Any charged particle in an electric field will feel a force proportional to the charge and field strength such that F → → = q E → → , where F is force, q is charge, and E is electric field strength. Similarly, any particle moving in a magnetic field will feel a force proportional to the velocity and charge of the particle. The force felt by any particle is then equal to F → → = q v → → × × B → → , where F is force, q is the charge on the particle, v is the velocity of the particle, B is the strength of the magnetic field, and × × is the cross product. In the case of a velocity selector, the magnetic field is always at 90 degrees to the velocity and the force is simplified to F = q v B in the direction described by the cross product.