JEE Main 2024 — Magnetic Effects of Current Question with Solution

Net force on proton inside an electric and a magnetic field is given by

${\overrightarrow{F}}_e+{\overrightarrow{F}}_m=q\overrightarrow{E}+q\left(\overrightarrow{v}\times \overrightarrow{B}\right) ...\left(1\right)$

If both $\overrightarrow{E}$ and $\overrightarrow{B}$ are zero, then ${\overrightarrow{F}}_e$ and ${\overrightarrow{F}}_m$ both are zero. Hence, velocity may remain constant. Therefore, option (A) is correct.

If $E=0, B\ne 0$, but velocity is parallel or antiparallel to the magnetic field, then also ${\overrightarrow{F}}_e$ and ${\overrightarrow{F}}_m$ both are zero. Hence, option (B) is also correct.

If $E\ne 0, B\ne 0$, but ${\overrightarrow{F}}_e + {\overrightarrow{F}}_m=0$, then also velocity may remain constant. Thus, option (D) is also correct.