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Physics · 2025 · 3 marks
CBSE 2025 · Region 4 · Set 2 · Q23
(a) A charged particle q moving with a velocity v is subjected to a uniform magnetic field B acting perpendicular to v. If a uniform electric field E…
(a)
A charged particle $\displaystyle q$ moving with a velocity $\displaystyle \vec{\mathrm{v}}$ is subjected to a uniform magnetic field $\displaystyle \overrightarrow{\mathrm{B}}$ acting perpendicular to $\displaystyle \overrightarrow{\mathrm{v}}$. If a uniform electric field $\displaystyle \overrightarrow{\mathrm{E}}$ is also set up in the region along the direction of $\displaystyle \overrightarrow{\mathrm{B}}$, describe the path followed by the particle and draw its shape.
(b)
How will the magnetic field inside a long solenoid be affected when :
(i)
the radius of the turns of the solenoid is increased,
(ii)
the length of solenoid as well as the total number of its turns are doubled?
Marking-scheme solution
(a)
Due to the magnetic field the particle will follow a circular path, and due to the electric field the particle will accelerate along the electric field.
As a result the particle will follow a helical path with constant radius but increasing pitch.
(b)(i)
$\displaystyle B=\frac{\mu_{0} N I}{l}$ — No change
(ii)
$\displaystyle \frac{N}{l}=\frac{2 N}{2 l}$ — No change
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CBSE Class 12 Physics past-paper question from the 2025 board exam, with the answer as CBSE’s own marking scheme gives it. Where our answers come from.