Question

The mono energetic beams of electrons moving along +y direction enter a region of uniform electric and magnetic fields. If the beam goes straight through these simultaneously, then field B and E are directed possibly along

• A. -y axis and -z axis
• B. +z axis and -x axis
• C. + x axis and - x axis
• D. -x axis and -y axis

Answer 1

Answer: (B)

+z axis and -x axis

$\overrightarrow{V}=V\left(\hat{j}\right)$
Hence , B and E should be perpendicular to each other and also perpendicular to the velocity . Hence , B and E should be in z - axis and x axis
If B is towards + z axis , Force = q(V \times B ) is in + x direction . To balance this , E has to be in -x direction