CBSE 2026 · Region 3 · Set 1 · Q30 · 4 marks

A charged particle +q in an electric field $\displaystyle \overrightarrow{\mathrm{E}}$ experiences a force in the direction of the electric field. As a result, its kinetic energy changes. Similarly, the charged particle also experiences a force when it moves in a magnetic field $\displaystyle \overrightarrow{\mathrm{B}}$. But this magnetic force is perpendicular to both velocity $\displaystyle \overrightarrow{\mathrm{v}}$ of the charged particle and the magnetic field $\displaystyle \overrightarrow{\mathrm{B}}$, so it cannot change the kinetic energy of the charged particle. Consider two charged particles $\displaystyle 1$ and $\displaystyle 2$ of masses m and $\displaystyle \frac{\mathrm{m}}{2}$ having charges -q and +$\displaystyle 2$ q respectively. They are accelerated from rest through the same potential difference V and acquire kinetic energy $\displaystyle \mathrm{K}_{1}$ and $\displaystyle \mathrm{K}_{2}$. Then they enter in a region of uniform magnetic field $\displaystyle \overrightarrow{\mathrm{B}}$ perpendicular to their velocities.

(i)

The ratio of their kinetic energies $\displaystyle \left(\frac{\mathrm{K}_{1}}{\mathrm{~K}_{2}}\right)$ is :

(A)

$\displaystyle \frac{1}{2}$

(B)

$\displaystyle \frac{1}{4}$

(C)

$\displaystyle 4$ (D) $\displaystyle 1$ (ii) The ratio of the radii of the circular paths described by them $\displaystyle \left(\frac{\mathrm{r}_{1}}{\mathrm{r}_{2}}\right)$ is :

(A)

$\displaystyle \frac{1}{\sqrt{2}}$

(B)

$\displaystyle \sqrt{2}$

(C)

$\displaystyle \frac{1}{2}$

(D)

$\displaystyle 2$ (iii) Suppose particles $\displaystyle 1$ and $\displaystyle 2$ enter the magnetic field $\displaystyle \overrightarrow{\mathrm{B}}=\mathrm{B}_{0} \widehat{\mathrm{k}}$ with velocities $\displaystyle \vec{\mathrm{v}}_{1}=\mathrm{v}_{1} \hat{i}$ and $\displaystyle \vec{\mathrm{v}}_{2}=\mathrm{v}_{2} \hat{i}$. Then :

(A)

both particles revolve clockwise

(B)

both particles revolve anticlockwise

(C)

particle $\displaystyle 1$ revolves clockwise while particle $\displaystyle 2$ revolves anticlockwise

(D)

particle $\displaystyle 1$ revolves anticlockwise while particle $\displaystyle 2$ revolves clockwise

(iv)

(a)OR(b)

If period of revolution for particle $\displaystyle 1$ is $\displaystyle 4$ s , then for particle $\displaystyle 2$, the period will be :

(A)

$\displaystyle 1$ s

(B)

$\displaystyle 2$ s

(C)

$\displaystyle 4$ s

(D)

$\displaystyle 8$ s

CBSE Class 12 Physics past-paper question from the 2026board exam, with the answer as CBSE’s own marking scheme gives it. Where our answers come from.