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JEE Main 2025 Jan 24 Shift 2, Physics Q3: Motion of a Charged Particle in a Magnetic Field

JEE Main2025Jan 24, Shift 2Physics
Q.

Given below are two statements. One is labelled as Assertion (A) and the other is labelled as Reason (R).

Assertion (A) : A electron in a certain region of uniform magnetic field is moving with constant velocity in a straight line path.

Reason (R) : The magnetic field in that region is along the direction of velocity of the electron.

In the light of the above statements, choose the correct answer from the options given below :

  1. A

    (A) is false but (R) is true

  2. B

    Both (A) and (R) are true and (R) is the correct explanation of (A)

  3. C

    Both (A) and (R) are true but (R) is NOT the correct explanation of (A)

  4. D

    (A) is true but (R) is false

Solution

The magnetic force on a charged particle is . The cross product vanishes when (or anti-parallel), i.e. when or .

If , the electron experiences no acceleration and continues with constant velocity in a straight line. So Assertion (A) is correct.

The Reason (R) provides exactly this mechanism: aligning the field with the velocity makes the magnetic force zero. So (R) is also correct and is the right explanation of (A).

Correct option: (2)

Concept behind this question

Motion of a Charged Particle in a Magnetic FieldNotes, formulas and examples →

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