Solved: PHYSICS Question for NEET

NEET PHYSICSEasy

A beam of electrons passes un-deflected through mutually perpendicular electric and magnetic fields. Where do the electrons move if the electric field is switched off and the same magnetic field is maintained?

in an elliptical orbit.

in a circular orbit.

along a parabolic path.

along a straight line.

Step-by-Step Solution

Initial State (Undeflected): When the electron beam passes undeflected, the electric force ( $F_E = qE$ ) balances the magnetic force ( $F_B = qvB$ ). This implies the velocity vector $\vec{v}$ , electric field $\vec{E}$ , and magnetic field $\vec{B}$ are mutually perpendicular.
Final State (E switched off): When the electric field is removed, only the magnetic force acts on the electrons: $\vec{F} = q(\vec{v} \times \vec{B})$ .
Resulting Motion: Since the velocity $\vec{v}$ is perpendicular to the magnetic field $\vec{B}$ , the magnetic force acts as a centripetal force ( $F = qvB$ ), perpendicular to both $\vec{v}$ and $\vec{B}$ at all instants. This causes the electrons to move in a uniform circular orbit .

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