A charged particle carrying a charge +Q is fixed at point 0, A uniform magnetic field of induction B→ exists in the region which points out of the plane of the page. An electron is projected from point P with velocity V→ as shown in the figure. Then path of the electron

From the diagram it is evident that electric force F=14π∈oQe(OP)2 acting on the electron is radially inward and the magnetic force F→=-eV→×B→ is also radially inward. Therefore net force acting on the electron is always radially in ward and always perpendicular to the direction of motion. Hence the path is a circle.