Question

A proton ( mass $m$, charge $e$) projected with a velocity $v$ passes undeviated through a region of crossed electric and magnetic fields. The velocity with which an alpha particle (mass $4m$, charge $2e$) should be projected so that it passes undeviated through the same region is:

• A. $V$
• B. $2V$
• C. $4V$
• D. $8V$

Answer 1

The correct option is A $V$

As, in case of no deviation, Force due to electric field= Force due to magnetic field
Thus, $qvB=qE$
Thus, $vB=E$
As, $E$ and $B$ are same, this implies , $v$ must be same irrespective of charge or mass