Question

An electron enters a chamber in which an uniform magnetic field is present as shown in figure. Ignore gravity.

During its motion inside the chamber

• A. the force on the electron remains constant
• B. the kinetic energy of the electron remains constant
• C. the momentum of the electron remains constant
• D. the speed of the electron increases at a uniform rate

Answer 1

Answer: (B)

Magnetic force $F$ acting on the electron is perpendicular to $v$ and $B.$
$F=q . (V\times B) $
As $F$ is perpendicular to $v$ so, $F \cdot v = 0$.
Hence, work done by force is zero.
As force is not doing any work so magnitude of velocity or kinetic energy of electron remains constant.