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NEET 2025 · Moving Charge in a Magnetic Field

Moving Charge in a Magnetic Field question from NEET 2025

Q21NEET 2025Medium

Moving Charge in a Magnetic Field

An electron (mass 9×10−31 kg9 \times 10^{-31}\text{ kg} and charge 1.6×10−19 C1.6 \times 10^{-19}\text{ C}) moving with speed c/100 (c=c/100\ (c = speed of light) is injected into a magnetic field B⃗\vec{B} of magnitude 9×10−4 T9 \times 10^{-4}\text{ T} perpendicular to its direction of motion. We wish to apply a uniform electric field E⃗\vec{E} together with the magnetic field so that the electron does not deflect from its path. Then (speed of light c=3×108 ms−1c = 3 \times 10^{8}\text{ ms}^{-1})

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Answer: (d)

  1. No deflection: electric and magnetic forces cancel, eE=evB⇒E=vBeE = evB \Rightarrow E = vB, with E⃗⊥B⃗\vec{E} \perp \vec{B} and ⊥v⃗\perp \vec{v}.
  2. v=3×108100=3×106v = \dfrac{3 \times 10^{8}}{100} = 3 \times 10^{6} m/s
  3. E=3×106×9×10−4=2700=27×102E = 3 \times 10^{6} \times 9 \times 10^{-4} = 2700 = 27 \times 10^{2} V/m
  4. So the answer is (d).
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