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Friday, 9 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 2× in all
  1. 7 Apr 2025, Shift 1 · Q40Uniform magnetic fields of different strengths (B1B_1 and B2B_2), both normal to the plane of the paper exist as shown in the figure. A…HardDiagram basedHas a figure

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 42 of the 174 JEE Main shifts held from 2017 to 2026.