A particle of charge q and mass m starts moving from the origin under the action of an electric field E→=E0i^ and B→=B0i^with a velocity v→=v0j^. The speed of the particle will become 2 v0 after a time

E→ is parallel to B→ and v→ is perpendicular to both. Therefore, path of the particle is a helix with increasing pitch. Speed of particle at any time t isv=vx2+vy2+vz2---(1)  Here, vy2+vz2=v02---(2)  and v=2v0------(3) from (1), (2) & (3)  we get vx=3v0 vx=axt⇒3v0=qEmt⇒t=3mv0qE