Question

A charged particle is released from rest in a region of uniform electric field and magnetic field, which are parallel to each other. The locus of the particle will be

• A. helix of constant pitch
• B. cycloid
• C. helix of varrying pitch
• D. straight line

Answer 1

The correct option is D straight line

Since, the particle starts from rest, at the beginning only electric force acts on the particle.

Now, this electric force provides an accelaration to the particle due to which its velocity increases.

However, the velocity is along the $\overrightarrow{E}$ as well as $\overrightarrow{B}$, so the magnetic force acting on the particle is always zero.

Hence, the particle will travel in a straight line path on account of the electric force acting on it.

Hence, option $\left(d\right)$ is the correct answer.

Why this Question? Note: Always remember that if the velocity is either parallel or anti-paralle to the magnetic field, the magnetic force acting on the particle is zero. However, the electric force is independent of the velocity of the particle.