Question

A charged particle is moved along a magnetic field line. The magnetic force on the particle is
(a) along its velocity
(b) opposite to its velocity
(c) perpendicular to its velocity
(d) zero.

Answer 1

(d) zero

The force on a charged particle q moving with velocity v in a magnetic field B is given by
$\overrightarrow{F}=q(\overrightarrow{v}\times \overrightarrow{B})$
As the charge is moving along the magnetic line of force, the velocity and magnetic field vectors will point in the same direction, making a cross product.
$$\begin{gathered} (\overrightarrow{v}\times \overrightarrow{B})=0 \\ \Rightarrow \overrightarrow{F}=0 \end{gathered}$$
So, the magnetic force on the particle will be zero.