Alpha, beta and gamma rays emitted by a radioactive substance are allowed to pass through a region containing a magnetic field at right angles to their path. The energy gained will be
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → zero for all of them
The magnetic force on a moving charge, \( \vec{F} = q\,\vec{v}\times\vec{B} \), is always perpendicular to the velocity, so it does no work: \( W = \vec{F}\cdot\vec{v}\,\Delta t = 0 \).
Alpha and beta particles are charged, so the field bends their paths, but their speed and kinetic energy stay the same. Gamma rays are uncharged, so the field does not act on them at all. The energy gained is zero for all three.
Option (4) zero for all of them — Correct. A magnetic field can change the direction of a charge, never its speed or energy.
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).”