Q. A proton (or charged particle) moving with velocity $v$ is acted upon by electric field $E$ and magnetic field $B$. The proton will move undeflected if
A) $E$ is perpendicular to $B$
B) $E$ is parallel to $v$ and perpendicular to $B$
C) $E,\ B$ and $v$ are mutually perpendicular and $v = \frac{E}{B}$
D) $E$ and $B$ both are parallel to $v$
Explanation
(c) In this case $\left| \overrightarrow{F_{e}} \right| = |\overrightarrow{F_{m}}|$ and both forces are opposite to each other
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.