An electron is moving in a region of electric field and magnetic field. The kinetic energy of the electron
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → would change due to electric field.
Kinetic energy changes only when a force does work on the particle. The magnetic force \( q(\vec{v} \times \vec{B}) \) is always perpendicular to the velocity, so it does no work; it can change the direction of motion but not the speed. The electric force \( q\vec{E} \) does not depend on the velocity and in general has a component along it, so it does work and changes the kinetic energy.
\( \displaystyle W_{\mathrm{magnetic}} = \vec{F} \cdot \vec{v}\,dt = q(\vec{v} \times \vec{B}) \cdot \vec{v}\,dt = 0 \)
Why the other options are incorrect
Option (2) would change due to magnetic field — Incorrect. The magnetic force is perpendicular to the velocity at every instant, so it does no work.
Option (3) would change due to both of these fields — Incorrect. Only the electric field contributes; the magnetic field adds nothing to the change in kinetic energy.
Option (4) would not change. — Incorrect. The electric force does work on the moving electron, so the kinetic energy changes in general.
NCERT (2026-27):
Motion in a Magnetic Field – “… In the case of motion of a charge in a magnetic field, the magnetic force is perpendicular to the velocity of the particle. So no work is done and no change in the magnitude of the velocity is produced (though the direction of momentum may be changed). [Notice that this is unlike the force due to an electric field, \( q\mathbf{E} \), which can have a component parallel (or antiparallel) to motion and thus can transfer energy in addition to momentum.]”