When a positively charged particle enters into a uniform magnetic field with uniform velocity its trajectory can be
i) a straight line
ii) a circle
iii) a helix

$\overrightarrow{\mathrm{F}}=\mathrm{q}(\overrightarrow{\mathrm{v}}\times \overrightarrow{\mathrm{B}})$

$\left|\overrightarrow{\mathrm{F}}\right|=\mathrm{q}(\left|\overrightarrow{\mathrm{v}}\right|\times \left|\overrightarrow{\mathrm{B}}\right|)\mathrm{sin\theta }$

if $\mathrm{\theta }=0^0 \mathrm{or} {180}^0$ , force will be zero. So, the particle will follow straight path.

If $\mathrm{\theta }={90}^0 , \mathrm{F}=\mathrm{qvB}$ and its direction will be perpendicular to both velocity and magnetic field. So, the path will be circular.

If angle between velocity and the magnetic field is other than 0 and 180 degrees, then component of the velocity along the line of field produces pitch and the component of the velocity perpendicular to the field direction produces the circular path of the particle. So, the path is helical.