An electron falling freely under the influence of gravity enters a uniform magnetic field directed towards south. The electron is initially deflected towards

• Correct answer
  A

  east
• B

  west
• C

  north
• D

  south

Solution

For the freely falling electron.
$\begin{aligned} & \overrightarrow{\mathrm{v}}=-v_0 \hat{\mathrm{k}}, \overrightarrow{\mathrm{B}}=-\mathrm{B}_0 \hat{\mathrm{j}} \\ & \therefore \overrightarrow{\mathrm{F}}=-\mathrm{e}(\overrightarrow{\mathrm{v}} \times \overrightarrow{\mathrm{B}})=-\mathrm{e}\left[\left(-v_0 \hat{\mathrm{k}}\right) \times\left(-\mathrm{B}_0 \hat{\mathrm{j}}\right)\right] \\ & =-\mathrm{v}_0 B_0(-\hat{\mathrm{i}})=\mathrm{v}_0 B_0 \hat{\mathrm{i}}=e v_0 B_0 \text { (East) }\end{aligned}$ $\therefore$ The electron is deflected to wards east.

Hence, the correct answer is option: 'A'.