moving charge have its own magnetic field becoz moving charges produces magnetic field and other one is the applied magnetic field when we place the charge in the magnetic field (the applied magnetic field and field which is produced by a moing charge interacts each other )and the charge q is in the applied magnetic field experiences a force and this force depends upon the three factor i,e the magnitude of charge the component of velocity perpendicular to the magnetic field and the magnitude of applied field on combining these we get force ,F= qvBsintheta and the direction of force is given by right hand thumb rule and the condition is that if the charge is perpendicular to the magnetic than the force acting on the charge is maximum the direction of this maximum force is given by flemming's left hand rule this max force is equal to ,F (max) = qvB only becoz the charge is perpendicular to the magnetic field u know sin90° = 1 .....if u understand that pls vote