Question

An electron moving with uniform velocity in x direction enters a region of uniform magnetic field along y direction. Which of the followed physical quantity(ies) is (are) non-zero and remain constant?
I. Velocity of the electron.
II Magnitude of the momentum of the election.
III. Force on the electron.
IV. The kinetic energy of electron.

• A. Only I and II
• B. Only III and IV
• C. All four
• D. Only II and IV

Answer 1

The correct option is C All four

When an electron moves in the external magnetic field, a magnetic force acts on the electron perpendicular to the direction of motion of the electron. Thus magnetic force does not do any work on the electron. Hence its kinetic energy remains constant.

Momentum of electron $p=\sqrt{2mE}$

$⟹$ Momentum of the electron is also constant as kinetic energy $E$ is constant.

Similarly, velocity of electron $v=\frac{p}{m}$

$⟹$ $v=$ constant (as $p$ is constant).

Now magnetic force acting on th electron $F=qvB$

$\therefore$ $(v,B)=$ constant $⟹F=$constant

Hence all four remain constant.