...........

An electron is launched with velocity v in a uniform magnetic field B. The angle θ between v and B lies between 0and л/2.Its velocity vector v returns to its initial value in a time interval of?

Hello
It is not at all a hard question but very interesting.It will be a fun if we discuss the question deeply.
Let consider B to be along +ve x-axis and V making an angle $ with the x-axis(0<$<PI/2).
Resolve v in x-axis and y-axis.As v_x is along with B, B will not at all effect it .but v_y will get a force which will enforce it to move in a circular path.But observe that both the velocity is going to remain unchanged.Thus just after every full revolution of electron U must be knowing about what is t for one revolution .$/frac{2pim}{eB}$
Thank u.