Question

A beam of cathode rays moving towards south is under the action of downward electric field. To prevent its deflection by the electric field, a magnetic field should be directed

• A. Eastward
• B. Westward
• C. Upward
• D. Downward

Answer 1

Answer: (A)

Eastward

For equilibrium of cathode rays write the force equation:
$q\overrightarrow{E}+q(\overrightarrow{v}\times \overrightarrow{B})=0$
$⟹\overrightarrow{B}\times \overrightarrow{v}=\overrightarrow{E}$
$\overrightarrow{v}$ is towards south and $\overrightarrow{E}$ is in downward direction, so $\overrightarrow{B}$ should be in east direction