CBSE 2025 · Region 1 · Set 3 · Q23 · 3 marks

A particle of charge $\displaystyle q$ is moving with a velocity $\displaystyle \vec{v}$ at a distance ' $\displaystyle d$ ' from a long straight wire carrying a current ' $\displaystyle I$ ' as shown in figure. At this instant, it is subjected to a uniform electric field $\displaystyle \overrightarrow{\mathrm{E}}$ such that the particle keeps moving undeviated. In terms of unit vectors $\displaystyle \hat{i}, \hat{j}$ and $\displaystyle \hat{k}$, find -

(a)

the magnetic field $\displaystyle \overrightarrow{\mathrm{B}}$,

(b)

the magnetic force $\displaystyle \overrightarrow{\mathrm{F}}_{\mathrm{m}}$, and

(c)

the electric field $\displaystyle \overrightarrow{\mathrm{E}}$, acting on the charge.

CBSE Class 12 Physics past-paper question from the 2025board exam, with the answer as CBSE’s own marking scheme gives it. Where our answers come from.