Question

An electron is fired parallel to a uniform electric and uniform magnetic field acting simultaneously and in the same direction. The electron

• A. moves along a circular path
• B. moves along a parabolic path
• C. loses kinetic energy
• D. gains kinetic energy

Answer 1

The correct option is C loses kinetic energy

As we know that,

$\overrightarrow{F_m}=q(\overrightarrow{V}\times \overrightarrow{B})$ and $\overrightarrow{F_e}=q\overrightarrow{E}$

Since electron is fired parallel to both $\overrightarrow{E}\&\overrightarrow{B}$

$\therefore \theta =0$

So, magnetic force will be

$F_m=(-e)vB\sin 0^{\circ }=0$

It means only electric force will act on the electron which is

$F_e=(-e)E$

Since, charge on the electron is negative, means force will act in the opposite to the direction of velocity because velocity of electron and electric field have same direction.

Therefore, velocity of electron decreases with time that means kinetic energy also decreases.

Hence, option $\left(c\right)$ is right.