Moving Charges And Magnetism

A positively charged particle \(q\) of mass $m$ is passed through a velocity selector. It moves horizontally rightward without deviation along the line \(y=\frac{2 m v}{q B}\) with a speed \(v\). The electric field is vertically downwards and magnetic field is into the plane of paper. Now, the electric field is switched off at \(t=0\). The angular momentum of the charged particle about origin \(O\) at \(t=\frac{\pi m}{q B}\) is