Question

A charged particle is moved along a magnetic field line. The magnetic force on the particle is 

Options

• along its velocity

•  opposite to its velocity

• perpendicular to its velocity

• zero

Answer 1

zero 

The force on a charged particle q moving with velocity v in a magnetic field B is given by

\[\vec{F} = q( \vec{v} \times \vec{B} )\] 

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.

\[( \vec{v}  \times  \vec{B} )   =   0\] 

\[ \Rightarrow  \vec{F}    =   0\]

So, the magnetic force on the particle will be zero.