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Saturday, 31 October

JEE Main 2025 · PhysicsMultiple choiceDiagram basedHardMulti-step

JEE Main 7 April 2025, Shift 1, Physics Q40

Question 40 of 75 in this shift, Physics question 15 of 25, Section A.

Uniform 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 charged particle of mass m and charge q, at the interface at an instant, moves into the region 2 with velocity vv and returns to the interface. It continues to move into region 1 and finally reaches the interface. What is the displacement of the particle during this movement along the interface ? (Consider the velocity of the particle to be normal to the magnetic field and B2>B1B_2 > B_1)

The figure, in words

Horizontal interface separating Region 1 (above, field B1) and Region 2 (below, field B2); crosses (field into the plane of the paper) fill both regions.
  1. (1)mvqB1(1−B2B1)×2\frac{mv}{qB_1}\left(1 - \frac{B_2}{B_1}\right) \times 2
  2. (2)mvqB1(1−B1B2)\frac{mv}{qB_1}\left(1 - \frac{B_1}{B_2}\right)
  3. (3)mvqB1(1−B1B2)×2\frac{mv}{qB_1}\left(1 - \frac{B_1}{B_2}\right) \times 2Official answer
  4. (4)mvqB1(1−B2B1)\frac{mv}{qB_1}\left(1 - \frac{B_2}{B_1}\right)

Official answer

Option 3

NTA final key.

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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  5. 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
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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.