Question

A uniform electric field and a uniform magnetic field exist in a region in the same direction. An electron is projected with a velocity pointed in the same direction. Then the electron will:

• A. be deflected to the left without increase in speed.
• B. be deflected to the right without increase in speed.
• C. not be deflected but its speed will decrease.
• D. not be deflected but its speed will increase.

Answer 1

Answer: (C)

Force on a charged particle in the electromagnetic field, $\overrightarrow{F}=q\left(\overrightarrow{v}\times \overrightarrow{B}\right)+q\overrightarrow{E}$
As the electron is moving parallel to both the fields, so magnetic force does not affect the electron's motion. But electric force $(qE)$ acts opposite to the motion of the electron. Hence, the electron will not be deflected, but its speed will decrease.