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Saturday, 31 October

JEE Main 2020 · PhysicsMultiple choiceSingle correctMediumMulti-step

JEE Main 9 January 2020, Shift 2, Physics Q14

Question 14 of 75 in this shift, Physics question 14 of 25, Section A.

An electron gun is placed inside a long solenoid of radius RR on its axis. The solenoid has nn turns/length and carries a current II. The electron gun shoots an electron along the radius of the solenoid with speed vv. If the electron does not hit the surface of the solenoid, maximum possible value of vv is (all symbols have their standard meaning) :

The figure, in words

Long solenoid of radius R drawn along the z-axis (x, y, z axes shown) with current arrows around the windings.
  1. (1)eμ0nIRm\frac{e\mu_0 nIR}{m}
  2. (2)eμ0nIR2m\frac{e\mu_0 nIR}{2m}Official answer
  3. (3)eμ0nIR4m\frac{e\mu_0 nIR}{4m}
  4. (4)2eμ0nIRm\frac{2e\mu_0 nIR}{m}

Official answer

Option 2

NTA final key (Jan 2020).

Chapter
Moving Charges and Magnetism
Topic
Lorentz Force
Idea tested
Circular Motion in Uniform B-Field

Same idea in other shifts

Asked 7× in all
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  3. 28 Jun 2022, Shift 1 · Q59A singly ionized magnesium atom (A=24) ion is accelerated to kinetic energy 5 keV, and is projected perpendicularly into a magnetic field…MediumNumerical value
  4. 4 Aug 2021, Shift 1 · Q9Assume there is a possibility of constant Electric field and Magnetic field and a charged particle under the influence of these forces…EasySingle correct
  5. 25 Jul 2021, Shift 2 · Q14Two ions having same mass have charges in the ratio 1 : 2. They are projected normally in a uniform magnetic field with their speeds in…EasySingle correct
  6. 18 Mar 2021, Shift 2 · Q16A proton and an α\alpha-particle, having kinetic energies KpK_p and KαK_\alpha respectively, enter into a magnetic field at right angles.…MediumSingle correct

Question text from the official JEE Main paper published by NTA; answer from the final answer key. Chapter, topic and idea tags, difficulty and skill are Lumi’s. This question needs a figure we do not reproduce; it is described in words above. Lumi analyses 108 of the 174 JEE Main shifts held from 2017 to 2026.