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Magnetics question

2023 · 13 Apr · Shift 2 · Q59
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Magnetics question

2023 · 13 Apr · Shift 2 · Q59

JEE MainPhysicsMagneticsMCQ+4 / −1
An electron is moving along the positive x\mathrm{x}x-axis. If the uniform magnetic field is applied parallel to the negative z-axis, then A. The electron will experience magnetic force along positive y-axis B. The electron will experience magnetic force along negative y-axis C. The electron will not experience any force in magnetic field D. The electron will continue to move along the positive x\mathrm{x}x-axis E. The electron will move along circular path in magnetic field Choose the correct answer from the options given below:
  1. A
    A and E only
  2. B
    B and D only
  3. C
    B and E only
  4. D
    C and D only
View written solutionFree

Correct answer: C

  1. Given

    • Electron velocity is along positive xxx-axis: v⃗=+i^\vec v = +\hat iv=+i^
    • Magnetic field is along negative zzz-axis: B⃗=−k^\vec B = -\hat kB=−k^
  2. Magnetic force formula F⃗=q(v⃗×B⃗)\vec F = q(\vec v \times \vec B)F=q(v×B) For an electron, charge is negative: q=−eq=-eq=−e

  3. Find v⃗×B⃗\vec v \times \vec Bv×B v⃗×B⃗=i^×(−k^)=−(i^×k^)\vec v \times \vec B = \hat i \times (-\hat k) = -(\hat i \times \hat k)v×B=i^×(−k^)=−(i^×k^)

    Using cross-product relations: i^×k^=−j^\hat i \times \hat k = -\hat ji^×k^=−j^​

    Therefore, v⃗×B⃗=−(−j^)=j^\vec v \times \vec B = -(-\hat j)=\hat jv×B=−(−j^​)=j^​

    So for a positive charge, force would be along positive yyy-axis.

  4. Since the particle is an electron The electron has negative charge, so the force direction is opposite to j^\hat jj^​: F⃗=−e(j^)\vec F = -e(\hat j)F=−e(j^​) Hence force is along negative yyy-axis.

    Therefore, statement B is correct and A is false.

  5. Nature of motion The velocity is perpendicular to the magnetic field: v⃗⊥B⃗\vec v \perp \vec Bv⊥B In a uniform magnetic field, when v⃗⊥B⃗\vec v \perp \vec Bv⊥B, the charged particle moves in a circular path.

    Therefore, statement E is correct.

  6. Check remaining statements

    • C: "The electron will not experience any force" → false
    • D: "The electron will continue to move along positive xxx-axis" → false, because magnetic force changes its direction continuously.
  7. Correct combination Correct statements are B and E only.

    From the given options, this corresponds to: Option C\boxed{\text{Option C}}Option C​

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