An electron is moving along positive x-axis. To get it moving on an anticlockwise circular path in xy-plane, a magnetic field is applied.

For the particle to move along anticlock wise path, force should be along   j^. Velocity is along   i^.Now F¯m=−eV¯×B¯in terms of unit vectors only. j^=−i^×?^ j^=?^×i^ Clearly, ?^ is  k^.