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Q. An electron gun is placed inside a long solenoid of radius $R$ on its axis. The solenoid has $n$ turns/length and carries a current $I$. The electron gun shoots an electron along the radius of the solenoid with speed $v$. If the electron does not hit the surface of the solenoid, maximum possible value of $v$ is ( all symbols have their standard meaning) :Physics Question Image

JEE MainJEE Main 2020Moving Charges and Magnetism

Solution:

Top view of solenoid
Maximum possible radius of electron $= \frac{R}{2}$
$\therefore \frac{R}{2} = \frac{mv}{qB} = \frac{mv_{max}}{e\left(\mu_{0}ni\right)}$
$v_{max} = \frac{R}{2} \frac{e\,\mu _{0}ni}{m}$

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