocity v, perpendicular to magnetic field B is given by,

r=mv/qB

r=mv/q

as new particle has same mass, same charge and same velocity v, therefore radius r will be same for new particle,

now if new particle has charge - q, then by Fleming's left hand rule, it will experience a force in the opposite direction as experienced by charge q, therefore path of new particle will be counter-clockwise.