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Question: Answered & Verified by Expert
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):

PhysicsMagnetic Effects of CurrentJEE MainJEE Main 2020 (09 Jan Shift 2)
Options:
  • A eμ0nIRm
  • B eμ0nIR2m
  • C eμ0nIR4m
  • D 2eμ0nIRm
Solution:
1769 Upvotes Verified Answer
The correct answer is: eμ0nIRm

Magnetic Field Inside Solenoid is B=μ0nI

Velocity ν has given to the electron 

So, Magnetic Force

F=qνB

as ν and B are perpendicular.

In a magnetic field, charge will move on a circular path. Here, If radius is just less than or equal to radius of solenoid then the electron does not hit the surface.

F=qνB=mν2Rmν=qBRas  q=eν=eBRmSo,V=eμ0nIRm
 

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