Question

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
• D. E and B both are parallel to v

Answer 1

Answer: (C)

E, B and v are mutually perpendicular and

Answer 2

In this case $\left| \overrightarrow { F _ { e } } \right| = \left| \overrightarrow { F _ { m } } \right|$ and both forces are opposite to each other.