Question bank/NEET UG/Physics

NEET UGPhysicsModerateNEET 2025

An electron (mass 9×10−319 \times 10^{-31} kg and charge 1.6×10−191.6 \times 10^{-19} C) moving with speed c/100 (cc = speed of light) is injected into a magnetic field B⃗\vec{B} of magnitude 9×10−49 \times 10^{-4} T perpendicular to its direction of motion. We wish to apply an 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 = 3×1083 \times 10^8 ms−1^{-1})

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Sol. For no deflection of electron, F⃗B=F⃗E\vec{F}_B = \vec{F}_E

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−e(v⃗×B⃗)=−eE⃗-e(\vec{v} \times \vec{B}) = -e\vec{E}

⇒E⃗=v⃗×B⃗⇒E⃗⊥B⃗\Rightarrow \vec{E} = \vec{v} \times \vec{B} \Rightarrow \vec{E} \perp \vec{B}

E=vB=c100×9×10−4E = vB = \frac{c}{100} \times 9 \times 10^{-4}

=3×108100×9×10−4= \frac{3 \times 10^8}{100} \times 9 \times 10^{-4}

=27×102= 27 \times 10^2 V m−1^{-1}

Source: geekyrakshit/indian-exam-questions / neet2025-code45

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